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Soil properties are resilient despite grass invasion, fire, and grazing

Soil properties are resilient despite grass invasion, fire, and grazing AbbreviationsKBGKentucky bluegrass‐dominated plotsMBCmicrobial biomass carbonPMNpotentially mineralizable nitrogenPOX Cpermanganate oxidizable carbon.INTRODUCTIONWhile agricultural expansion continues to advance in the northern Great Plains (Johnston, 2014; Wright & Wimberly, 2013), modern anthropogenic influence in this region has recently shifted from unsustainable depletion of natural resources towards stewardship (Chapin et al., 2009). While landowners historically perceived grasslands as a limitless resource for grazing (Briske, 2017) and fire as a liability (Bendel et al., 2020; Wonkka et al., 2015), landowners are gaining an appreciation for natural disturbances and heterogeneity in these grassland systems (Briske, 2017). Unfortunately, we must still cope with unforeseen ecological consequences of historical management.One such repercussion is the invasion of nonnative species such as Kentucky bluegrass (Poa pratensis L.; hereafter referred to as bluegrass). Often, when there are shifts in fire regime, conditions develop that facilitate the invasion of nonnative species such as bluegrass (D'Antonio, 2000). In addition, bluegrass is a grazing‐tolerant species, so historically over‐grazing grasslands may have displaced native species while facilitating the invasion of bluegrass (Toledo et al., 2014). Bluegrass has rapidly become one of the predominant species in the northern Great Plains (USDA‐NRCS, n.d.‐a), threatening biodiversity of native grassland communities, ecosystem services, and forage production (Gasch et al., 2020).Bluegrass also has the potential to alter soil properties, in part due to the turfgrass growth habit and formation of a thickened thatch layer (Etter, 1951). Thatch refers to the accumulation of litter and root biomass on top of, and intermingled with, the soil surface. In the case of bluegrass, this excess litter may alter the microclimate of the soil by acting as a buffer against atmospheric moisture and temperature (Bosy & Reader, 1995; Murray & Juska, 1977; Pierson et al., 2002). Furthermore, plant community shifts from native, diverse mixed‐grass prairie to a more uniform stand of turfgrass is likely to change organic matter inputs to the soil in terms of quantity, quality, and distribution. For example, monocultures of bluegrass have been found to cause lower soil carbon‐to‐nitrogen ratios than native C4 grass species (Wedin & Pastor, 1993). As the carbon‐to‐nitrogen ratio of residue inputs shifts, subsequent soil processes may also be affected. Bradley et al. (2006) found that increased nitrogen levels in grassland systems led to significant changes in the composition and relative abundance of soil microbial communities by increasing bacterial and decreasing fungal abundance. Increased nitrogen is also generally associated with increased decomposition rates; however, Knorr et al. (2005) suggests that increased nitrogen can have an inhibitory effect on certain decomposers, which may in turn dampen the stimulating effect of increased nitrogen. Overall, bluegrass dominance has the potential to affect a wide array of soil properties and ecological processes, due to its invasion extent and dominance, as well as its morphological and phenological differences from the historic native plant community.To minimize detrimental effects of bluegrass on the ecosystem, grassland management strategies such as fire, grazing, and their combination can diminish the competitive advantage of bluegrass. In general, studies show that grazing alone has a limited effect on bluegrass control (Biondini et al., 1998; Otfinowski, et al., 2017; Rogers et al., 2005). Both overgrazing and the exclusion of grazing can increase bluegrass levels (Grant et al., 2009). Fire, on the other hand, has been found to consistently reduce bluegrass cover and thatch, although the degree of control depends on fire regime as well as initial bluegrass invasion level (Ereth et al., 2017; Kral et al., 2018). Kral et al. (2018), McMurphy & Anderson, 1965, and Towne & Kemp, 2008 found that the decline in bluegrass cover following late‐growing season and dormant‐season burns persisted at least three years postfire. A combination of grazing and burning has been shown to increase plant species diversity and forage quality more than either process alone (Hartnett et al., 1996).Core IdeasFire and grazing were applied to Kentucky bluegrass‐invaded grasslands.Most carbon, nitrogen, and microbial properties did not differ between idle or treated sites.Labile carbon and decomposition rates were temporarily reduced after fire.Soil properties were remarkably stable in a northern prairie despite invasion and management.Fire and grazing have varied effects on soil. A 2018 review paper found that the effects of prescribed fires on soil properties depended on parameters such as fire regime, soil type, seasonality, residence time, and periodicity (Alcañiz et al., 2018). This review evaluated the effect of prescribed fire on a variety of soil properties, including chemical and biological properties. Soil nitrogen generally increased after fire, though excessively high fire temperatures (>200 °C) may lead to nitrogen volatilization (Alcañiz et al., 2018). Likewise, carbon stocks in the soil often increased after low‐intensity fires due to the incorporation of partially burned organic matter, while high‐intensity fires typically decreased carbon stocks (Alcañiz et al., 2018). The effect of fire on soil organisms was extremely variable. Results ranged from complete elimination of soil organisms (Barreiro et al., 2015; Choromanska & DeLuca, 2001; Switzer et al., 2012; Williams et al., 2012), to increased microbial activity resulting from the influx of ash materials (Blankenship & Arthur, 1999; Fultz et al., 2016; Gray & Dighton, 2009).Grazing may also play a role in shaping soil microbial communities, based on the results from a 2017 meta‐analysis on the subject (Zhao et al., 2017). Zhao et al. (2017) found that heavy grazing significantly decreased total microbial, bacterial, and fungal community size, yet brief, light, and moderate grazing had no effect on these properties. Furthermore, grazing intensity may also affect soil carbon and nitrogen pools. In a grazing intensity trial in the mixed‐grass prairie of North Dakota, Frank et al. (1995) found that soils in sites excluded from grazing had higher nitrogen content than in moderately or heavily grazed pastures. In this study, moderate grazing also decreased carbon content, while heavy grazing did not, likely due to changes in species composition resulting from the disturbance (Frank et al., 1995).Since fire and grazing often occur simultaneously on a landscape, we must consider the combined effect that these two disturbances may have on soil properties. A study in the tall‐grass prairie of Kansas found that grazing increased net nitrogen mineralization, while annual burning decreased it (Johnson & Matchett, 2001). These results suggest that fire and grazing may have contrasting effects on soil properties, and these disturbances may affect nutrient transformation rates. There is currently limited research on the coupled effect that fire and grazing (and subsequent shifts in plant species assemblages) may have on soil properties in the grasslands of the northern Great Plains.While the eradication of bluegrass from grasslands in the northern Great Plains is not a realistic option due to the extent of the invasion, Dornbusch et al. (2020) found that combined fire and grazing reduces the cover of bluegrass, as well as the influence of bluegrass on plant community composition over time compared with traditional management. With bluegrass established as a permanent fixture in these ecosystems and the need for adaptive management required on a greater scale, we must evaluate ecosystem effects, including any potential effects to belowground soil properties resulting from bluegrass dominance and associated vegetation management practices.The goal of our research was to measure the effect that bluegrass, and management with fire and grazing, may have on soil nutrient pools, microbial communities, and biological processes. We designed a survey to evaluate these soil properties in areas either (a) dominated by bluegrass and historically left idle, or (b) hosting a mix of bluegrass and native species and managed with fire and grazing. Between the idle and managed systems, we compared carbon and nitrogen fractions in both labile and stable soil pools, as well as microbial abundance and community structure. These measurements were stratified through time to understand the effects of fire and grazing on soil properties before and immediately after fire, as well as one year postfire. Additionally, we conducted in‐field litterbag incubations to determine if litter decomposition differed across these treatments. Results will help us understand how both bluegrass and land management techniques may affect soil properties in a northern mixed‐grass prairie of the Great Plains.MATERIALS AND METHODSStudy areaThis study was conducted during the summer of 2018 in mixed‐grass prairie at North Dakota State University Central Grasslands Research Extension Center in south‐central North Dakota (46°42′55.6″ N, 99°26′51.6″ W). Prairies in this region are characterized by the dominance of cool‐season, perennial grasses (Toledo et al., 2014). For data consistency, we limited our study to thin loamy ecological sites, which have a historical climax plant community that includes green needlegrass [Nassella viridula (Trin.) Barkworth], little bluestem [Schizachyrium scoparium (Nash) E.P. Bickwell], and western wheatgrass [Pascopyrum smithii (Rydb.) Á. Löve] (USDA‐NRCS n.d.‐b). However, following extended periods of idle and overgrazing management and increased moisture patterns, bluegrass proliferates and litter accumulates (USDA‐NRCS n.d.‐b).Extreme temperatures and a semi‐arid precipitation regime are characteristic in this region. Rainfall is typically the limiting factor in determining the plant community composition, with annual precipitation ranging from approximately 38–51 cm per year (USDA‐NRCS n.d.‐b), with the majority occurring during the growing‐season months. During the 2018 growing season, the study area received above‐average rainfall, with precipitation totals reaching 44.2 cm between April and September (NDAWN, 2019).Soils in the study area are typically well‐drained with low runoff depending on slope and vegetative cover (USDA‐NRCS n.d.‐b). Twelve monitoring sites were selected from map units identified within the Zahl‐Williams‐Zahill complex (fine‐loamy, mixed, superactive, frigid Typic Calciustolls, or Argiustolls) (Soil Survey Staff, n.d.). Typical soil profiles for these units include a thin layer of loam (0–15 cm) on top of clay loam extending through the substrata (Soil Survey Staff, n.d.). For the 2018 growing season, soil temperature under turfgrass vegetation averaged 14 °C (NDAWN, 2019).Experimental designWe identified 12 monitoring sites distributed across the research station. The plots were selected based on the historical management of the land and a visual assessment of expressed plant community, which we validated through plant community surveys. Four of the plots were located within grazing exclosures and were dominated by thick stands of bluegrass. The other eight sites (four in 2017 and four in 2018) were historically subject to grazing and had greater expression of native plant species, though bluegrass was present at all sample locations. Exact plot locations considered a combination of factors, including pasture history, predetermined fire and grazing treatment structure, and ecological site description, so distances between plots and treatments was variable (plots within treatments ranged from 0.5–7 km apart).The idle bluegrass‐dominated plots (hereafter referred to as KBG, n = 4) were excluded from grazing and not burned in recent decades. The average bluegrass cover across the four plots was 39% (average of 44% nonnative cover overall), accompanied by a thick thatch layer; the average cover of native species in those plots was 21%. The composition of idle plots was consistent across years. The plots established in 2017 were predominantly native plant species (averages of 41% native species cover and 19% bluegrass cover of 25% nonnative cover overall) managed with fire and grazing using a patch‐burn–grazing management system (hereafter referred to as ’17 burn, n = 4). These plots were burned in the spring of 2017 just after green‐up. They were grazed with cross‐bred Angus cow‐calf pairs from mid‐May through late‐October using season‐long grazing stocked at a full‐use stocking rate (40–50% degree of disappearance). Four additional plots were added in 2018 (hereafter referred to as ‘18 burn, n = 4) and subject to the same fire and grazing treatments. Vegetation surveys were conducted after fires when plant species could be identified. In 2018, all treated plots had similar vegetation composition to one another (averages of 12–13% bluegrass cover and 17–24% native cover) but lower overall cover than previous years (averages of 40–42% total cover). Our goal was to compare soil properties in the long‐term idle plots with those in areas that were burned and grazed one year prior to sampling (’17 burn) and immediately prior to sampling (’18 burn). These plots represented a range of conditions that are likely to occur across the bluegrass‐invaded northern Great Plains landscape.At the beginning of the 2018 growing season and within three hours following the prescribed burns to the ’18 burn treatment, soil samples were collected from all treatments for analysis of general, chemical, and biological properties. Additional soil samples were collected from the ’18 burn treatment throughout the 2018 growing season to provide insight as to how soil properties changed over time in response to management. These plots were sampled immediately before and after the spring burn occurred and then one and three months after spring fires and exposure to grazing.For each sampling event throughout the season, we collected six samples (each divided into depths of 0–5 and 5–15 cm) to form one composite sample at each depth for each study plot, resulting in a total of 144 samples throughout the 2018 season. The six samples per plot were collected randomly from a 5‐m radius around a center‐point.Data collectionWe measured general soil properties including texture, pH, electrical conductivity, bulk density (ρb), and water content. Soil texture was determined based on the USDA soil particle size classification system, and we used the hydrometer method outlined by Gee and Or (2002) to analyze particle size. We determined electrical conductivity and pH using the 1:1 soil/water ratio methods outlined respectively by Rhoades (1996) and Thomas (1996). We measured bulk density by averaging three replicate soil samples of known volume (2.5‐cm radius x 15‐cm height cylinder) from 0‐to‐15‐cm depth, collected via hammer corer from each study plot. Bulk density samples were collected and processed according to protocol established in Blake and Hartge (1986). Lastly, water content (θg) was determined on a gravimetric dry‐mass basis through mass loss (g) of field‐moist soil samples oven‐dried to constant weight, as outlined in Gardner (1986). Volumetric water content (θv) was calculated by multiplying gravimetric water content by the bulk density of the soil (θv = θg × ρb).Carbon properties measured include total carbon, organic carbon, inorganic carbon, permanganate oxidizable carbon (POX C), and microbial biomass carbon (MBC). For total, organic, and inorganic carbon fractions, soil samples were air‐dried and then ground to pass a 250‐micron sieve, as recommended in Nelson and Sommers (1996). Total carbon was determined by dry combustion through elemental analysis (Vario Macro CNS, Elementar Americas, Inc.); inorganic carbon was also determined analytically (Primacs, Skalar Analytical). We then calculated organic carbon as the difference between total and inorganic carbon. The remaining carbon pools, POX C and MBC, represent labile soil carbon fractions. Permanganate oxidizable C was determined following the procedure from Weil et al. (2003). We measured MBC concentrations through the chloroform fumigation‐extraction method (Beck et al., 1997; Joergensen, 1996; Vance et al., 1987).Nitrogen pools measured include total nitrogen, inorganic nitrogen (NH4 and NO3), and potentially mineralizable nitrogen (PMN). Soil samples for these analyses were air‐dried rapidly following collection and then stored in air‐tight containers to limit microbial processes that may alter nitrogen levels. Air drying soils has become a widely adopted method in preserving soil for the analysis of inorganic nitrogen forms, but some research suggests that analysis of field‐moist soils, frozen in air‐tight containers, is a more effective method (Nelson & Bremner, 1972). However, due to the low initial levels of inorganic nitrogen in our study plots, we determined the air‐drying method to be sufficient. Samples for analysis of total nitrogen were ground to pass a 250‐micron sieve, while the samples for inorganic nitrogen were ground to pass a 2.00‐mm sieve. Total nitrogen was determined through elemental analysis (Vario Macro CNS, Elementar Americas, Inc.). We performed potassium chloride extractions on samples prior to inorganic nitrogen analysis (on an auto‐analyzer) following procedures outlined in Robertson et al. (1999). Lastly, we measured PMN, an indicator of the nitrogen mineralization capacity of the soil's microbial community. We determined PMN using the 7‐d anaerobic incubation method outlined by Drinkwater et al. (1996). The difference between inorganic soil nitrogen measured in incubated and control samples represents PMN.We used phospholipid fatty acid analysis to determine microbial community structure. This method provides estimates of the number and relative abundance of microbial groups at a broad taxonomic level. Samples were stored on ice during transport and then frozen until processing. Upon processing, frozen samples were lyophilized and then ground to pass a 2.00‐mm sieve. The phospholipid fatty acid analysis was conducted by Microbial Identification Labs, Inc., which follows lipid extraction procedures described by Buyer and Sasser (2012), quantitative analysis with gas chromatography (HP6890, Hewlett Packard), peak identification using Sherlock software version 6.2 (MIDI, Inc.), and the PLFAD2 version 2.0 peak‐naming table. We analyzed microbial groups in terms of absolute abundance (nmol fatty acid g−1 soil), relative abundance (group/total abundance, as percent), and fungal/bacterial ratio across treatments.To estimate decomposition rates across treatments, we installed litterbags, stratified by depth, at each plot. Litterbags constructed of plastic window‐screen material (2‐mm mesh) were fitted on top of surface vegetation and at depths of 5 and 15 cm in the soil profile for a year‐long incubation period (three replicate litterbags per depth, per plot). At each bluegrass plot, an additional set of triplicate litterbags were installed within the bluegrass thatch layer, positioned above the soil surface but below accumulated vegetative biomass. We filled the litterbags with bluegrass litter collected from our study area. Standing and fallen bluegrass litter was included, while bluegrass thatch that was intermingled with the soil surface was excluded in order to minimize the inclusion of mineral soil. Chemical analysis of standing and fallen bluegrass biomass revealed that the litter had an average carbon‐to‐nitrogen ratio of 29:1. Protocol for the preparation, installation, retrieval, and processing of the litterbags was modified from methods described by Bocock and Gilbert (1957). Decomposition was estimated from mass loss (%) of litter after the in‐field incubation period.Data analysisStatistical differences across treatments were analyzed using a one‐way ANOVA test with Tukey's honest significance test used for mean comparisons. To explore how variables changed through time in the ’18 burn treatment, we used a repeated‐measures ANOVA. Additional analyses included: a) computation of pairwise correlation coefficients to better understand relationships between selected properties, b) principal component analysis of microbial communities across plots and treatments, and c) a paired t‐test to compare decomposition within a study plot between the bluegrass thatch canopy and the exposed surface. All analyses were performed in JMP© (Version 13; SAS Institute Inc.), and statistically significant differences were determined at a p ≤ .05 threshold.One‐way ANOVAs assume that a population is normally distributed, has uniform variances, and that samples are independent. We have no reason to believe that populations in this study are not normally distributed or that the variances are not equal, and we are limited in our ability to assess these characteristics across n = 4 samples. While the unique pasture histories across experimental plots in this study could possibly affect the independence of samples, many studies show that soil properties become independent within 5–10 m across many different ecosystems, including grasslands (summarized by Ettema & Wardle, 2002; Ritz et al., 2004). Additional differences in topography, land use history, and soils all increased the spatial independence of experimental units.RESULTSSince soil properties selected for measurement in this study vary with soil depth, we mainly assessed treatment differences within depth, rather than between depths. A summary of general soil properties is reported in Table 1. Measured properties were fairly variable, despite best efforts to minimize inherent soil disparities across plots during experimental design.1TABLEGeneral soil properties across treatments at the North Dakota State University Central Grasslands Research Extension CenterTreatmentDepthpHECSandClayBulk densitycmμS cm−1%g cm−1KBG0–56.6 (6.2–7.2)354 (278–486)36 (30–47)29 (24–34)0.90 (0.85–0.98)5–156.8 (6.3–7.7)334 (198–448)42 (36–54)27 (22–30)’17 burn0–56.8 (6.3–7.2)395 (262–505)37 (25–53)29 (22–35)0.91 (0.78–1.05)5–156.8 (6.3–7.5)348 (216–474)45 (32–68)29 (21–36)’18 burn0–56.6 (6.3–7.0)411 (257–535)34 (28–39)20 (11–29)0.84 (0.79–0.90)5–157.1 (6.6–7.7)516 (252–805)33 (21–42)25 (14–39)Note. Mean values (with ranges reported in parentheses) of soil pH, electrical conductivity (EC), sand (%), and clay (%) within depths of 0–5 and 5–15 cm at North Dakota State University Central Grasslands Research Extension Center from plots dominated by Kentucky bluegrass (KBG, n = 4) and plots managed with a spring fire and grazing regime in 2017 (‘17 burn, 1‐yr postburn, n = 4) and 2018 (‘18 burn, new burn, n = 4). Bulk density of the soil was measured for the depth 0–15 cm (n = 12, 3 cores at 4 plots per treatment)Overall, there were few significant differences in carbon and nitrogen pools across treatments. However, we did find significant differences in POX C, a measure of labile carbon pools. Permanganate oxidizable C was significantly lower in the ’18 burn treatment immediately following fire, at both 0–5‐ and 5–15‐cm depths, when compared with the KBG and ’17 burn treatments (Table 2). Although no other significant differences were measured, an interesting trend was observed in shallow soils (0–5 cm) between the KBG and ’17 burn treatments. The KBG treatment mean values were consistently lower than those of the ’17 burn treatment in shallow soils, with the singular exception of NH4–N being higher in the KBG treatment. While not as uniform, this trend was also present at 5–15 cm, with exceptions being the PMN, NH4–N, and NO3–N pools.2TABLENutrient pool mean values from sites dominated by invasive Kentucky bluegrass (KBG) and sites managed with fire and grazing in 2017 and 2018NutrientKBG'17 burn'18 burnmg kg−10‐to‐5‐cm depthTotal C56,800 (5,239)a59,625 (6,523)a56,600 (13,227)aOrganic C56,400 (5,114)a58,800 (6,908)a56,475 (13,118)aPOX C1,420 (79)a1,431 (111)a1,134 (191)bMBC1,117 (169)a1,228 (170)a1,120 (234)aTotal N5,175 (465)a5,525 (287)a5,225 (981)aPMN160 (32)a165 (19)a169 (5.00)aNH4–N22 (6.04)a17 (5.32)a20 (6.57)aNO3–N2.15 (0.01)a3.73 (2.00)a2.67 (1.06)a5‐to‐15‐cm depthTotal C36,350 (3,426)a38,425 (4,424)a35,475 (7,871)aOrganic C34,625 (1,338)a35,775 (2,170)a35,350 (8,093)aPOX C967 (82)a1,007 (75)a768 (109)bMBC663 (21)a720 (189)a677 (82)aTotal N3,675 (206)a3,725 (222)a3,525 (680)aPMN74 (8.71)a72 (15)a84 (14)aNH4–N12 (3.08)a12 (5.81)a13 (1.39)aNO3–N2.13 (0.03)a2.13 (0.04)a2.66 (1.06)aNote. Mean absolute values (with standard deviations reported in parentheses) of soil carbon and nitrogen pools (mg/kg) at depths of 0–5 cm (top) and 5–15 cm (bottom) from plots dominated by Kentucky bluegrass (n = 4) and plots managed with a spring fire and grazing regime in 2017 (‘17 burn, one‐year postburn, n = 4) and 2018 (‘18 burn, new burn, n = 4). Carbon fractions measured include total carbon (Total C), organic carbon (Organic C), permanganate oxidizable carbon (POX C), and microbial biomass carbon (MBC). Nitrogen fractions measured include total nitrogen (Total N), potentially mineralizable nitrogen (PMN), ammonium–N (NH4–N), and nitrate–N (NO3–N). Measurements were taken in April 2018 at the North Dakota State University Central Grasslands Research Extension Center. Superscripted letters (a, b) represent statistically significant differences across treatments within a nutrient pool at an individual depth (p ≤ .05)Our analysis revealed no significant changes over time within the 2018 season or in response to the ’18 fire treatment (Table 3). To better understand if there were any meaningful trends, or relationships driving the fluctuations in these pools, we analyzed correlations between variables and included volumetric soil moisture as an additional variable in the analysis. The results of the correlation analysis are reported in Figure 1. We found significant correlations between volumetric water content, PMN, and NH4–N as well as between total nitrogen and several carbon pools.3TABLENutrient pool mean values over time from sites managed with fire and grazing in 2018NutrientPreburnPostburn1 mo3 momg kg−10‐to‐5‐cm depthTotal C54,300 (14,266)56,600 (13,227)60,075 (16,592)58,675 (11,067)Organic C53,975 (13,812)56,475 (13,118)59,800 (16,476)58,550 (11,173)POX C1,141 (362)1,135 (191)1,271 (157)1,121 (120)MBC1,252 (175)1,120 (234)1,193 (220)1,004 (126)Total N5,100 (1,036)5,225 (981)5,575 (1,417)5,375 (885)PMN144 (31)169 (5.00)165 (12)171 (29)NH4–N25 (15)21 (6.57)36 (9.70)9.14 (1.10)NO3–N13 (18)2.67 (1.06)4.42 (2.60)2.15 (0.01)5‐to‐15‐cm depthTotal C34,275 (7,730)35,475 (7,871)34,675 (8,894)35,475 (8,159)Organic C33,625 (7,762)35,350 (8,093)34,300 (8,765)35,050 (8,545)POX C626 (74)768 (109)834 (122)694 (157)MBC650 (80)677 (82)652 (79)574 (57)Total N3,350 (580)3,525 (680)3,450 (819)3,600 (796)PMN63 (28)84 (14)74 (15)76 (15)NH4–N7.54 (3.62)13 (1.39)22 (7.86)7.50 (2.29)NO3–N5.64 (5.56)2.66 (1.06)2.68 (1.07)2.14 (0.02)Note. Mean absolute values (with standard deviations reported in parentheses) of soil carbon and nitrogen pools (mg/kg) at depths of 0–5 cm (top) and 5–15 cm (bottom) from plots managed under a spring fire and grazing regime in 2018 (n = 4). Values were measured immediately before (preburn) and after (postburn) the prescribed burn and at increasing time steps throughout the 2018 season (1 and 3 mo). Carbon fractions measured include total carbon (Total C), organic carbon (Organic C), permanganate oxidizable carbon (POX C), and microbial biomass carbon (MBC). Nitrogen fractions measured include total nitrogen (Total N), potentially mineralizable nitrogen (PMN), ammonium (NH4–N), and nitrate (NO3–N). Measurements were taken between April and August, 2018 at the North Dakota State University Central Grasslands Research Extension Center. No statistically significant differences were observed at p ≤ .05 (within each depth)1FIGURECorrelations between measured variables. Pairwise correlation coefficients (n = 24 observations, including all sample dates and all treatments for each depth increment) were computed for soil data collected at the North Dakota State University Central Grasslands Research Extension Center in Streeter, ND throughout the 2018 growing season. Statistically significant correlations are indicated by *, **, and *** (p ≤ .05, p ≤ .01, and p ≤ .001, respectively). Correlation coefficients greater than .40 are considered moderate (Aklogu, 2018) and displayed in bold red. Carbon fractions measured include total carbon (Total C), organic carbon (Organic C), permanganate oxidizable carbon (POX C), and microbial biomass carbon (MBC). Nitrogen fractions measured include total nitrogen (Total N), potentially mineralizable nitrogen (PMN), ammonium (NH4–N), and nitrate (NO3–N). Volumetric water content (Vol. H2O) was also included in the matrixAnalysis of the absolute abundance of microbial groups and the fungal/bacterial ratios across treatments revealed no significant differences in total microbial abundance, individual groups across treatments, or in response to fire (Figure 2). Principal component analysis did not reveal any distinction in microbial group community shifts between treatments.2FIGUREAbsolute abundance of soil microbial community distribution and fungal/bacterial ratios. Distribution of soil microbial communities by broad taxonomic groups in terms of absolute abundance (nmol/g) at depths of 0–5 cm and 5—15 cm. Graphs on the left (a and c) represent values from plots dominated by Kentucky bluegrass (KBG, n = 4) and plots managed with spring fire and grazing in 2017 (‘17 burn, one‐year postburn, n = 4.) and 2018 (‘18 burn, new burn, n = 4). Graphs on the right (b and d) represent mean values measured immediately before (preburn) and after (postburn) the prescribed burn performed in the spring of 2018 and at increasing time steps throughout the 2018 season (1 and 3 mo). Microbial groups analyzed include Gram‐negative and Gram‐positive bacteria (Gram ‐ and Gram +), actinomycetes, fungi, arbuscular mycorrhizal fungi (AM Fungi), and eukaryotes (protists). The numbers at the base of the bars represent fungal/bacterial ratios. Statistical differences within groups (assessed at p ≤ .05) were not observed between treatments or time stepsAlthough we did not observe changes in the microbial community in response to vegetation or management treatments, litterbag incubation results showed that decomposition of bluegrass litter varied across treatments and within depth (Figure 3). Significantly less decomposition occurred in the ’18 burn treatment at 5 cm than in the KBG treatment at the same depth. However, no differences (p > .05) were found across treatments at the surface or at 15 cm. Overall, annual decomposition ranged from 25–43% at the surface, 34–83% at 5 cm, and 38–82% at 15 cm. Litterbags installed within the thatch layer of the KBG treatment lost on average between 27–58% of total litter mass; however, a t‐test comparison to decomposition occurring at the surface of those same plots revealed no significant differences (p > .05).3FIGUREDecomposition of Kentucky bluegrass litter by depth. Average decomposition (calculated by litter mass loss [%]) of bluegrass litter at the soil surface (n = 3 per plot), 5‐cm depth (n = 3 per plot), and 15‐cm depth (n = 3 per plot) across plots dominated by Kentucky bluegrass (KBG, n = 4) and plots managed with spring fire and grazing in 2017 (‘17 burn, one‐year postburn, n = 4.) and 2018 (‘18 burn, new burn, n = 4). Decomposition was measured over a year‐long incubation period from 2018 to 2019 at the North Dakota State University Central Grasslands Research Extension Center. Letters (a, b) represent statistically significant treatment differences among means within depth (p ≤ .05)DISCUSSIONOverall, belowground pools of carbon, nitrogen, and microbial communities proved to be fairly resilient to changes in management practices and the composition of expressed vegetative cover and surface litters. The majority of carbon and nitrogen fractions, as well as measures of microbial groups, abundance, and activity, showed no significant differences in soils across bluegrass‐dominated idle management areas or under burned and grazed plots. There were also no significant changes in response to fire and grazing management across time steps within the 2018 season. The data did not support the original hypothesis that shifts in vegetation patterns, namely the change in quantity of bluegrass and associated litter production resulting from burning and grazing, would alter soil chemical and biological properties. These results are surprising considering that exotic plant invasions, as well as fire and grazing, are known to have significant effects on soil properties (Alcañiz et al., 2018; Ehrenfeld, 2003; Zhao et al., 2017).We did find soil POX C to respond to treatments, with lower values in managed areas immediately following a prescribed burn than in plots given a year to recover after the application of a burn and in bluegrass‐dominated plots with no active management. This suggests that the prescribed burns in this experiment could have caused the decline in POX C. These results are in contrast to many studies assessing the response of total soil carbon to prescribed burns, which generally report increased values following fire (Brye, 2006; Scharenbroch et al., 2012; Úbeda et al., 2005).The measurement of POX C does differ from total carbon in the sense that it is a highly active, labile form of carbon. Our results are in line with those of a study conducted by Muqaddas et al. (2015), which found decreasing values of soil POX C as the frequency of burns increased. This form of carbon likely decreases following fire due to the aboveground removal of vegetation and thus the decreased carbon inputs belowground from root exudates. However, POX C measured in our study a year after prescribed burning had the highest mean values, suggesting that any decline in this pool as a result of fire is temporary and prescribed fire may actually increase this pool with time as plant communities recover. This deviation between low and high levels of labile carbon can act as a source of heterogeneity in the ecosystem in contrast to more steady and homogenous nutrient pools below idle management areas.While not statistically significant, bluegrass‐dominated plots had overall lower carbon and nitrogen values at 0–5 cm for nearly all measured pools than plots managed with fire and grazing one year after burning. The only exception to this trend was greater NH4–N values in bluegrass‐dominated areas. Since this trend was most pronounced in shallow soils, where bluegrass litter and the effects of aboveground management such as fire and grazing would have the greatest influence, this suggests that, over time, the removal of bluegrass and its associated thatch through fire and grazing may increase carbon and nitrogen levels in the soil, with the exception of NH4–N, which may decrease.Further testing would be required to determine if changes in NH4–N concentrations were the result of bluegrass stimulating this pool or fire and grazing depleting it. Often, fire is thought of as a disturbance that reduces soil nitrogen through volatilization; however, several studies suggest otherwise. Kennard and Gholz (2001) and Gundale et al. (2005) found extractable ions of NH4–N to increase following applications of prescribed burns, which both studies attributed to postfire mineralization. Higher levels of NH4–N in bluegrass‐dominated areas compared with burned areas suggest that litter processing rates may be greater in bluegrass conditions, although it is hard to capture this dynamic process just by measuring the abundance of NH4–N, as this pool is very small and changes rapidly over short time scales. If bluegrass monocultures are causing increased levels of NH4–N, this would promote a synergistic positive feedback cycle. However, since these values were not statistically significant in our study, this narrative is mainly speculative.Correlation analysis between variables revealed many of the measured carbon pools were strongly tied with total nitrogen. This relationship is logical considering nitrogen is often the limiting nutrient for plant growth and microbial processes, which take up carbon and nitrogen together at a relatively consistent ratio (Cleveland & Liptzin, 2007). Weaker correlations were observed between the volumetric water content of the soil and both NH4–N and PMN. Soil water may be altered as a result of bluegrass dominance and the accumulation of thatch on top of the soil surface (Toledo et al., 2014), which could in turn affect these correlated labile nitrogen pools. However, if soil water in this study was affected by the bluegrass monocultures or management with fire and grazing, our results suggest the treatment effects were not significant enough to result in differences in either of these nutrient pools.Although there were few changes in the measured soil nutrient pools and no changes observed in microbial populations, we did find decomposition to be significantly lower at 5 cm in soils that had been exposed to a prescribed burn within the same year than in soils that were dominated by bluegrass monocultures. A combination of factors contributes to decomposition, including abiotic conditions of the physical soil environment, microbial communities, and substrate availability (Conant et al., 2011). Differences in the combination of factors that drive decomposition were presumably great enough between these two treatments to result in significant differences in the breakdown of litter. Since the composition and availability of litter was constant in the litterbag incubations and no changes were observed in microbial communities across treatments, it is likely the soil abiotic conditions in the burned and grazed plots were driving differences in decomposition observed at 5 cm.Warmer and dryer soil conditions at shallow depths, characteristic of recently burned areas, may have played a role in the decreased decomposition we observed. Typically, higher temperatures correspond to higher rates of decomposition, provided enough moisture is also available (Sierra et al., 2017). The link between temperature and moisture in relation to decomposition suggests that since temperature was not limiting in our study, reduced soil moisture (averaging 0.18 m3 m−3 in recently burned plots compared with 0.30 m3 m−3 in bluegrass‐dominated plots at 5 cm throughout the 2018 growing season) was likely the factor hindering decomposition in recently burned areas.In contrast to sites that had been burned in 2018, no differences in decomposition were observed between soils dominated by bluegrass monocultures and areas burned in 2017, suggesting decomposing conditions rebound relatively quickly from fire. The decomposition study revealed another interesting trend, which was decomposition of bluegrass litter was fairly high (over 80% mass loss in soil) across all study areas, regardless of treatment. This was surprising considering bluegrass is known to accumulate litter and develop a thick thatch layer, and decreased levels of decomposition are often implicated as the cause. The high levels of decomposition in our study are consistent with findings from Hendrickson et al. (2001), which showed monocultures of improved grass cultivars had significantly greater rates of decomposition than native grass species occurring in undisturbed rangeland. Additionally chemical analysis of standing and fallen bluegrass biomass from our study revealed the litter had an average carbon‐to‐nitrogen ratio of 29:1, which would not be in a range prohibitive to decomposition due to nitrogen limitation. The results suggest decreased decomposition is not the reason for the characteristic accumulation of bluegrass litter, but the cause is more likely due to disproportionately high levels of bluegrass biomass production relative to litter decomposition rates.The observed differences in decomposition suggest soil processes in this study may be affected by vegetative characteristics, yet our analysis of nutrient and biochemical pools found abundances to remain fairly stable across treatments. While the total abundance of soil nutrient and biochemical pools may not change significantly, it is possible the processes driving the turnover rate of these pools is different. This idea is supported by findings from Wedin and Tilman (1990) and Wedin and Pastor (1993), which showed monocultures of perennial grasses could affect annual net nitrogen mineralization up to 10‐fold after three years (Wedin & Tilman, 1990), yet monocultures of the same species did not change total soil carbon or nitrogen after four years (Wedin &, Pastor,1993). Sanderson et al. (2017) found bluegrass also changed the isotopic composition of soil carbon compared with native prairie species. Future studies on the subject should focus on quantifying the cycling rates, rather than the sizes, of these pools.Overall, our study found that neither the difference between bluegrass and native plant dominance nor fire and grazing had a strong effect on the soil properties studied in this survey. The prescribed burn may have caused a brief decline of labile POX C as well as slowed decomposition in shallow soil, but both properties rebounded by one year after burning. Soil properties measured repeatedly over time in this study did not reveal any seasonal shifts within the same season following the application of prescribed burns. Future research should characterize long‐term soil response to management with fire and grazing, as the fire and grazing system in this study was newly implemented.Results from this study revealed belowground properties in the study area were strikingly uniform, suggesting significant resilience to shifts in aboveground characteristics. While marginal differences were noted between areas recently burned and grazed and areas dominated by bluegrass under idle management, these differences were short‐lived. The plasticity of the measured soil conditions in response to management, and their tendency to return to stable conditions, suggests one of two possibilities: (a) prior to bluegrass invasion, the soil conditions in this area were already similar to conditions measured in this study, and they have not shifted significantly in response to changes in plant assemblages, or (b) the soil properties studied in this experiment may have reached a point where they are no longer responsive to management, and they have been fundamentally changed as a result of bluegrass dominance. Under either of these circumstances, we have hope that management strategies targeting a more favorable plant community are not going to be challenged or delayed because of any negative effects to the soil. The use of fire and grazing as a management practice to reduce bluegrass and promote native plant diversity appears to be a promising strategy, as this study found no long‐term negative effects on the studied soil properties in areas under this management regime.DATA AVAILABILITY STATEMENTThese data are available from the authors upon request.ACKNOWLEDGMENTSThis work was financially supported by the Central Grasslands Research Extension Center (CGREC), the North Dakota Agricultural Experiment Station, and the School of Natural Resource Sciences at North Dakota State University. The authors are grateful for assistance in the field and laboratory from Joel Bell, Rebecca Hebron, Alec Deschene, Megan Dornbusch, Pamela Block, Mackenzie Ries, and the entire CGREC prescribed burn crew, and for comments on a previous version of this manuscript by three faculty members in the School of Natural Resource Sciences at North Dakota State University. We collectively acknowledge that we gather at NDSU, a land grant institution, on the traditional lands of the Oceti Sakowin (Dakota, Lakota, Nakoda) and Anishinaabe Peoples in addition to many diverse Indigenous Peoples still connected to these lands. We honor with gratitude Mother Earth and the Indigenous Peoples who have walked with her throughout generations. We will continue to learn how to live in unity with Mother Earth and build strong, mutually beneficial, trusting relationships with Indigenous Peoples of our region.AUTHOR CONTRIBUTIONSLeslie Gerhard: Conceptualization; Formal analysis; Investigation; Visualization; Writing – original draft; Writing – review & editing. Caley K. Gasch: Conceptualization; Resources; Supervision; Visualization; Writing – review & editing. Kevin Sedivec: Resources; Writing – review & editing.CONFLICT OF INTERESTThe authors declare no conflict of interest.REFERENCESAkoglu, H. (2018). User's guide to correlation coefficients. 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Soil properties are resilient despite grass invasion, fire, and grazing

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Wiley
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© 2022 Crop Science Society of America and American Society of Agronomy
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2639-6696
DOI
10.1002/agg2.20257
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Abstract

AbbreviationsKBGKentucky bluegrass‐dominated plotsMBCmicrobial biomass carbonPMNpotentially mineralizable nitrogenPOX Cpermanganate oxidizable carbon.INTRODUCTIONWhile agricultural expansion continues to advance in the northern Great Plains (Johnston, 2014; Wright & Wimberly, 2013), modern anthropogenic influence in this region has recently shifted from unsustainable depletion of natural resources towards stewardship (Chapin et al., 2009). While landowners historically perceived grasslands as a limitless resource for grazing (Briske, 2017) and fire as a liability (Bendel et al., 2020; Wonkka et al., 2015), landowners are gaining an appreciation for natural disturbances and heterogeneity in these grassland systems (Briske, 2017). Unfortunately, we must still cope with unforeseen ecological consequences of historical management.One such repercussion is the invasion of nonnative species such as Kentucky bluegrass (Poa pratensis L.; hereafter referred to as bluegrass). Often, when there are shifts in fire regime, conditions develop that facilitate the invasion of nonnative species such as bluegrass (D'Antonio, 2000). In addition, bluegrass is a grazing‐tolerant species, so historically over‐grazing grasslands may have displaced native species while facilitating the invasion of bluegrass (Toledo et al., 2014). Bluegrass has rapidly become one of the predominant species in the northern Great Plains (USDA‐NRCS, n.d.‐a), threatening biodiversity of native grassland communities, ecosystem services, and forage production (Gasch et al., 2020).Bluegrass also has the potential to alter soil properties, in part due to the turfgrass growth habit and formation of a thickened thatch layer (Etter, 1951). Thatch refers to the accumulation of litter and root biomass on top of, and intermingled with, the soil surface. In the case of bluegrass, this excess litter may alter the microclimate of the soil by acting as a buffer against atmospheric moisture and temperature (Bosy & Reader, 1995; Murray & Juska, 1977; Pierson et al., 2002). Furthermore, plant community shifts from native, diverse mixed‐grass prairie to a more uniform stand of turfgrass is likely to change organic matter inputs to the soil in terms of quantity, quality, and distribution. For example, monocultures of bluegrass have been found to cause lower soil carbon‐to‐nitrogen ratios than native C4 grass species (Wedin & Pastor, 1993). As the carbon‐to‐nitrogen ratio of residue inputs shifts, subsequent soil processes may also be affected. Bradley et al. (2006) found that increased nitrogen levels in grassland systems led to significant changes in the composition and relative abundance of soil microbial communities by increasing bacterial and decreasing fungal abundance. Increased nitrogen is also generally associated with increased decomposition rates; however, Knorr et al. (2005) suggests that increased nitrogen can have an inhibitory effect on certain decomposers, which may in turn dampen the stimulating effect of increased nitrogen. Overall, bluegrass dominance has the potential to affect a wide array of soil properties and ecological processes, due to its invasion extent and dominance, as well as its morphological and phenological differences from the historic native plant community.To minimize detrimental effects of bluegrass on the ecosystem, grassland management strategies such as fire, grazing, and their combination can diminish the competitive advantage of bluegrass. In general, studies show that grazing alone has a limited effect on bluegrass control (Biondini et al., 1998; Otfinowski, et al., 2017; Rogers et al., 2005). Both overgrazing and the exclusion of grazing can increase bluegrass levels (Grant et al., 2009). Fire, on the other hand, has been found to consistently reduce bluegrass cover and thatch, although the degree of control depends on fire regime as well as initial bluegrass invasion level (Ereth et al., 2017; Kral et al., 2018). Kral et al. (2018), McMurphy & Anderson, 1965, and Towne & Kemp, 2008 found that the decline in bluegrass cover following late‐growing season and dormant‐season burns persisted at least three years postfire. A combination of grazing and burning has been shown to increase plant species diversity and forage quality more than either process alone (Hartnett et al., 1996).Core IdeasFire and grazing were applied to Kentucky bluegrass‐invaded grasslands.Most carbon, nitrogen, and microbial properties did not differ between idle or treated sites.Labile carbon and decomposition rates were temporarily reduced after fire.Soil properties were remarkably stable in a northern prairie despite invasion and management.Fire and grazing have varied effects on soil. A 2018 review paper found that the effects of prescribed fires on soil properties depended on parameters such as fire regime, soil type, seasonality, residence time, and periodicity (Alcañiz et al., 2018). This review evaluated the effect of prescribed fire on a variety of soil properties, including chemical and biological properties. Soil nitrogen generally increased after fire, though excessively high fire temperatures (>200 °C) may lead to nitrogen volatilization (Alcañiz et al., 2018). Likewise, carbon stocks in the soil often increased after low‐intensity fires due to the incorporation of partially burned organic matter, while high‐intensity fires typically decreased carbon stocks (Alcañiz et al., 2018). The effect of fire on soil organisms was extremely variable. Results ranged from complete elimination of soil organisms (Barreiro et al., 2015; Choromanska & DeLuca, 2001; Switzer et al., 2012; Williams et al., 2012), to increased microbial activity resulting from the influx of ash materials (Blankenship & Arthur, 1999; Fultz et al., 2016; Gray & Dighton, 2009).Grazing may also play a role in shaping soil microbial communities, based on the results from a 2017 meta‐analysis on the subject (Zhao et al., 2017). Zhao et al. (2017) found that heavy grazing significantly decreased total microbial, bacterial, and fungal community size, yet brief, light, and moderate grazing had no effect on these properties. Furthermore, grazing intensity may also affect soil carbon and nitrogen pools. In a grazing intensity trial in the mixed‐grass prairie of North Dakota, Frank et al. (1995) found that soils in sites excluded from grazing had higher nitrogen content than in moderately or heavily grazed pastures. In this study, moderate grazing also decreased carbon content, while heavy grazing did not, likely due to changes in species composition resulting from the disturbance (Frank et al., 1995).Since fire and grazing often occur simultaneously on a landscape, we must consider the combined effect that these two disturbances may have on soil properties. A study in the tall‐grass prairie of Kansas found that grazing increased net nitrogen mineralization, while annual burning decreased it (Johnson & Matchett, 2001). These results suggest that fire and grazing may have contrasting effects on soil properties, and these disturbances may affect nutrient transformation rates. There is currently limited research on the coupled effect that fire and grazing (and subsequent shifts in plant species assemblages) may have on soil properties in the grasslands of the northern Great Plains.While the eradication of bluegrass from grasslands in the northern Great Plains is not a realistic option due to the extent of the invasion, Dornbusch et al. (2020) found that combined fire and grazing reduces the cover of bluegrass, as well as the influence of bluegrass on plant community composition over time compared with traditional management. With bluegrass established as a permanent fixture in these ecosystems and the need for adaptive management required on a greater scale, we must evaluate ecosystem effects, including any potential effects to belowground soil properties resulting from bluegrass dominance and associated vegetation management practices.The goal of our research was to measure the effect that bluegrass, and management with fire and grazing, may have on soil nutrient pools, microbial communities, and biological processes. We designed a survey to evaluate these soil properties in areas either (a) dominated by bluegrass and historically left idle, or (b) hosting a mix of bluegrass and native species and managed with fire and grazing. Between the idle and managed systems, we compared carbon and nitrogen fractions in both labile and stable soil pools, as well as microbial abundance and community structure. These measurements were stratified through time to understand the effects of fire and grazing on soil properties before and immediately after fire, as well as one year postfire. Additionally, we conducted in‐field litterbag incubations to determine if litter decomposition differed across these treatments. Results will help us understand how both bluegrass and land management techniques may affect soil properties in a northern mixed‐grass prairie of the Great Plains.MATERIALS AND METHODSStudy areaThis study was conducted during the summer of 2018 in mixed‐grass prairie at North Dakota State University Central Grasslands Research Extension Center in south‐central North Dakota (46°42′55.6″ N, 99°26′51.6″ W). Prairies in this region are characterized by the dominance of cool‐season, perennial grasses (Toledo et al., 2014). For data consistency, we limited our study to thin loamy ecological sites, which have a historical climax plant community that includes green needlegrass [Nassella viridula (Trin.) Barkworth], little bluestem [Schizachyrium scoparium (Nash) E.P. Bickwell], and western wheatgrass [Pascopyrum smithii (Rydb.) Á. Löve] (USDA‐NRCS n.d.‐b). However, following extended periods of idle and overgrazing management and increased moisture patterns, bluegrass proliferates and litter accumulates (USDA‐NRCS n.d.‐b).Extreme temperatures and a semi‐arid precipitation regime are characteristic in this region. Rainfall is typically the limiting factor in determining the plant community composition, with annual precipitation ranging from approximately 38–51 cm per year (USDA‐NRCS n.d.‐b), with the majority occurring during the growing‐season months. During the 2018 growing season, the study area received above‐average rainfall, with precipitation totals reaching 44.2 cm between April and September (NDAWN, 2019).Soils in the study area are typically well‐drained with low runoff depending on slope and vegetative cover (USDA‐NRCS n.d.‐b). Twelve monitoring sites were selected from map units identified within the Zahl‐Williams‐Zahill complex (fine‐loamy, mixed, superactive, frigid Typic Calciustolls, or Argiustolls) (Soil Survey Staff, n.d.). Typical soil profiles for these units include a thin layer of loam (0–15 cm) on top of clay loam extending through the substrata (Soil Survey Staff, n.d.). For the 2018 growing season, soil temperature under turfgrass vegetation averaged 14 °C (NDAWN, 2019).Experimental designWe identified 12 monitoring sites distributed across the research station. The plots were selected based on the historical management of the land and a visual assessment of expressed plant community, which we validated through plant community surveys. Four of the plots were located within grazing exclosures and were dominated by thick stands of bluegrass. The other eight sites (four in 2017 and four in 2018) were historically subject to grazing and had greater expression of native plant species, though bluegrass was present at all sample locations. Exact plot locations considered a combination of factors, including pasture history, predetermined fire and grazing treatment structure, and ecological site description, so distances between plots and treatments was variable (plots within treatments ranged from 0.5–7 km apart).The idle bluegrass‐dominated plots (hereafter referred to as KBG, n = 4) were excluded from grazing and not burned in recent decades. The average bluegrass cover across the four plots was 39% (average of 44% nonnative cover overall), accompanied by a thick thatch layer; the average cover of native species in those plots was 21%. The composition of idle plots was consistent across years. The plots established in 2017 were predominantly native plant species (averages of 41% native species cover and 19% bluegrass cover of 25% nonnative cover overall) managed with fire and grazing using a patch‐burn–grazing management system (hereafter referred to as ’17 burn, n = 4). These plots were burned in the spring of 2017 just after green‐up. They were grazed with cross‐bred Angus cow‐calf pairs from mid‐May through late‐October using season‐long grazing stocked at a full‐use stocking rate (40–50% degree of disappearance). Four additional plots were added in 2018 (hereafter referred to as ‘18 burn, n = 4) and subject to the same fire and grazing treatments. Vegetation surveys were conducted after fires when plant species could be identified. In 2018, all treated plots had similar vegetation composition to one another (averages of 12–13% bluegrass cover and 17–24% native cover) but lower overall cover than previous years (averages of 40–42% total cover). Our goal was to compare soil properties in the long‐term idle plots with those in areas that were burned and grazed one year prior to sampling (’17 burn) and immediately prior to sampling (’18 burn). These plots represented a range of conditions that are likely to occur across the bluegrass‐invaded northern Great Plains landscape.At the beginning of the 2018 growing season and within three hours following the prescribed burns to the ’18 burn treatment, soil samples were collected from all treatments for analysis of general, chemical, and biological properties. Additional soil samples were collected from the ’18 burn treatment throughout the 2018 growing season to provide insight as to how soil properties changed over time in response to management. These plots were sampled immediately before and after the spring burn occurred and then one and three months after spring fires and exposure to grazing.For each sampling event throughout the season, we collected six samples (each divided into depths of 0–5 and 5–15 cm) to form one composite sample at each depth for each study plot, resulting in a total of 144 samples throughout the 2018 season. The six samples per plot were collected randomly from a 5‐m radius around a center‐point.Data collectionWe measured general soil properties including texture, pH, electrical conductivity, bulk density (ρb), and water content. Soil texture was determined based on the USDA soil particle size classification system, and we used the hydrometer method outlined by Gee and Or (2002) to analyze particle size. We determined electrical conductivity and pH using the 1:1 soil/water ratio methods outlined respectively by Rhoades (1996) and Thomas (1996). We measured bulk density by averaging three replicate soil samples of known volume (2.5‐cm radius x 15‐cm height cylinder) from 0‐to‐15‐cm depth, collected via hammer corer from each study plot. Bulk density samples were collected and processed according to protocol established in Blake and Hartge (1986). Lastly, water content (θg) was determined on a gravimetric dry‐mass basis through mass loss (g) of field‐moist soil samples oven‐dried to constant weight, as outlined in Gardner (1986). Volumetric water content (θv) was calculated by multiplying gravimetric water content by the bulk density of the soil (θv = θg × ρb).Carbon properties measured include total carbon, organic carbon, inorganic carbon, permanganate oxidizable carbon (POX C), and microbial biomass carbon (MBC). For total, organic, and inorganic carbon fractions, soil samples were air‐dried and then ground to pass a 250‐micron sieve, as recommended in Nelson and Sommers (1996). Total carbon was determined by dry combustion through elemental analysis (Vario Macro CNS, Elementar Americas, Inc.); inorganic carbon was also determined analytically (Primacs, Skalar Analytical). We then calculated organic carbon as the difference between total and inorganic carbon. The remaining carbon pools, POX C and MBC, represent labile soil carbon fractions. Permanganate oxidizable C was determined following the procedure from Weil et al. (2003). We measured MBC concentrations through the chloroform fumigation‐extraction method (Beck et al., 1997; Joergensen, 1996; Vance et al., 1987).Nitrogen pools measured include total nitrogen, inorganic nitrogen (NH4 and NO3), and potentially mineralizable nitrogen (PMN). Soil samples for these analyses were air‐dried rapidly following collection and then stored in air‐tight containers to limit microbial processes that may alter nitrogen levels. Air drying soils has become a widely adopted method in preserving soil for the analysis of inorganic nitrogen forms, but some research suggests that analysis of field‐moist soils, frozen in air‐tight containers, is a more effective method (Nelson & Bremner, 1972). However, due to the low initial levels of inorganic nitrogen in our study plots, we determined the air‐drying method to be sufficient. Samples for analysis of total nitrogen were ground to pass a 250‐micron sieve, while the samples for inorganic nitrogen were ground to pass a 2.00‐mm sieve. Total nitrogen was determined through elemental analysis (Vario Macro CNS, Elementar Americas, Inc.). We performed potassium chloride extractions on samples prior to inorganic nitrogen analysis (on an auto‐analyzer) following procedures outlined in Robertson et al. (1999). Lastly, we measured PMN, an indicator of the nitrogen mineralization capacity of the soil's microbial community. We determined PMN using the 7‐d anaerobic incubation method outlined by Drinkwater et al. (1996). The difference between inorganic soil nitrogen measured in incubated and control samples represents PMN.We used phospholipid fatty acid analysis to determine microbial community structure. This method provides estimates of the number and relative abundance of microbial groups at a broad taxonomic level. Samples were stored on ice during transport and then frozen until processing. Upon processing, frozen samples were lyophilized and then ground to pass a 2.00‐mm sieve. The phospholipid fatty acid analysis was conducted by Microbial Identification Labs, Inc., which follows lipid extraction procedures described by Buyer and Sasser (2012), quantitative analysis with gas chromatography (HP6890, Hewlett Packard), peak identification using Sherlock software version 6.2 (MIDI, Inc.), and the PLFAD2 version 2.0 peak‐naming table. We analyzed microbial groups in terms of absolute abundance (nmol fatty acid g−1 soil), relative abundance (group/total abundance, as percent), and fungal/bacterial ratio across treatments.To estimate decomposition rates across treatments, we installed litterbags, stratified by depth, at each plot. Litterbags constructed of plastic window‐screen material (2‐mm mesh) were fitted on top of surface vegetation and at depths of 5 and 15 cm in the soil profile for a year‐long incubation period (three replicate litterbags per depth, per plot). At each bluegrass plot, an additional set of triplicate litterbags were installed within the bluegrass thatch layer, positioned above the soil surface but below accumulated vegetative biomass. We filled the litterbags with bluegrass litter collected from our study area. Standing and fallen bluegrass litter was included, while bluegrass thatch that was intermingled with the soil surface was excluded in order to minimize the inclusion of mineral soil. Chemical analysis of standing and fallen bluegrass biomass revealed that the litter had an average carbon‐to‐nitrogen ratio of 29:1. Protocol for the preparation, installation, retrieval, and processing of the litterbags was modified from methods described by Bocock and Gilbert (1957). Decomposition was estimated from mass loss (%) of litter after the in‐field incubation period.Data analysisStatistical differences across treatments were analyzed using a one‐way ANOVA test with Tukey's honest significance test used for mean comparisons. To explore how variables changed through time in the ’18 burn treatment, we used a repeated‐measures ANOVA. Additional analyses included: a) computation of pairwise correlation coefficients to better understand relationships between selected properties, b) principal component analysis of microbial communities across plots and treatments, and c) a paired t‐test to compare decomposition within a study plot between the bluegrass thatch canopy and the exposed surface. All analyses were performed in JMP© (Version 13; SAS Institute Inc.), and statistically significant differences were determined at a p ≤ .05 threshold.One‐way ANOVAs assume that a population is normally distributed, has uniform variances, and that samples are independent. We have no reason to believe that populations in this study are not normally distributed or that the variances are not equal, and we are limited in our ability to assess these characteristics across n = 4 samples. While the unique pasture histories across experimental plots in this study could possibly affect the independence of samples, many studies show that soil properties become independent within 5–10 m across many different ecosystems, including grasslands (summarized by Ettema & Wardle, 2002; Ritz et al., 2004). Additional differences in topography, land use history, and soils all increased the spatial independence of experimental units.RESULTSSince soil properties selected for measurement in this study vary with soil depth, we mainly assessed treatment differences within depth, rather than between depths. A summary of general soil properties is reported in Table 1. Measured properties were fairly variable, despite best efforts to minimize inherent soil disparities across plots during experimental design.1TABLEGeneral soil properties across treatments at the North Dakota State University Central Grasslands Research Extension CenterTreatmentDepthpHECSandClayBulk densitycmμS cm−1%g cm−1KBG0–56.6 (6.2–7.2)354 (278–486)36 (30–47)29 (24–34)0.90 (0.85–0.98)5–156.8 (6.3–7.7)334 (198–448)42 (36–54)27 (22–30)’17 burn0–56.8 (6.3–7.2)395 (262–505)37 (25–53)29 (22–35)0.91 (0.78–1.05)5–156.8 (6.3–7.5)348 (216–474)45 (32–68)29 (21–36)’18 burn0–56.6 (6.3–7.0)411 (257–535)34 (28–39)20 (11–29)0.84 (0.79–0.90)5–157.1 (6.6–7.7)516 (252–805)33 (21–42)25 (14–39)Note. Mean values (with ranges reported in parentheses) of soil pH, electrical conductivity (EC), sand (%), and clay (%) within depths of 0–5 and 5–15 cm at North Dakota State University Central Grasslands Research Extension Center from plots dominated by Kentucky bluegrass (KBG, n = 4) and plots managed with a spring fire and grazing regime in 2017 (‘17 burn, 1‐yr postburn, n = 4) and 2018 (‘18 burn, new burn, n = 4). Bulk density of the soil was measured for the depth 0–15 cm (n = 12, 3 cores at 4 plots per treatment)Overall, there were few significant differences in carbon and nitrogen pools across treatments. However, we did find significant differences in POX C, a measure of labile carbon pools. Permanganate oxidizable C was significantly lower in the ’18 burn treatment immediately following fire, at both 0–5‐ and 5–15‐cm depths, when compared with the KBG and ’17 burn treatments (Table 2). Although no other significant differences were measured, an interesting trend was observed in shallow soils (0–5 cm) between the KBG and ’17 burn treatments. The KBG treatment mean values were consistently lower than those of the ’17 burn treatment in shallow soils, with the singular exception of NH4–N being higher in the KBG treatment. While not as uniform, this trend was also present at 5–15 cm, with exceptions being the PMN, NH4–N, and NO3–N pools.2TABLENutrient pool mean values from sites dominated by invasive Kentucky bluegrass (KBG) and sites managed with fire and grazing in 2017 and 2018NutrientKBG'17 burn'18 burnmg kg−10‐to‐5‐cm depthTotal C56,800 (5,239)a59,625 (6,523)a56,600 (13,227)aOrganic C56,400 (5,114)a58,800 (6,908)a56,475 (13,118)aPOX C1,420 (79)a1,431 (111)a1,134 (191)bMBC1,117 (169)a1,228 (170)a1,120 (234)aTotal N5,175 (465)a5,525 (287)a5,225 (981)aPMN160 (32)a165 (19)a169 (5.00)aNH4–N22 (6.04)a17 (5.32)a20 (6.57)aNO3–N2.15 (0.01)a3.73 (2.00)a2.67 (1.06)a5‐to‐15‐cm depthTotal C36,350 (3,426)a38,425 (4,424)a35,475 (7,871)aOrganic C34,625 (1,338)a35,775 (2,170)a35,350 (8,093)aPOX C967 (82)a1,007 (75)a768 (109)bMBC663 (21)a720 (189)a677 (82)aTotal N3,675 (206)a3,725 (222)a3,525 (680)aPMN74 (8.71)a72 (15)a84 (14)aNH4–N12 (3.08)a12 (5.81)a13 (1.39)aNO3–N2.13 (0.03)a2.13 (0.04)a2.66 (1.06)aNote. Mean absolute values (with standard deviations reported in parentheses) of soil carbon and nitrogen pools (mg/kg) at depths of 0–5 cm (top) and 5–15 cm (bottom) from plots dominated by Kentucky bluegrass (n = 4) and plots managed with a spring fire and grazing regime in 2017 (‘17 burn, one‐year postburn, n = 4) and 2018 (‘18 burn, new burn, n = 4). Carbon fractions measured include total carbon (Total C), organic carbon (Organic C), permanganate oxidizable carbon (POX C), and microbial biomass carbon (MBC). Nitrogen fractions measured include total nitrogen (Total N), potentially mineralizable nitrogen (PMN), ammonium–N (NH4–N), and nitrate–N (NO3–N). Measurements were taken in April 2018 at the North Dakota State University Central Grasslands Research Extension Center. Superscripted letters (a, b) represent statistically significant differences across treatments within a nutrient pool at an individual depth (p ≤ .05)Our analysis revealed no significant changes over time within the 2018 season or in response to the ’18 fire treatment (Table 3). To better understand if there were any meaningful trends, or relationships driving the fluctuations in these pools, we analyzed correlations between variables and included volumetric soil moisture as an additional variable in the analysis. The results of the correlation analysis are reported in Figure 1. We found significant correlations between volumetric water content, PMN, and NH4–N as well as between total nitrogen and several carbon pools.3TABLENutrient pool mean values over time from sites managed with fire and grazing in 2018NutrientPreburnPostburn1 mo3 momg kg−10‐to‐5‐cm depthTotal C54,300 (14,266)56,600 (13,227)60,075 (16,592)58,675 (11,067)Organic C53,975 (13,812)56,475 (13,118)59,800 (16,476)58,550 (11,173)POX C1,141 (362)1,135 (191)1,271 (157)1,121 (120)MBC1,252 (175)1,120 (234)1,193 (220)1,004 (126)Total N5,100 (1,036)5,225 (981)5,575 (1,417)5,375 (885)PMN144 (31)169 (5.00)165 (12)171 (29)NH4–N25 (15)21 (6.57)36 (9.70)9.14 (1.10)NO3–N13 (18)2.67 (1.06)4.42 (2.60)2.15 (0.01)5‐to‐15‐cm depthTotal C34,275 (7,730)35,475 (7,871)34,675 (8,894)35,475 (8,159)Organic C33,625 (7,762)35,350 (8,093)34,300 (8,765)35,050 (8,545)POX C626 (74)768 (109)834 (122)694 (157)MBC650 (80)677 (82)652 (79)574 (57)Total N3,350 (580)3,525 (680)3,450 (819)3,600 (796)PMN63 (28)84 (14)74 (15)76 (15)NH4–N7.54 (3.62)13 (1.39)22 (7.86)7.50 (2.29)NO3–N5.64 (5.56)2.66 (1.06)2.68 (1.07)2.14 (0.02)Note. Mean absolute values (with standard deviations reported in parentheses) of soil carbon and nitrogen pools (mg/kg) at depths of 0–5 cm (top) and 5–15 cm (bottom) from plots managed under a spring fire and grazing regime in 2018 (n = 4). Values were measured immediately before (preburn) and after (postburn) the prescribed burn and at increasing time steps throughout the 2018 season (1 and 3 mo). Carbon fractions measured include total carbon (Total C), organic carbon (Organic C), permanganate oxidizable carbon (POX C), and microbial biomass carbon (MBC). Nitrogen fractions measured include total nitrogen (Total N), potentially mineralizable nitrogen (PMN), ammonium (NH4–N), and nitrate (NO3–N). Measurements were taken between April and August, 2018 at the North Dakota State University Central Grasslands Research Extension Center. No statistically significant differences were observed at p ≤ .05 (within each depth)1FIGURECorrelations between measured variables. Pairwise correlation coefficients (n = 24 observations, including all sample dates and all treatments for each depth increment) were computed for soil data collected at the North Dakota State University Central Grasslands Research Extension Center in Streeter, ND throughout the 2018 growing season. Statistically significant correlations are indicated by *, **, and *** (p ≤ .05, p ≤ .01, and p ≤ .001, respectively). Correlation coefficients greater than .40 are considered moderate (Aklogu, 2018) and displayed in bold red. Carbon fractions measured include total carbon (Total C), organic carbon (Organic C), permanganate oxidizable carbon (POX C), and microbial biomass carbon (MBC). Nitrogen fractions measured include total nitrogen (Total N), potentially mineralizable nitrogen (PMN), ammonium (NH4–N), and nitrate (NO3–N). Volumetric water content (Vol. H2O) was also included in the matrixAnalysis of the absolute abundance of microbial groups and the fungal/bacterial ratios across treatments revealed no significant differences in total microbial abundance, individual groups across treatments, or in response to fire (Figure 2). Principal component analysis did not reveal any distinction in microbial group community shifts between treatments.2FIGUREAbsolute abundance of soil microbial community distribution and fungal/bacterial ratios. Distribution of soil microbial communities by broad taxonomic groups in terms of absolute abundance (nmol/g) at depths of 0–5 cm and 5—15 cm. Graphs on the left (a and c) represent values from plots dominated by Kentucky bluegrass (KBG, n = 4) and plots managed with spring fire and grazing in 2017 (‘17 burn, one‐year postburn, n = 4.) and 2018 (‘18 burn, new burn, n = 4). Graphs on the right (b and d) represent mean values measured immediately before (preburn) and after (postburn) the prescribed burn performed in the spring of 2018 and at increasing time steps throughout the 2018 season (1 and 3 mo). Microbial groups analyzed include Gram‐negative and Gram‐positive bacteria (Gram ‐ and Gram +), actinomycetes, fungi, arbuscular mycorrhizal fungi (AM Fungi), and eukaryotes (protists). The numbers at the base of the bars represent fungal/bacterial ratios. Statistical differences within groups (assessed at p ≤ .05) were not observed between treatments or time stepsAlthough we did not observe changes in the microbial community in response to vegetation or management treatments, litterbag incubation results showed that decomposition of bluegrass litter varied across treatments and within depth (Figure 3). Significantly less decomposition occurred in the ’18 burn treatment at 5 cm than in the KBG treatment at the same depth. However, no differences (p > .05) were found across treatments at the surface or at 15 cm. Overall, annual decomposition ranged from 25–43% at the surface, 34–83% at 5 cm, and 38–82% at 15 cm. Litterbags installed within the thatch layer of the KBG treatment lost on average between 27–58% of total litter mass; however, a t‐test comparison to decomposition occurring at the surface of those same plots revealed no significant differences (p > .05).3FIGUREDecomposition of Kentucky bluegrass litter by depth. Average decomposition (calculated by litter mass loss [%]) of bluegrass litter at the soil surface (n = 3 per plot), 5‐cm depth (n = 3 per plot), and 15‐cm depth (n = 3 per plot) across plots dominated by Kentucky bluegrass (KBG, n = 4) and plots managed with spring fire and grazing in 2017 (‘17 burn, one‐year postburn, n = 4.) and 2018 (‘18 burn, new burn, n = 4). Decomposition was measured over a year‐long incubation period from 2018 to 2019 at the North Dakota State University Central Grasslands Research Extension Center. Letters (a, b) represent statistically significant treatment differences among means within depth (p ≤ .05)DISCUSSIONOverall, belowground pools of carbon, nitrogen, and microbial communities proved to be fairly resilient to changes in management practices and the composition of expressed vegetative cover and surface litters. The majority of carbon and nitrogen fractions, as well as measures of microbial groups, abundance, and activity, showed no significant differences in soils across bluegrass‐dominated idle management areas or under burned and grazed plots. There were also no significant changes in response to fire and grazing management across time steps within the 2018 season. The data did not support the original hypothesis that shifts in vegetation patterns, namely the change in quantity of bluegrass and associated litter production resulting from burning and grazing, would alter soil chemical and biological properties. These results are surprising considering that exotic plant invasions, as well as fire and grazing, are known to have significant effects on soil properties (Alcañiz et al., 2018; Ehrenfeld, 2003; Zhao et al., 2017).We did find soil POX C to respond to treatments, with lower values in managed areas immediately following a prescribed burn than in plots given a year to recover after the application of a burn and in bluegrass‐dominated plots with no active management. This suggests that the prescribed burns in this experiment could have caused the decline in POX C. These results are in contrast to many studies assessing the response of total soil carbon to prescribed burns, which generally report increased values following fire (Brye, 2006; Scharenbroch et al., 2012; Úbeda et al., 2005).The measurement of POX C does differ from total carbon in the sense that it is a highly active, labile form of carbon. Our results are in line with those of a study conducted by Muqaddas et al. (2015), which found decreasing values of soil POX C as the frequency of burns increased. This form of carbon likely decreases following fire due to the aboveground removal of vegetation and thus the decreased carbon inputs belowground from root exudates. However, POX C measured in our study a year after prescribed burning had the highest mean values, suggesting that any decline in this pool as a result of fire is temporary and prescribed fire may actually increase this pool with time as plant communities recover. This deviation between low and high levels of labile carbon can act as a source of heterogeneity in the ecosystem in contrast to more steady and homogenous nutrient pools below idle management areas.While not statistically significant, bluegrass‐dominated plots had overall lower carbon and nitrogen values at 0–5 cm for nearly all measured pools than plots managed with fire and grazing one year after burning. The only exception to this trend was greater NH4–N values in bluegrass‐dominated areas. Since this trend was most pronounced in shallow soils, where bluegrass litter and the effects of aboveground management such as fire and grazing would have the greatest influence, this suggests that, over time, the removal of bluegrass and its associated thatch through fire and grazing may increase carbon and nitrogen levels in the soil, with the exception of NH4–N, which may decrease.Further testing would be required to determine if changes in NH4–N concentrations were the result of bluegrass stimulating this pool or fire and grazing depleting it. Often, fire is thought of as a disturbance that reduces soil nitrogen through volatilization; however, several studies suggest otherwise. Kennard and Gholz (2001) and Gundale et al. (2005) found extractable ions of NH4–N to increase following applications of prescribed burns, which both studies attributed to postfire mineralization. Higher levels of NH4–N in bluegrass‐dominated areas compared with burned areas suggest that litter processing rates may be greater in bluegrass conditions, although it is hard to capture this dynamic process just by measuring the abundance of NH4–N, as this pool is very small and changes rapidly over short time scales. If bluegrass monocultures are causing increased levels of NH4–N, this would promote a synergistic positive feedback cycle. However, since these values were not statistically significant in our study, this narrative is mainly speculative.Correlation analysis between variables revealed many of the measured carbon pools were strongly tied with total nitrogen. This relationship is logical considering nitrogen is often the limiting nutrient for plant growth and microbial processes, which take up carbon and nitrogen together at a relatively consistent ratio (Cleveland & Liptzin, 2007). Weaker correlations were observed between the volumetric water content of the soil and both NH4–N and PMN. Soil water may be altered as a result of bluegrass dominance and the accumulation of thatch on top of the soil surface (Toledo et al., 2014), which could in turn affect these correlated labile nitrogen pools. However, if soil water in this study was affected by the bluegrass monocultures or management with fire and grazing, our results suggest the treatment effects were not significant enough to result in differences in either of these nutrient pools.Although there were few changes in the measured soil nutrient pools and no changes observed in microbial populations, we did find decomposition to be significantly lower at 5 cm in soils that had been exposed to a prescribed burn within the same year than in soils that were dominated by bluegrass monocultures. A combination of factors contributes to decomposition, including abiotic conditions of the physical soil environment, microbial communities, and substrate availability (Conant et al., 2011). Differences in the combination of factors that drive decomposition were presumably great enough between these two treatments to result in significant differences in the breakdown of litter. Since the composition and availability of litter was constant in the litterbag incubations and no changes were observed in microbial communities across treatments, it is likely the soil abiotic conditions in the burned and grazed plots were driving differences in decomposition observed at 5 cm.Warmer and dryer soil conditions at shallow depths, characteristic of recently burned areas, may have played a role in the decreased decomposition we observed. Typically, higher temperatures correspond to higher rates of decomposition, provided enough moisture is also available (Sierra et al., 2017). The link between temperature and moisture in relation to decomposition suggests that since temperature was not limiting in our study, reduced soil moisture (averaging 0.18 m3 m−3 in recently burned plots compared with 0.30 m3 m−3 in bluegrass‐dominated plots at 5 cm throughout the 2018 growing season) was likely the factor hindering decomposition in recently burned areas.In contrast to sites that had been burned in 2018, no differences in decomposition were observed between soils dominated by bluegrass monocultures and areas burned in 2017, suggesting decomposing conditions rebound relatively quickly from fire. The decomposition study revealed another interesting trend, which was decomposition of bluegrass litter was fairly high (over 80% mass loss in soil) across all study areas, regardless of treatment. This was surprising considering bluegrass is known to accumulate litter and develop a thick thatch layer, and decreased levels of decomposition are often implicated as the cause. The high levels of decomposition in our study are consistent with findings from Hendrickson et al. (2001), which showed monocultures of improved grass cultivars had significantly greater rates of decomposition than native grass species occurring in undisturbed rangeland. Additionally chemical analysis of standing and fallen bluegrass biomass from our study revealed the litter had an average carbon‐to‐nitrogen ratio of 29:1, which would not be in a range prohibitive to decomposition due to nitrogen limitation. The results suggest decreased decomposition is not the reason for the characteristic accumulation of bluegrass litter, but the cause is more likely due to disproportionately high levels of bluegrass biomass production relative to litter decomposition rates.The observed differences in decomposition suggest soil processes in this study may be affected by vegetative characteristics, yet our analysis of nutrient and biochemical pools found abundances to remain fairly stable across treatments. While the total abundance of soil nutrient and biochemical pools may not change significantly, it is possible the processes driving the turnover rate of these pools is different. This idea is supported by findings from Wedin and Tilman (1990) and Wedin and Pastor (1993), which showed monocultures of perennial grasses could affect annual net nitrogen mineralization up to 10‐fold after three years (Wedin & Tilman, 1990), yet monocultures of the same species did not change total soil carbon or nitrogen after four years (Wedin &, Pastor,1993). Sanderson et al. (2017) found bluegrass also changed the isotopic composition of soil carbon compared with native prairie species. Future studies on the subject should focus on quantifying the cycling rates, rather than the sizes, of these pools.Overall, our study found that neither the difference between bluegrass and native plant dominance nor fire and grazing had a strong effect on the soil properties studied in this survey. The prescribed burn may have caused a brief decline of labile POX C as well as slowed decomposition in shallow soil, but both properties rebounded by one year after burning. Soil properties measured repeatedly over time in this study did not reveal any seasonal shifts within the same season following the application of prescribed burns. Future research should characterize long‐term soil response to management with fire and grazing, as the fire and grazing system in this study was newly implemented.Results from this study revealed belowground properties in the study area were strikingly uniform, suggesting significant resilience to shifts in aboveground characteristics. While marginal differences were noted between areas recently burned and grazed and areas dominated by bluegrass under idle management, these differences were short‐lived. The plasticity of the measured soil conditions in response to management, and their tendency to return to stable conditions, suggests one of two possibilities: (a) prior to bluegrass invasion, the soil conditions in this area were already similar to conditions measured in this study, and they have not shifted significantly in response to changes in plant assemblages, or (b) the soil properties studied in this experiment may have reached a point where they are no longer responsive to management, and they have been fundamentally changed as a result of bluegrass dominance. Under either of these circumstances, we have hope that management strategies targeting a more favorable plant community are not going to be challenged or delayed because of any negative effects to the soil. The use of fire and grazing as a management practice to reduce bluegrass and promote native plant diversity appears to be a promising strategy, as this study found no long‐term negative effects on the studied soil properties in areas under this management regime.DATA AVAILABILITY STATEMENTThese data are available from the authors upon request.ACKNOWLEDGMENTSThis work was financially supported by the Central Grasslands Research Extension Center (CGREC), the North Dakota Agricultural Experiment Station, and the School of Natural Resource Sciences at North Dakota State University. The authors are grateful for assistance in the field and laboratory from Joel Bell, Rebecca Hebron, Alec Deschene, Megan Dornbusch, Pamela Block, Mackenzie Ries, and the entire CGREC prescribed burn crew, and for comments on a previous version of this manuscript by three faculty members in the School of Natural Resource Sciences at North Dakota State University. We collectively acknowledge that we gather at NDSU, a land grant institution, on the traditional lands of the Oceti Sakowin (Dakota, Lakota, Nakoda) and Anishinaabe Peoples in addition to many diverse Indigenous Peoples still connected to these lands. We honor with gratitude Mother Earth and the Indigenous Peoples who have walked with her throughout generations. We will continue to learn how to live in unity with Mother Earth and build strong, mutually beneficial, trusting relationships with Indigenous Peoples of our region.AUTHOR CONTRIBUTIONSLeslie Gerhard: Conceptualization; Formal analysis; Investigation; Visualization; Writing – original draft; Writing – review & editing. Caley K. Gasch: Conceptualization; Resources; Supervision; Visualization; Writing – review & editing. Kevin Sedivec: Resources; Writing – review & editing.CONFLICT OF INTERESTThe authors declare no conflict of interest.REFERENCESAkoglu, H. (2018). User's guide to correlation coefficients. 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Journal

"Agrosystems, Geosciences & Environment"Wiley

Published: Jan 1, 2022

References