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J. Hydrol. Hydromech., 69, 2021, 4, 378–386 ©2021. This is an open access article distributed DOI: 10.2478/johh-2021-0026 under the Creative Commons Attribution ISSN 1338-4333 NonCommercial-NoDerivatives 4.0 License Biocrust effects on soil infiltrability in the Mu Us Desert: Soil hydraulic properties analysis and modeling 1,2,3* 4 Hongjie Guan , Xinyu Liu Yanchi Research Station, School of Soil and Water Conservation, Beijing Forestry University, Beijing 100083, China. Key Laboratory of State Forestry Administration on Soil and Water Conservation, Beijing Forestry University, Beijing 100083, China. Beijing Engineering Research Center of Soil and Water Conservation, Beijing Forestry University, Beijing 100083, China. State Key Laboratory of Earth Surface Processes and Resource Ecology, MOE Engineering Research Center of Desertification and Blown-sand Control, Faculty of Geographical Science, Beijing Normal University, Beijing 100875, China. Corresponding author. E-mail: firstname.lastname@example.org Abstract: The presence of biocrusts changes water infiltration in the Mu Us Desert. Knowledge of the hydraulic proper- ties of biocrusts and parameterization of soil hydraulic properties are important to improve simulation of infiltration and soil water dynamics in vegetation-soil-water models. In this study, four treatments, including bare land with sporadic cy- anobacterial biocrusts (BL), lichen-dominated biocrusts (LB), early-successional moss biocrusts (EMB), and late- successional moss biocrusts (LMB), were established to evaluate the effects of biocrust development on soil water infil- tration in the Mu Us Desert, northwest of China. Moreover, a combined Wooding inverse approach was used for the es- timation of soil hydraulic parameters. The results showed that infiltration rate followed the pattern BL > LB > EMB > LMB. Moreover, the LB, EMB, and LMB treatments had significantly lower infiltration rates than the BL treatment. The saturated soil moisture (θs) and shape parameter (αVG) for the EMB and LMB treatments were higher than that for the BL and LB treatments, although the difference among four treatments was insignificant. Water retention increased with bi- ocrust development at high-pressure heads, whereas the opposite was observed at low-pressure heads. The development of biocrusts influences van Genuchten parameters, subsequently affects the water retention curve, and thereby alters available water in the biocrust layer. The findings regarding the parameterization of soil hydraulic properties have im- portant implications for the simulation of eco-hydrological processes in dryland ecosystems. Keywords: Cyanobacteria; Lichen; Moss; Infiltration; Inverse approach; Hydraulic parameter. 1 INTRODUCTION soil, climate, and biocrusts differ from other areas. In this area, a large number of shrubs died with the occurrence of late- Dryland landscape is usually characterized by woody plant successional biocrusts, possibly due to the reduced soil water patches, which is separated by open patches (Ludwig et al., infiltration caused by biocrusts (Guan and Liu, 2019; Xiao and 2005). The surface of these open patches is usually occupied by Hu, 2017). However, no conclusive data are yet available and biocrusts (also named biological soil crusts), which result from our understanding of the mechanisms of biocrusts in altering a configuration between soil particles and cyanobacteria, algae, soil hydrology remains unknown. fungi, lichens, and mosses within the upper millimeters of the In addition, the presence of biocrusts alters soil physical soil (Belnap, 2006; Eldridge et al., 2020; Weber et al., 2016). properties, and subsequently influences soil hydrological pro- Biocrusts change physicochemical properties of topsoils and cesses, and thus changes plant growth (Havrilla et al., 2019; have a significant impact on water infiltration (Chamizo et al., Kidron, 2019; Xiao and Hu, 2017). Improved knowledge of the hydraulic properties of biocrusts and estimation of soil hydrau- 2016; Jiang et al., 2018). A few studies have been performed to evaluate the effects of lic parameters are crucial for improving simulation of infiltra- biocrusts on water infiltration. Nevertheless, contradicting tion, soil water dynamics, and soil water uptake in vegetation- results exist regarding the roles of biocrusts on soil water infil- soil-water models (Coppola et al., 2011; Wang et al., 2007; Yu tration (Warren, 2003; Weber et al., 2016). Biocrusts modify et al., 2010), and have important implications for the simulation soil infiltrability through different pathways. Biocrusts increase of eco-hydrological processes in dryland ecosystems (Chen et soil porosity, roughness and aggregate stability and improve al., 2018). We hypothesized that biocrust development has a physical structure, which in turn increase soil water infiltration negative influence on water infiltration. Accordingly, the pur- (Felde et al., 2014; Jiang et al., 2018). On the other hand, bi- poses of our study were: (1) to evaluate whether biocrust devel- ocrusts increase water repellency and cause pore clogging due opment influence soil water infiltration; (2) to estimate the to exopolysaccharide excretion, which impede soil water infil- hydraulic parameters in biocrust-covered soils. tration (Keck et al., 2016; Kidron et al., 2020; Xiao et al., 2019). These contradictory results were attributed to the differ- 2 MATERIALS AND METHODS ence in soil properties (soil structure and texture), biocrust 2.1 Experimental site characteristics (cover, morphology, and composition), and climate (mainly rainfall characteristics) (Weber et al., 2016). This study was undertaken at the Yanchi Research Station, Therefore, further studies are necessary to test these contradict- Ningxia Province, northwestern China (106°30′−107°47′ E and ing results in other areas, such as the Mu Us Desert where the 37°04′−38°10′ N, 1550 m above the sea level). The site is lo- 378 Biocrust effects on soil infiltrability in the Mu Us Desert: Soil hydraulic properties analysis and modeling cated on the southwestern fringe of the Mu Us Desert and is consisted of Microcoleus vaginatus. The lichen-dominated characterized by a mid-temperate semi-arid climate with mean biocrusts are mainly composed of Collema tenax species with a annual temperature of 8.1 °C. Mean annual precipitation in this low cover of cyanobacteria. The moss-dominated biocrusts, in area is 287 mm, most of which occurs from July to September. addition to Byumargenteum p., include a certain amount of Soil texture is classified as sand. The average sand, silt, and lichens and cyanobacteria. A vernier caliper and line intercept clay content in the shallow soil profile (0- to 10-cm depth) are transects were used for measuring the thickness and cover of 79.1%, 18.5%, and 2.4%, respectively. The mean percentages biocrusts, respectively. Biocrust samples, which were manually of sand, silt and clay in lower soil (10- to 60-cm depth) are screened through a 2-mm screen and dried at 65 °C for 24 h, 93.0%, 4.3%, and 2.7%, respectively. The dominant shrubs are were used for measuring the biomass. The polysaccharide con- Artemisia ordosica, Caragana korshinskii, Salix psammophila, tent of biocrust samples were also measured by the phenol- and Hedysarum mongolicum, which are distributed in patches sulfuric acid method. Additionally, particle size distribution of covering 30–70% of the soil surface. The inter-canopy soil the biocrust layer and the subsurface (at 5-cm depth under surface is commonly covered by biocrusts. biocrust layer) was determined. 2.2 Experimental design 2.3. Infiltration experiments In this study, infiltration experiments were conducted in Au- The infiltration experiments were performed using a disc in- gust 2017 to evaluate the influence of biocrust development on filtrometer with a 15 cm-diameter disc. The height and diameter water infiltration. In this experiment, four treatments, including of the water reservoir tower was 100 and 3.5 cm, respectively. bare land with sporadic cyanobacterial biocrusts (BL) (Fig. In order to estimate van Genuchten parameters, three pressure 1A), lichen-dominated biocrusts (LB) (Fig. 1B), early- heads (h) of –3, –6, and –12 cm at each infiltration point were successional moss biocrusts (EMB) (Fig. 1C), and late- used. Prior to each measurement, a layer of fine sand with successional moss biocrusts (LMB) (Fig. 1D), were established thickness of 2 mm was laid on soil surface at each infiltration to evaluate the effects of biocrust development on soil water point and then the disc infiltrometer was put on the fine sand. infiltration. Five replicates with similar soil and topographic The water level in the reservoir tower was recorded until it conditions were established for each treatment, thus totalling 20 reached steady state. The time interval for observation was 10 s experimental sites. during the first 3 min of the infiltration experiment. However, There are three main types of biocrusts in this area: cyano- the time interval for observation was 30 s when the infiltration bacteria, lichens, and mosses. The cyanobacterial biocrusts are time reached 3 min. C D Fig. 1. Landscapes with the four treatments for study including (A) bare land with sporadic cyanobacterial biocrusts (BL), (B) lichen- dominated biocrusts (LB), (C) early-successional moss biocrusts (EMB), (D) late-successional moss biocrusts (LMB). 379 Hongjie Guan, Xinyu Liu 2.4 Calculation of soil hydraulic parameters (Coppola et al., 2011). The microscopic pore radius (λ , mm) was calculated The following method was adopted to analyze the infiltra- through Eq. (8) according to White and Sully (1987). tion data. Over time, the infiltration from a circular source with a constant pressure head can be described by the Wooding's σα GRD λ = (8) solution (Wooding, 1968): ρ g 4λ −1 2 c where σ is the surface tension (N m ); ρ is the density of water Qr=+ π(K h)[1 ] (1) −1 −1 πr (kg m ); and g is the acceleration due to gravity (N kg ). with λ is calculated according to White and Sully (1987): 2.5 Estimation of van Genuchten parameters using a combined Wooding inverse approach bS λ = (2) Soil hydraulic properties are usually expressed by the soil [Kh ( )(θθ − )] fin ini hydraulic conductivity (K(h)) and water retention curve (θ(h)). The most important work in determining the functional rela- where r0 is the radius of the disc (cm); h is the pressure head tionships between the soil hydraulic conductivity, water con- (cm), which was –3, –6, and –12 cm; λc is the macroscopic tent, and matric potential is to obtain van Genuchten parame- capillary length; K(h) is the unsaturated conductivity under a ters, which are estimated using a coupled Wooding inverse −1 given pressure head (cm min ); Q is the steady-state infiltra- approach that combined the results from Wooding’s analytical 3 −1 tion rate (cm min ); b is a shape parameter between 1/2 and solution with a parameter estimation method using a numerical π/4 (Smettem and Clothier, 1989); θfin is the final soil water solution of the Richards equation (Coppola et al., 2011; Laza- 3 −3 content (cm cm ); θini is the initial surface soil water content rowitch et al., 2007). 3 −3 −1/2 (cm cm ); and S is the sorptivity (cm min ). The following form of the Richards equation is usually used The Q can be calculated as the following form by substitut- to characterize the radially symmetric isothermal Darcian flow ing Eq. (2) into (1): in a variably saturated isotropic rigid porous medium (Warrick, 1992): Qb 4S =+ Kh () (3) π[rrπ(θθ − )] ∂∂ θ 1 ∂hh ∂ ∂ 00finini =+ (( rK h) ) (K(h)−1) (9) ∂∂ tr r ∂r ∂z ∂z with ic is expressed as: where t is time; r is the radial coordinate; and z is the vertical coordinate positive downward. Initial and boundary conditions i = (4) πr that are appropriate for a disc tension infiltrometer experiment are expressed by the following equations (Šimůnek and van From Eqs. (3) and (4), the K(h) can be calculated as the fol- Genuchten, 2000): lowing form by replacing Q with constant infiltration rate (ic, −1 hr ( ,z,t)== h (z) t 0 (10) cm min ) (White and Sully, 1987): ini 4bS hr ( ,z,t)=< h (t) 0 r< r , z= 0 (11) Kh ()=− i (5) [π( r θθ − )] 0 fin ini ∂hr (,z,t) => 1 rr , z= 0 (12) In Eq. (5), S is estimated by the intercept of the regression ∂z 1/2 1/2 line between ΔI/Δt and t according to Vandervaere et al. 1/2 (1997), where Δt is the variable quantity of the square root of hr ( ,z,t)=+ h r z→∞ (13) ini time (min) and ΔI is the variable quantity of cumulative infiltra- tion (cm); and ic is calculated by the slope of the linear fitted where h is the initial pressure head (cm); r is the disc radius ini 0 cumulative infiltration curves with the stable infiltration data. (cm); and h0 is the time-variable supply pressure head (cm). According to the quasi-linear Gardner model (Gardner, The van Genuchten model (van Genuchten, 1980) was cho- −1 1958), the K(h) (cm min ) could be expressed as: sen to express the soil water retention, θ(h), and hydraulic conductivity, K(θ): α h GRD Kh () = K e (6) θθ − −m −1 Sh == [1+ α ] (14) where K is the saturated hydraulic conductivity (cm min ); eVG θθ − sr and αGRD is the exponential slope. The Ks can be expressed using Eqs. (5) and (6). n −1 m = (15) 4bS n α h GRD Ke =− i (7) sc [π( r θθ − )] 0 fin ini lm 1/ 2 KS ( )=− K S [1 (1−S )m] (16) ese e The Ks and αGRD in Eq. (7) are only the two unknown param- where θ and θ are the saturated and residual soil moisture eters, which could be calculated by two consecutive pressure s r 3 −3 −1 (cm cm ), respectively; α (cm ), m and n are the shape heads. The approach assumes that parameter αGRD is constant VG parameters; S is the effective fluid saturation; and l is the tortu- over the interval between two consecutive pressure heads e 380 Biocrust effects on soil infiltrability in the Mu Us Desert: Soil hydraulic properties analysis and modeling osity parameter, which is usually fixed at 0.5 (Mualem, 1976). the EMB and LMB treatments, but significantly lower than for The transient tension disc infiltration data, together with ini- the LB treatment. Specifically, the i under the pressure head of tial and final soil moisture, were used for the numerical inverse –3 cm followed the pattern BL > LB > EMB > LMB. Further- determination of van Genuchten parameters, by fixing K at the more, for the BL treatment under the pressure head of –3 cm, value obtained using Wooding’s analytical solution. Given that the ic was 92.3% higher than that for the LB treatment (P < all optimizations initially gave θr estimates very close to zero, 0.05) (Table 2). The ic for the LB treatment under the pressure we decided to fix θr to zero for all cases. The objective func- head of –3 cm was 25.8% and 38.7% higher than that for the tion, Φ, for the numerical inverse approach is: EMB and LMB treatments, although there was no significant difference in the infiltration rate among the three treatments (Table 2). ∗ 2 Φ= ()ββ WI[ (t )−I( ,t )] (17) ii i i In addition, the K(h), S, K , and λ for the BL treatment were s c i=1 obviously higher than that for the other treatments. In detail, the K(h) and K for the BL treatment under the pressure head of –3 where M is the number of measurements in a particular set; β is cm were significantly higher than that for the LB treatment by the vector of optimized parameters; Wi is the weight of a partic- * 2.0- and 2.1-fold, respectively (Table 2). Similarly, the S aver- ular measured point; Ii (cm) is the measured cumulative infil- aged over three pressure heads followed the pattern BL > LB > tration at time ti; and Ii (cm) is the simulated cumulative infil- EMB > LMB. On the contrary, the λ followed the pattern BL tration at time t . < LB < EMB < LMB, although the difference among them was Water retention curves can be obtained through the estimat- insignificant. The two-way ANOVA results indicated that the ed van Genuchten parameters. The plant available water, which influences of biocrust type or head on iini, ic, S, K(h), Ks, and λc was defined as the difference in soil moisture (Δθ) within a were significant at the 0.01 or 0.05 level; however, the influ- pressure head ranged from –1 to –1000 cm, was estimated by ence of biocrust type on λm was insignificant at the 0.05 level water retention curves (Wang et al., 2007). (Table 3). In addition, the influences of the interaction between the biocrust type and pressure head on iini, ic, S, K(h), and Ks 2.6 Data analyses were insignificant at the 0.05 level. A two-way ANOVA was used for analyzing the effects of 3.3 Determination of van Genuchten parameters biocrust type and pressure head on the soil water infiltrability at the 5% probability level. The differences in soil hydraulic pa- As can be seen in Table 4, the θs and αVG increased with bi- rameters among these treatments were analyzed using the least ocrust development. More specifically, the θs and αVG for the significant difference (LSD) tests at the 5% probability level. EMB and LMB treatments were higher than that for the BL and All statistical analyses were performed using the SPSS 19.0 LB treatments, although the difference among four treatments software (SPSS, Chicago, IL, USA). was insignificant. For instance, the θ for the LMB treatment was 28.0% higher than that for the LB treatment. Moreover, 3 RESULTS there was no significant difference in θ and α between the s VG 3.1 Characteristics of biocrusts BL and LB treatments. In addition, the relation between the parameter of the n and biocrust development was found to be Generally, with the development of biocrusts, the cover, non-unique. In detail, the BL and EMB treatments had slightly thickness, and biomass of the biocrusts significantly increased; higher n than the LB and LMB treatments. Furthermore, there however, the difference in biocrust cover between the EMB and was no obvious difference in n between the BL and EMB LMB treatments was insignificant at the 0.05 level (Table 1). In treatments or between the LB and LMB treatments. The one- comparison to the bare land, biocrusted soils had more silt and way ANOVA results indicated that the influences of biocrust clay content. Moreover, higher fine content (i.e., silt and clay) development on θs, αVG and n were insignificant at the 0.05 was observed in the moss-covered biocrusts compared to li- level (data not shown) chen-covered biocrusts (Table 1). As shown in Fig. 3, biocrust development had a significant influence on water retention and hydraulic conductivity. Water 3.2 Effects of biocrust development on water infiltration retention increased with biocrust development at high-pressure heads, whereas the opposite was observed at low-pressure As shown in Fig. 2, from initial to steady state, the BL heads (Fig. 3A and B). In detail, water retention at high- treatment had significantly higher infiltration rates than the pressure heads followed the pattern LMB > EMB > BL > LB. other treatments. Furthermore, infiltration rate was similar for Table 1. Characteristics of the biocrusts in the four treatments. Treatments Cover Thickness Biomass Polysaccha- Sand Silt Clay –2 (%) (mm) (mg cm ) ride content (%) (%) (%) –1 (μg mg ) Bare land with sporadic cyanobacterial 85.79±5.09 a 12.71±1.51 b 1.50±0.16 b – – – – biocrusts (BL) Lichen-dominated biocrusts (LB) 70±9.7 b 4.8±0.9 c 0.81±0.08 c 2.26±0.03 b 84.59±4.43 a 13.53±2.21 b 1.88±0.13 b Early-successional moss biocrusts 76.8±3.5 a 8.9±0.7 b 1.84±0.36 b 3.76±0.05 a 77.50±5.26 b 20.60±2.56 a 1.90±0.21 b (EMB) Late-successional moss biocrusts (LMB) 95.8±1.8 a 15.2±0.8 a 2.94±0.57 a 3.94±0.02 a 75.34±4.82 b 21.33±2.29 a 3.33±0.18 a Different letters in the same column indicate significant differences at the probability level of 0.05. 381 Hongjie Guan, Xinyu Liu pared to the bare land (Table 2; Fig. 2). The negative biocrust 0.02 effects on water infiltration are likely due to the higher amount BL LB EMB LMB of clay and silt in the biocrusted soils, which could reduce soil pores and result in a decrease in water infiltration (Table 1). Additionally, the biocrust effects could attributed to the higher 0.01 biocrust thickness and the polysaccharide content (Table 1). We should note that the polysaccharides measured in this study is different from exopolysaccharides (EPS), and it can only serve as a measure for biocrust biomass (similar to chlorophyll con- tent). Namely, the polysaccharides cannot be an indication of 0.00 0 100 200300 400500 600700 the capacity of the biocrusts to absorb water. Although not Time (s) measured in this study, the involvement of the EPS cannot be neglected. Given that the ability of the EPS to absorb copious Fig. 2. Infiltration rate of the four treatments including bare land amounts of water (Chenu, 1993; Colica et al., 2014; Mazor et with sporadic cyanobacterial biocrusts (BL), lichen-dominated biocrusts (LB), early-successional moss biocrusts (EMB), and late- al., 1996), the cyanobacterial capacity to excrete copious successional moss biocrusts (LMB). amounts of EPS (Kidron et al., 2003), and the fact that cyano- bacteria also inhabit the lichen- and moss-dominated biocrusts On the contrary, Water retention at low-pressure heads fol- (Gentili et al., 2005; Paulsrud and Lindblad, 1998; Rossi et al., lowed the pattern LB > BL > LMB > EMB. Hydraulic conduc- 2018), an efficient impediment of infiltration is expected. This tivity for the BL treatment, as shown in Fig. 3C, was obviously issue deserves further research. higher than that of the other treatments. Furthermore, there was Our results correspond to the results of other authors (Cop- no obvious difference among the LB, EMB, and LMB treat- pola et al., 2011; Xiao et al., 2019; Yang et al., 2016), who ments, and the difference was dependent on the pressure head. concluded that the lichen- and moss-covered biocrusts led to a In addition, the development of biocrusts influences the wa- decrease in water infiltration (Table 5). However, some other ter retention curve, and thus alters the available water. In detail, studies have shown the opposite: biocrusts increased water Δθ for the moss-covered treatments were obviously higher than infiltration (Jiang et al., 2018; Kakeh et al., 2021) (Table 5). that for the cyanobacteria- or lichen-covered treatments. For The difference in soil surface roughness caused by biocrusts 3 −3 example, the available water increased from Δθ = 0.251 m m could partly explain these contradictive results (Warren, 2003). 3 −3 and Δθ = 0.214 m m for the BL and LB treatments to Δθ = In detail, rugose or smooth biocrusts such as cyanobacteria are 3 −3 0.281 m m for the EMB treatment. Moreover, for moss bi- common in the areas without frost-heaving, thus resulting in a 3 −3 ocrusts, the available water increased from Δθ = 0.281 m m low surface roughness. Nevertheless, in the areas where soils 3 −3 for the EMB treatment to Δθ = 0.290 m m for the LMB freeze, frost-heaving of biocrusted surfaces results in a highly treatment. However, the available water decreased from Δθ = roughened surface. Higher roughness allows more residence 3 −3 3 −3 0.251 m m for the BL treatment to Δθ = 0.214 m m for the time, and thus leads to an increased infiltration (Warren, 2003). LB treatment (Fig. 3A and B). In this study site with no obvious frost-heaving, low surface roughness resulted in a negative effect of biocrusts on water 4 DISCUSSION infiltration. 4.1 Influence of biocrust development on water infiltration Additionally, our results concluded that moss-dominated biocrusts had much lower soil infiltrability compared to the Our study shows that the presence and development of bi- lichen-dominated biocrusts (Table 2; Fig. 2). In comparison to ocrusts had a negative effect on water infiltration when com- lichen-covered biocrusts, higher cover, thickness, biomass, and Table 2. Soil hydraulic parameters of biocrusts and bare soil under the different biocrust development. Pressure –1 –1 –1 –1/2 -1 Treatments i (mm min ) i (mm min ) K(h) (mm min ) S (mm min ) K (mm min ) λ (mm) λ (mm) ini c s c m head –3 cm BL 1.36±0.37 bc 0.75±0.09 a 0.79±0.10 a 0.89±0.48 ab 1.44±0.18 b 4.87±3.27 bc 21.79±13.05 b LB 1.00±0.28 bc 0.39±0.06 b 0.39±0.06 b 0.18±0.06 b 0.70±0.11 c 0.54±0.24 c 453.81±390.96 b EMB 0.93±0.15 bc 0.31±0.04 bc 0.31±0.04 bc 0.11±0.02 b 0.57±0.08 c 0.14±0.05 c 168.72±109.64 b LMB 1.05±0.32 bc 0.27±0.07 bc 0.26±0.07 bc 0.09±0.06 b 0.48±0.13 c 0.31±0.26 c 3603.71±2192.01 a –6 cm BL 1.36±0.26 bc 0.72±0.19 a 0.69±0.16 a 0.61±0.34 b 2.30±0.52 a 3.89±3.01 bc 108.74±95.49 b LB 0.99±0.37 bc 0.33±0.07 b 0.32±0.07 b 0.25±0.04 b 1.07±0.23 bc 0.85±0.19 c 12.18±4.02 b EMB 0.49±0.16 c 0.23±0.06 bc 0.24±0.06 bc 0.07±0.02 b 0.79±0.19 c 0.13±0.08 c 1667.40±1192.36 ab LMB 0.49±0.13 c 0.18±0.05 bc 0.19±0.06 bc 0.12±0.04 b 0.64±0.20 c 0.35±0.14 c 187.57±162.26 b –12 cm BL 2.95±0.70 a 0.20±0.05 bc 0.12±0.02 c 1.20±0.29 a 1.32±0.19 bc 63.96±26.20 a 0.47±0.25 b LB 1.61±0.47 b 0.11±0.03 c 0.08±0.02 c 0.69±0.19 ab 0.91±0.18 bc 25.63±11.06 b 0.55±0.19 b EMB 0.79±0.26 bc 0.05±0.02 c 0.05±0.01 c 0.32±0.10 b 0.53±0.15 c 7.27±3.30 bc 1.78±0.51 b LMB 0.91±0.29 bc 0.06±0.02 c 0.05±0.01 c 0.37±0.12 b 0.57±0.16 c 12.40±5.96 bc 1.80±0.73 b Note. Different letters in the same column indicate significant differences at the probability level of 0.05. BL: bare land with sporadic cyanobacte- ria biocrusts; LB: lichen-dominated biocrusts; EMB: early-successional moss biocrusts; LMB: late-successional moss biocrusts. -1 Infiltration rate (mm s ) Biocrust effects on soil infiltrability in the Mu Us Desert: Soil hydraulic properties analysis and modeling ity when compared to the lichen-covered biocrusts (Table 1; A 0.35 Xiao et al., 2019). Similarly, Wu et al. (2012) concluded that BL 0.30 soil infiltrability was lower for the moss-covered biocrusts LB compared to the lichen-covered biocrusts. On the contrary, EMB 0.25 some other studies have found that infiltration increased when LMB 0.20 biocrusts shifted from cyanobacteria or algae to lichens or mosses, which is usually characterized by increased biomass 0.15 (Barger et al., 2006; Chamizo et al., 2012). The different re- 0.10 sponse of infiltration to biocrust type was dependent upon surface roughness. In these regions, greater roughness which 0.05 may have been caused by frost-heaving for lichen- or moss- dominated biocrusts than those for the cyanobacteria or algae 0.00 0 200 400 600 800 1000 1200 could explain the contradictory results (Belnap et al., 2013). -h (cm) It should be noted that the use of a disc infiltrometer does not adequately reflect water flow under natural conditions. 0.35 Firstly, it does not reflect surface roughness, because a layer of BL 0.30 fine sand was laid on soil surface before infiltration experi- LB ments were performed. Therefore, the use of a disc infiltrome- 0.25 EMB ter could alleviate the influence of surface roughness caused by LMB biocrusts on water infiltration. Secondly, unlike one- 0.20 dimensional vertical flow under natural conditions, the uncon- 0.15 fined three-dimensional infiltration (vertical and lateral flow) usually take place under tension condition when using a disc 0.10 infiltrometer (Vandervaere et al., 1997). Although there are 0.05 some above-mentioned limitations, the disc infiltrometer under tension condition can still reflect biocrust effect on soil infiltra- 0.00 0 5 10 15 20 25 bility, as verified by a similar study of Xiao et al. (2019) that -h (cm) reported consistent results of the double-ring infiltrometer and the disc infiltrometer. Considering the above-mentioned issues, 1.8 further research is needed to evaluate the difference between the BL 1.6 double-ring infiltrometer and disc infiltrometer in our study site. LB 1.4 EMB 4.2 Determination of van Genuchten parameters 1.2 LMB 1.0 Parametrization of hydraulic properties on biocrusted soils 0.8 and estimation of soil hydraulic parameters are crucial for im- 0.6 proving simulation of infiltration and soil water dynamics in 0.4 vegetation-soil-water models (Coppola et al., 2011; Yu et al., 2010), and thus the understanding of eco-hydrological process- 0.2 es in dryland ecosystems (Rodríguez-Caballero et al., 2015). 0.0 -3 -2 -1 0 1 2 3 Among them, the most important factor is to estimate the pa- rameters of van Genuchten model. Our results indicated that log|h| (cm) moss-covered soils had lower K than lichen-covered soils; however, the θ for the moss-covered soils was higher than that Fig. 3. Water retention (A and B) and hydraulic conductivity (C) of s the four treatments including bare land with sporadic cyanobacteri- for lichen-covered soils, although the differences were insignif- al biocrusts (BL), lichen-dominated biocrusts (LB), early- icant. This result could be attributed to the higher fine content successional moss biocrusts (EMB), and late-successional moss and polysaccharide content of the moss-covered soils when biocrusts (LMB). compared to lichen-covered soils (Table 1). Furthermore, simi- lar to our simulation study, the results of field experiments by polysaccharide content for the moss-covered biocrusts could Guan and Liu (2019) also found that well-developed moss- explain this phenomenon (Table 1; Kidron et al., 2003; Chami- covered biocrusts had higher θs than lichen-covered biocrusts, zo et al., 2012). Furthermore, higher fine content (i.e., silt and suggesting a higher retention capacity for the moss-covered clay) in the moss-covered biocrusts may reduce soil infiltrabil- biocrusts. Table 3. P values from the two-way ANOVA to test the influences of biocrust type and pressure head as well as their interaction effects on soil hydraulic parameters. –1 –1 –1/2 –1 –1 Effects iini (mm min ) ic (mm min ) S (mm min ) K(h) (mm min ) Ks (mm min ) λc (mm) λm (mm) Biocrusts 0.001** <0.001** <0.001** <0.001** <0.001** 0.013* 0.175 Pressure head 0.015* <0.001** <0.001** 0.024* 0.023* <0.001** 0.133 Biocrusts × Pressure head 0.188 0.149 0.020* 0.431 0.949 0.031* 0.040* Note. i , initial infiltration rate; i , steady-state infiltration rate; S, sorptivity; K(h), unsaturated hydraulic conductivity at h pressure head; ini c Ks, saturated hydraulic conductivity; λc, macroscopic capillary length; λm, microscopic pore radius. * Effect is significant at .05 level of probability. ** Effect is significant at .01 level of probability. -1 K (mm min ) θ (-) θ (-) Hongjie Guan, Xinyu Liu Table 4. Water retention and hydraulic conductivity parameters for the biocrusted soils under the different biocrust development. –1 Treatments θs αVG (cm ) n Bare land with sporadic cyanobacterial biocrusts (BL) 0.28±0.02 a 0.11±0.03 a 1.72±0.14 a Lichen-dominated biocrusts (LB) 0.25±0.01 a 0.12±0.03 a 1.51±0.07 a Early-successional moss biocrusts (EMB) 0.30±0.02 a 0.21±0.04 a 1.72±0.08 a Late-successional moss biocrusts (LMB) 0.32±0.04 a 0.27±0.09 a 1.54±0.07 a Different letters in the same column indicate significant differences at the probability level of 0.05. Table 5. Regional difference in the biocrust effects on soil water infiltrability (in increasing order of annual precipitation). Climate (annual Measured Biocrust effects Locations Soil texture Biocrust type References precipitation in mm) parameters on infiltration Heihe River Hyper arid (71) Silty loam Infiltration rate Decreasing Yang et al. (2016) Basin Central-Western Hyper arid (90) Sand Cyanobacteria Infiltration rate Increasing under Eldridge et al. Negev Desert tension; no signifi- (2000) cant effect under ponding Western Negev Hyper arid (95) Sand Cyanobacteria Runoff yield Decreasing Kidron (2016) Desert Mojave Desert Arid (101) Sand; loamy sand Cyanobacteria Infiltration rate Decreasing Herrick et al. (2010) Tengger Desert Arid (191) Sand Mosses Infiltration rate Increasing Coppola et al. (2011) Tabernas Desert Semiarid (200–235) Silty loam; sandy Cyanobacteria; Infiltration rate Decreasing with Chamizo et al. loam lichens; mosses biocrust develop- (2012) ment Colorado Plateau Semiarid (215) Loamy sand Cyanobacteria; Runoff coefficient Decreasing Faist et al. (2017) cyanolichens Southeastern Semiarid (220–235) Silty loam; sandy Cyanobacteria Runoff coefficient Decreasing with Cantón et al. (2020) Spain loam biocrust develop- ment Western New Semiarid (244) Sand Cyanobacteria; Infiltration rate Increasing Bowker et al. (2013) South Wales lichens; mosses Northern Chihua- Semiarid (250) Sandy clay loam Cyanobacteria Infiltration depth Increasing Chung et al. (2019) huan Desert Quara Qir range- Semiarid (273) Loamy Lichens; mosses Infiltration rate Increasing Kakeh et al. (2021) lands Qinghai-Tibet Semiarid (389) Silty loam Mosses Infiltration rate Increasing Jiang et al. (2018) Plateau Loess Plateau Semiarid (409) Loamy sand Cyanobacteria; Infiltration rate Decreasing Xiao et al. (2019) mosses Additionally, our results indicated that the shape parameter covered soils allows for a longer residence time of the water, of α followed the pattern LMB > EMB > LB > BL. This which could improve water retention at the soil surface. More- VG result suggests that the shape parameters of α increased with over, higher content of clay and silt in the moss-covered bi- VG biocrust development. A higher α corresponds to a lower air ocrusts reduce the soil pores, and lead to a decrease in soil VG entry value, and implies a lower water holding capacity near hydraulic conductivity. Accordingly, the presence of lichen or saturation (van Genuchten, 1980). Moreover, this result indi- moss biocrusts on the soil surface enhances water retention, but cates that the higher αVG was consistent with a higher content of inhibits soil hydraulic conductivity. clay and silt. On the contrary, Wang et al. (2007) reported that The occurrence and development of biocrusts change van the shape parameter of αVG declined as the time since stabiliza- Genuchten parameters and affect the water retention curve, and tion increased, and the lower αVG was related to a higher propor- subsequently alters the available water in the biocrust layer. We tion of silt- and clay-sized particle. The difference in the inverse found that available water in the biocrust layer increased with a method and model uncertainty may lead to this discrepancy. shift in biocrusts from cyanobacteria to lichens or mosses (Fig. The development of biocrusts leads to different van Genuch- 3A and B). It is noted that available water mentioned above ten parameters, which alter the θ(h) and K(h) curves. Our re- focused on the soil surface (i.e., biocrust layer). The increased sults indicate that θ(h) on the moss-covered soils was higher water content on the soil surface implies a lower soil water than on the lichen-covered soils at high-pressure heads; howev- content of the shrub root layer. This suggests that the presence er, the opposite was observed at low-pressure heads (Fig. 3B). and development of biocrusts could facilitate the growth of In addition, moss-covered soils had lower K(h) than the lichen- annual plants, but inhibit the growth of vascular plants. Moreo- covered soils. Decrease in K(h) and increase in θ(h) are likely ver, higher soil surface water content has important implica- due to the characteristics of biocrusts when compared to bare tions for dryland productivity. Therefore, our results provide land. In comparison to the bare land, greater water-holding strong support for the explicit inclusion of biocrusts in hydro- capacity on the biocrust-covered soils could lead to an im- logical and ecohydrological models. proved water conditions on the soil surface (Fig. 3B; Guan and Our study focused on the effects of biocrust development on Liu, 2019). Furthermore, the anchoring structures on lichen- soil infiltrability. The presence and development of biocrusts 384 Biocrust effects on soil infiltrability in the Mu Us Desert: Soil hydraulic properties analysis and modeling change the water infiltration during rainfall events and alter soil Chenu, C., 1993. Clay- or sand-polysaccharide associations as evaporation during the drying periods, and consequently have models for the interface between micro-organisms and soil: the potential to influence plant water uptake, and eventually water related properties and microstructure. Geoderma, 56, impact plant growth (Xiao and Hu, 2017; Zhang and Belnap, 143–156. 2015; Zhuang et al., 2015). In addition, biocrusts are very frag- Chung, Y.A., Thornton, B., Dettweiler-Robinson, E., Rudgers, ile and are susceptible to disturbance, which could affect water J.A., 2019. Soil surface disturbance alters cyanobacterial bi- infiltration during rainfall events (Barger et al., 2006; Chamizo ocrusts and soil properties in dry grassland and shrubland et al., 2012). However, how the biocrust development and its ecosystems. Plant soil, 441, 147–159. disturbance influence soil water availability and shrub growth Colica, G., Li, H., Rossi, F., Li, D., Liu, Y., De Philippis, R., have not yet been well understood (Chamizo et al., 2016; Ki- 2014. Microbial secreted exopolysaccharides affect the hy- dron and Aloni, 2018). Accordingly, further studies are neces- drological behavior of induced biological soil crusts in de- sary to evaluate the role of the biocrust development and its sert sandy soils. Soil Biol. Biochem., 68, 62–70. disturbance in soil water uptake and growth of vascular plants Coppola, A., Basile, A., Wang, X., Comegna, V., Tedeschi, A., in drylands. Mele, G., Comegna, A., 2011. Hydrological behaviour of microbiotic crusts on sand dunes: Example from NW China 5 CONCLUSIONS comparing infiltration in crusted and crust-removed soil. Soil Till. Res., 117, 34–43. 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Journal of Hydrology and Hydromechanics – de Gruyter
Published: Dec 1, 2021
Keywords: Cyanobacteria; Lichen; Moss; Infiltration; Inverse approach; Hydraulic parameter
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