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Impact of a Cultivation System upon the Weed Seedbank Size and Composition in a Mediterranean Environment

Impact of a Cultivation System upon the Weed Seedbank Size and Composition in a Mediterranean... agriculture Article Impact of a Cultivation System upon the Weed Seedbank Size and Composition in a Mediterranean Environment Alessia Restuccia , Sara Lombardo * and Giovanni Mauromicale Dipartimento di Agricoltura, Alimentazione e Ambiente (Di3A), University of Catania, via Valdisavoia 5, 95123 Catania, Italy * Correspondence: saralomb@unict.it; Tel.: +39-0954783421 Received: 16 July 2019; Accepted: 2 September 2019; Published: 5 September 2019 Abstract: The knowledge of the soil seedbank is crucial to predict the dynamics of weed communities and potential future problems in agroecosystem weed management. Therefore, the aim of this study was to evaluate the qualitative and quantitative variation of the potential and real weed flora as a function of di erent cultivation systems (namely organic, conventional and uncultivated) in a Mediterranean environment (Sicily, south Italy). The results proved that soil seedbank density was significantly di erent in superficial (0–10 cm) and deeper soil layers (10–15 cm) in both organic and conventional cultivation systems. Portulacaceae and Amaranthaceae were the dominant botanical families, although they achieved a higher total number of seeds m under a conventional cultivation system than under organic and uncultivated ones. The whole weed flora was represented by 45 taxa, but the presence of the crop reduced the qualitative and quantitative composition of real weed flora. In conclusion, the knowledge of the seedbank size and composition, as well as the variation in time and space of real flora, may contribute to predict the dynamics of weed emergence and their possible interference with crops. In particular, information on the weed dynamics is essential to develop sustainable control protocols, especially under organic farming. Keywords: seedbank; botanical family; cultivation system; soil layer depth; real weed flora 1. Introduction Despite the significant advances in weed management technologies reached in the latest years, weed flora remains one of the most detrimental factors a ecting crops’ performance, as they are able to cause significant yield and quality losses [1]. While in developing countries, manual or mechanical weed management practices are prevalent, in developed ones the use of herbicides is dominant. Weed management outcomes strictly depend upon infestation size and composition [1], so that it is often necessary to adopt an integrated control approach (based on a combination of mechanical, cultural, biological and chemical management tools) to achieve an acceptable level of weed suppression [2]. As already reported in literature, cultivation systems may influence weed flora dynamics [3], and particularly, there is a growing interest by growers in the limitation of chemical weed control. Therefore, these eco-friendly cultivation systems rely more on cultural (crop rotation diversification and cover cropping) and mechanical weed control strategies [2]. In particular, the composition and abundance of weed flora in crop fields is mainly a ected by crop rotation and soil tillage systems [4]. Crop rotation is an e ective method of controlling weeds, since this modifies the weed seedbank and above-ground weed communities. However, the choice and sequence of crops may significantly a ect weed community dynamics, because of their di erent biological cycle, end use, competition against weeds and crop management practices [1]. This should prevent the proliferation and dominance Agriculture 2019, 9, 192; doi:10.3390/agriculture9090192 www.mdpi.com/journal/agriculture Agriculture 2019, 9, 192 2 of 14 of certain weed species, by proving an unsuitable environment for them. Also tillage systems can influence the size and composition of a weed seedbank, since it may determine a vertical redistribution of seeds through the soil layers [5]. In addition, tillage modifies the soil environment, particularly its temperature, moisture, aeration and biological activity. For example, conservation tillage systems are frequently reported to increase weed infestations [5,6] and to rely upon higher herbicides use [7]. Therefore, there is a growing interest in the use of environmental sound strategies for weed suppression. In particular, the ecologically-based weed management practices (e.g., the use of cover crops, no-tillage or reduced tillage systems) are increasingly adopted in organic farming [2]. These nonchemical weed management tools may reduce the problems of herbicide resistance, environmental pollution, weed diversification and crop performance. However, the e ectiveness of such strategies is more variable than the chemical weed management practices. In particular, the size of the weed seedbank is critical to the success of this ecologically-based weed control [8], since it can have long-lasting e ects on crop yields; but it is not just the size of the seedbank, as seed germination may be impaired by allelopathic e ects due to the phytotoxic activity of allelochemicals, introduced through crop rotations or by extracts applications [9]. Keeping in view all of the above-mentioned aspects, the knowledge of potential flora composition is essential to predict the weed cover of future infestations, and to develop integrated strategies for the management and control of weed flora [4,10–12], respecting environmental and economic constraints. The distribution of the seed stocks depends, indeed, upon the response of the spontaneous plant species (in terms of germination, seedlings survival, di usion, etc.) to the di erent agricultural practices [13]. Although it is known that weeds’ infestation strongly reduces crop yields [13,14], no experimental data on the relationship between potential and real flora for organic agroecosystems, (in terms of weed seedbank and weed flora dynamics using a phytosociological approach), is reported. Therefore, the purpose of this study was to assess the qualitative and quantitative composition of the potential (seedbank) and real flora in a Mediterranean agroecosystem, as a ected by the cultivation system (conventional, organic and uncultivated). A thorough knowledge of the impact of agricultural management practices on weed flora is fundamental to simultaneously developing control protocols capable of reducing the use of herbicides and to select a weed community with little impact on the agroecosystems. 2. Materials and Methods 2.1. Experimental Site, Climate and Soil Field experiments were carried out in the coastal plain of Siracusa (Sicily—South Italy, 37 03 N, 15 18 E, 10 m a.s.l.) in a moderately deep (~0.7 m) calcixerollic xerochrepts soil [15] in the 2012 and 2013 seasons. The pre-experimental soil composition was: Clay 30%, silt 25%, sand 35%, organic matter 2.0%, pH 8.4 (slightly alkali), total nitrogen 0.18%, available phosphorus 78 ppm, exchangeable potassium 337 ppm. The local climate is semiarid-Mediterranean, with mild winters and hot, rainless summers. On average, the coolest month is January (11.2 C), while October (111 mm) and July (2 mm) are, respectively, the rainiest and the driest months. The mean temperature during the hottest month (August) is 25.1 C; the local mean annual temperature is 18 C (Figure 1). On the basis of climatic data from 1965 to 1994 and the Rivas-Martinez bioclimatic indices, the whole sampling area may be classified within the thermo-Mediterranean inferior bioclimatic belt, with upper dry ombrotype [16]. Agriculture 2019, 9, 192 3 of 14 Agriculture 2019, 9, x FOR PEER REVIEW 3 of 15 Figure 1. Climatic ombrothermic diagram at the experimental field site (average data from 1965 to Figure 1. Climatic ombrothermic diagram at the experimental field site (average data from 1965 to 1994). 1994). 2.2. Experimental Design and Management Practices 2.2. Experimental Design and Management Practices The experiments included three treatments: Organic cultivation system (OCS), conventional The experiments included three treatments: Organic cultivation system (OCS), conventional cultivation system (CCS) and uncultivated plots (UCP), as a control. Each treatment, which included cultivation system (CCS) and uncultivated plots (UCP), as a control. Each treatment, which included three replicate plots of 100 m (4 25 m), were separated by 25 m-wide rows of Trifolium subterraneum three replicate plots of 100 m (4 × 25 m), were separated by 25 m-wide rows of Trifolium subterraneum L. cover cropping. The plots used for OCS and UCP were fully converted three years before our L. cover cropping. The plots used for OCS and UCP were fully converted three years before our experimental trials by a period of crop rotation involving potato, globe artichoke and faba bean. experimental trials by a period of crop rotation involving potato, globe artichoke and faba bean. The The same crop rotation was adopted in the CCS. same crop rotation was adopted in the CCS. For both OCS and CCS, in November 2012, seeds of Vicia faba L. subsp. major cv. Aguadulce, an For both OCS and CCS, in November 2012, seeds of Vicia faba L. subsp. major cv. Aguadulce, an early-maturing Spanish commercial genotype used for both fresh and dry seeds production, were early-maturing Spanish commercial genotype used for both fresh and dry seeds production, were hand sown at a depth of 4 cm; inter-row spacing was 75 cm, and the inter-plant spacing within the row hand sown at a depth of 4 cm; inter-row spacing was 75 cm, and the inter-plant spacing within the was 33 cm. Since faba bean had been previously grown in our experimental field, no supplemental row was 33 cm. Since faba bean had been previously grown in our experimental field, no inoculation of Rhizobium spp. was given in the soil. Prior to sowing, CCS was managed according to supplemental inoculation of Rhizobium spp. was given in the soil. Prior to sowing, CCS was managed the local farming practices: tillage consisted of a 25 cm (10”) deep moldboard ploughing, followed by according to the local farming practices: tillage consisted of a 25 cm (10”) deep moldboard ploughing, tine harrowing during the late summer; fertilization with 50 kg ha of P O (as superphosphate) and 2 5 −1 followed by tine harrowing during the late summer; fertilization with 50 kg ha of P2O5 (as 20 kg ha of N (as ammonium nitrate); weeds were controlled by hand when necessary. For OCS, −1 superphosphate) and 20 kg ha of N (as ammonium nitrate); weeds were controlled by hand when in which synthetic herbicides are banned, soil was ploughed to a 10 cm (4”) depth during the late spring. necessary. For OCS, in which synthetic herbicides are banned, soil was ploughed to a 10 cm (4”) depth The crop management (i.e., crop rotation, fertilization and protection of plants) in the OCS followed during the late spring. The crop management (i.e., crop rotation, fertilization and protection of plants) current European Union (EU) regulations (Regulation CE 834/2007, 889/2008, 967/2008, 1235/2008, in the OCS followed current European Union (EU) regulations (Regulation CE 834/2007, 889/2008, 1254/2008 and their modifications). UCP had no cultivation. 967/2008, 1235/2008, 1254/2008 and their modifications). UCP had no cultivation. 2.3. Seedbank Sampling and Analysis 2.3. Seedbank Sampling and Analysis InIn order to as order to assess sess the the sp spatial atial arrangement arrangement of weed of weed se seeds eds in the in thesoi soil, l, in al in all l thr thr ee tre ee tr aeatments, tments, soil soil samples sample wer s were col e collected lected ev every ery mont month h (fr(om from November November to to September) September) at at thr th ee ree di di er fferent ent depths depths ( (0–5 0–5 cm, cm, 5–10 cm and 10–15 cm) using a 4 cm diameter steel probe. Along the diagonals of each 100 m 5–10 cm and 10–15 cm) using a 4 cm diameter steel probe. Along the diagonals of each 100 m sampling sampling area (25 m length × 4 m width), a total of 990 soil cores (10 cores × 3 soil depth layers × 11 area (25 m length 4 m width), a total of 990 soil cores (10 cores 3 soil depth layers 11 months 3 months × 3 cultivation systems) were collected. The distance between sampling sites was 2.5 m. Soil cultivation systems) were collected. The distance between sampling sites was 2.5 m. Soil samples were samples were stored for a few days at 4 °C in the dark, to prevent seed germination prior to stored for a few days at 4 C in the dark, to prevent seed germination prior to identification. identification. Soil samples from each of the three soil layer depths were well-mixed and the inert fraction (stones, pebbles, etc.) was manually removed. For the direct seed extraction, seeds were separated from the soil by washing the sample with water. In order to have a better representation of the entire soil core, four Agriculture 2019, 9, 192 4 of 14 (150 g) replicates for each soil layer were previously pre-treated with 5 g of sodium hexametaphosphate solution for 20 min to disperse the colloid matrix. To evaluate Orobanche seeds, because of their small size and specific weight, only the suspension was filtered through a series of di erent mesh sieves (from 5 to 300 m) and then, after this sieving/floatation technique as described by Benvenuti and Macchia [11], they were observed by a MS5 stereo-microscope (Leica Microsystems, Wetzlar, Germany). The remaining solutions of each of the sub-samples were placed into a metal tube with a removable cap comprising a 250 m steel mesh and then were washed through an electric adjustable pressure (20–120 bar max) washer (Kärcher, K 3500 model, Winnenden, Germany). The extracted material was transferred to Petri dishes and then air-dried for the separation, identification and counting of the weed seeds with a stereo-microscope. The extracted seeds were submitted to the “seed crushing test” through slight finger pressuring for the assessment of viable seeds [17]. The values of viable seeds in each subsample were expressed as the number of seeds m of surface area for each soil depth horizon [18]. The potential flora data were grouped as function of the botanical family, and each species was assigned to a life-form category considering the Raunkiaer system [19]. Then, with the aim of evaluating the e ects of the cultivation system and soil layer on weed dynamics, we grouped these species into main botanical families (i.e., Portulacaceae, Amaranthaceae, Asteraceae, Primulaceae, Scrophulariaceae) and included the remaining ones into the ‘others’ group. The Orobanchaceae family was considered separately because of the heavy infestation of faba bean fields by Orobanche crenata Forsk [20]. 2.4. Weed Flora Analysis For the study of the real weed flora, in each plot (OCS, CCS and UCP) 16 permanent 0.5 m squares were randomly marked and monthly surveyed according to the scale proposed by Braun-Blanquet [21]. The reported technique has been widely applied in studies on agricultural systems and arable fields [22]. Such scale provides the following classes of cover: 5 = 75–100%; 4 = 50–75%; 3 = 25–50%; 2 = 5–25%; 1 = numerous individuals, but less than 5%, or scattered, with cover up to 5%; + = few individuals, with small cover; r = rare, solitary, with small cover. The study allows the calculation of the number of plants rooted within each quadrat, as a measurement of the abundance or density of a single species. Each species was identified using di erent identification keys [23,24], and the nomenclature was given to each of them [25]. For all of the surveys, the normalized frequency of occurrence for each species was determined with the aim to assign each species to one of the five presence classes (I: 0–20%; II: 20–40%; III: 40–60%; IV: 60–80%; V: 80–100%). Frequency, expressed as the proportion of total quadrats containing at least one rooted individual of a given species, was also used to determine the biological spectrum of the whole weed community analyzed. 2.5. Statistical Analysis Levene’s test was used to test homoscedasticity (or homogeneity of variance); data were then subjected to a three-way (cultivation system  soil layer  botanical family) analysis of variance (ANOVA). Means were separated by the least significance di erence (LSD) test when the F-test was significant. Percent values were transformed to arcsin x (Bliss transformation) prior to being statistically analyzed; untransformed data were reported and discussed. All calculations and ANOVA were performed using CoStat version 6.003 (CoHort Software, Monterey, CA, USA). Agriculture 2019, 9, 192 5 of 14 3. Results 3.1. Weed Species Richness The relative density (expressed as percentage of total seeds m ) of each weed species in relation to the cultivation system and soil layer is reported in Table 1. In total, 32 taxa belonging to 18 botanical families were recorded in the three soil layer depths; 78% of which were therophytes, while the remainder were hemi-cryptophytes. No significant di erences in terms of weed seedbank composition at di erent soil layer depths were found among the evaluated cultivation systems. Indeed, the data about weed seedbank showed that one species (Lolium multiflorum Lam.) was unique to the OCS, one (Veronica polita Fr.) was exclusively found in UCP and the others were present in all the cultivation systems (Table 1). In particular, Portulaca oleracea L. was the most dominant species in all the soil layer depths and cultivation systems with mean values ranging from 1883 to 4967 seeds m , followed by Amaranthus retroflexus L. with values ranging from 1583 to 2767 seed m . Both weed species mainly contributed to the total seed m under CCS than under OCS and UCP (Table 1). For A. retroflexus, it is interesting to note that the number of seeds m increased under both CCS and OCS, going from the superficial to the deeper soil layer, while the opposite trend was observed under UCP. Other weed species, i.e., Anagallis arvensis L., Fumaria ocinalis L., Helminthotheca echioides (L.) Holub and Sonchus asper (L.) Hill were also widespread in all of the cultivation systems and soil layer depths, but showing mean values of density below 900 seeds m (data not shown). Among the sporadic weeds, Fallopia convolvulus (L.) À. Love, Papaver rhoeas L. and Verbena ocinalis L. were found in all the cultivation systems under study, despite the fact that they were absent in specific soil layer depths; while e.g., L. multiflorum and Sonchus oleraceus L. were recorded only in a few plots (Table 1). 3.2. Weed Seedbank Density and Distribution for the Main Botanical Families ANOVA results indicated that, regardless of soil layer depth and botanical family, the total weed seedbank density was significantly higher in CCS (31,650 seeds m ) than in OCS and UCP (26,200 and 23,250 seeds m , respectively) (data not shown). However, both weed seedbank density and distribution were a ected by the ‘botanical family x cultivation system’ (Figure 2) and ‘botanical family x soil layer depth’ (Figure 3) interactions. Agriculture 2019, 9, 192 6 of 14 Table 1. Weed seedbank species composition and distribution (% of total seeds) in relation to cultivation system and soil layer depth. Cultivation System CCS OCS UCP Biological Group Botanical Family Soil Layer Depth (cm) Soil Layer Depth (cm) Soil Layer Depth (cm) Species 0–5 5–10 10–15 0–5 5–10 10–15 0–5 5–10 10–15 T Amaranthaceae Amaranthus retroflexus L. 32.3 2.9 27.4 1.3 17.1 1.2 27.8 2.5 18.2 1.4 15.7 2.4 16.8 1.3 21.6 1.2 23.5 1.1 T Primulaceae Anagallis arvensis L. 5.2 0.7 4.6 0.7 - 6.8 1.0 6.1 0.8 4.1 0.5 7.8 0.9 9.9 0.9 15.4 1.0 H Chenopodiaceae Beta vulgaris L. - 2.3 0.4 - - - - 1.2 0.2 0.6 0.1 0.7 0.1 T Chenopodiaceae Chenopodium album L. 1.3 0.2 0.9 0.1 - 0.5 0.1 - - - - - T Asteraceae Conyza spp. 0.9 0.1 0.5 0.1 0.5 0.1 1.5 0.3 - - - - - H Asteraceae Crepis vescicaria L. 0.9 0.2 0.5 0.1 - 0.5 0.1 1.7 0.3 1.7 0.3 1.2 0.2 - - T Euphorbiaceae Euphorbia helioscopia L. 0.9 0.1 - 0.5 0.1 - - - 1.8 0.4 1.9 0.2 2.9 0.5 T Polygonaceae 0.9 0.1 0.9 0.1 - - 0.5 0.1 - 2.4 0.3 0.6 0.1 1.5 0.4 Fallopia convolvulus (L.) Á. Löve T Papaveraceae Fumaria ocinalis L. 0.9 0.1 1.4 0.2 1.1 0.3 1.0 0.2 3.5 0.6 4.1 0.3 4.8 0.3 3.1 0.5 2.9 0.2 T Papaveraceae Fumaria parviflora Lam. - 1.4 0.3 1.1 0.4 - - - 6.0 0.4 2.5 0.4 7.4 0.6 H Asteraceae Galactites elegans (All.) Soldano 1.3 0.2 - 0.5 0.1 - - - 1.2 0.2 3.1 0.2 0.7 0.1 T Boraginaceae Heliotropum europeum L. 0.4 0.1 - 0.5 0.1 - - 0.8 0.1 - - - T Asteraceae Helminthotheca echioides (L.) Holub 1.3 0.5 1.4 0.2 1.6 0.5 1.5 0.3 5.6 0.5 3.3 0.4 0.6 0.1 4.3 0.3 2.2 0.4 T Lamiaceae Lamium amplexicaule L. - - - - 2.0 0.1 0.8 0.1 0.6 0.1 3.1 0.4 1.5 0.1 T Malvaceae Lavatera trimestris L. 0.4 0.1 0.9 0.2 - - 0.5 0.1 - - - - T Poaceae Lolium multiflorum Lam. - - - - 0.5 0.1 - - - - T Fabaceae Lotus ornithopodioides L. 0.4 0.1 - - - - - - 0.6 0.1 - H Malvaceae Malva rotundifolia L. - - 0.5 0.1 - - - - - 0.7 0.1 H Orobanchaceae Orobanche crenata Forsk. 3.5 0.3 4.1 0.6 3.7 0.6 4.9 0.8 4.5 0.3 4.1 0.4 - - - T Papaveraceae Papaver rhoeas L. 0.9 0.2 - 1.1 0.5 1.0 0.2 - - 1.2 0.1 4.9 0.4 - T Poaceae Polypogon monspeliensis (L.) Desf. - - 0.5 0.1 0.5 0.1 - - - - - T Portulacaceae Portulaca oleracea L. 43.7 1.5 45.2 1.5 52.9 2.8 46.3 5.2 43.9 1.7 51.2 2.7 28.7 2.8 24.1 1.1 19.1 1.6 H Asteraceae Reichardia picroides (L.) Roth - 0.5 0.1 0.5 0.1 - 0.5 0.1 - - - - T Rubiaceae Galium aparine L. - 0.5 0.1 - - 0.5 0.1 - - - - T Solanaceae Solanum nigrum L. - - 0.5 0.1 - - - 3.0 0.5 1.9 0.3 2.2 0.3 T Asteraceae Sonchus asper (L.) Hill 3.5 0.9 4.6 0.4 6.4 0.8 2.4 0.5 8.1 0.8 8.3 0.9 13.8 1.0 13.0 0.9 18.4 1.1 T Asteraceae Sonchus oleraceus L. - - 3.2 0.7 - - - - 4.3 0.4 - H Verbenaceae Verbena ocinalis L. - - 1.1 0.3 0.5 0.1 1.0 0.1 - 0.6 0.1 - - T Scrophulariaceae Veronica cymbalaria Bodard - - 1.1 0.3 - - - 5.4 1.0 - 0.7 0.1 T Scrophulariaceae Veronica hederifolia L. 1.3 0.4 3.2 0.5 3.7 0.3 3.9 0.3 4.0 0.7 4.1 0.4 - - - T Scrophulariaceae Veronica persica Poiret - 1.6 0.3 1.0 0.2 - 1.7 0.2 - - - T Scrophulariaceae Veronica polita Fr. - - - - - - 3.0 0.4 0.6 0.1 - 1 2 H = hemicryptophytes; T = terophytes; CCS = conventional cultivation system; OCS = organic cultivation system; UCP = uncultivated plots. Agriculture 2019, 9, x FOR PEER REVIEW 7 of 15 3.2. Weed Seedbank Density and Distribution for the Main Botanical Families ANOVA results indicated that, regardless of soil layer depth and botanical family, the total weed −2 seedbank density was significantly higher in CCS (31,650 seeds m ) than in OCS and UCP (26,200 −2 and 23,250 seeds m , respectively) (data not shown). However, both weed seedbank density and Agricultur distribueti2019 on ,were a 9, 192 ffected by the ‘botanical family x cultivation system’ (Figure 2) and ‘botan 7 of ical 14 family x soil layer depth’ (Figure 3) interactions. Figure 2. Total weed seedbank distribution (%) and density (values in brackets and expressed as Figure 2. Total weed seedbank distribution (%) and density (values in brackets and expressed −2 2 number of seeds m ) as affected by the ‘botanical family x cultivation system’ interaction. CCS: as number of seeds m ) as a ected by the ‘botanical family x cultivation system’ interaction. Conventional Cultivation System; OCS: Organic Cultivation System; UCP: Uncultivated Plot. CCS: Conventional Cultivation System; OCS: Organic Cultivation System; UCP: Uncultivated Plot. LSDinteraction (P < 0.05) = 460. LSD (P 0.05) = 460. interaction Agriculture 2019, 9, 192 8 of 14 Agriculture 2019, 9, x FOR PEER REVIEW 8 of 15 −2 Figure 3. Total weed seedbank density (seed m ) as affected by the ‘botanical family × soil layer depth’ Figure 3. Total weed seedbank density (seed m ) as a ected by the ‘botanical family  soil layer interaction. depth’ interaction. In both CCS and OCS Portulacaceae represented the main family accounting for, respectively, In both CCS and OCS Portulacaceae represented the main family accounting for, respectively, −2 47% 47% and and 26% 26% of the of the total total seed seedss m m , f , followed ollowed b by y Amara Amaranthaceae nthaceae ((r respect espectively ively, 26 , 26% % a and nd 21 21%) %) and and Asteraceae (respectively, 9% and 11%) (Figure 2). Conversely, in UCP such botanical families were Asteraceae (respectively, 9% and 11%) (Figure 2). Conversely, in UCP such botanical families were −2 uniformly uniformly represented represented (24–20 (24–20 and and 21 21%). %). In a In addition, ddition, b both oth Por Portulacaceae tulacaceae ( (4967 4,967vs. vs. 4067 4,067 seeds seeds m m ) ) and and 2 2 −2 −2 Amaranthaceae (2767 vs. 1867 seeds m ) achieved a higher total number of seed m in CCS than in Amaranthaceae (2,767 vs. 1,867 seeds m ) achieved a higher total number of seed m in CCS than in OCS. OCS. Primulaceae Primulaceaewer wee re mo morere present in present in UCP UCP than than in the in the twotwo cultivated cultivate plots d pl (Figur ots (F eig 2). ure The 2).same The s tra end me was observed for Papaveraceae, which contributed more to the ‘others’ group than the other botanical trend was observed for Papaveraceae, which contributed more to the ‘others’ group than the other families botanical fa (i.e.,mili Chenopodiaceae es (i.e., Chen , op Euphorbiaceae odiaceae, Euphorbiace , Fabaceae a,e, Lamiaceae Fabaceae, , Malvaceae Lamiaceae, , Polygonaceae Malvaceae, Polygonacea , Solanaceae e,, Verbenaceae) (data not shown). Solanaceae, Verbenaceae) (data not shown). The The weed weed seedbank seedbank density density wa was s significantly significantly a affec ected ted by by the the interaction interaction betwe between en the the bot botanical anical family and the soil layer depth (Figure 3). In each soil layer depth, Portulacaceae was the predominant family and the soil layer depth (Figure 3). In each soil layer depth, Portulacaceae was the predominant botanical botanical fa family mily followed followed by by Amaranthaceae Amaranthaceae and and Asteraceae Asteraceae.. G Going oing f fr rom om the the sup superficial erficial ( (0–5 0–5 cm cm) ) to to the the deep (10–15 cm) soil layer, the amount of seeds per square meter was respectively reduced by 23.0% deep (10–15 cm) soil layer, the amount of seeds per square meter was respectively reduced by 23.0% and and 47 47.8% .8% for for Portulaca Portulacaceae ceae and and Amara Amaranthaceae nthaceae,, re respectively spectively. In . In contr contrast, ast, the the w weed eed seed seed d density ensity of of th the e other botanical families did not significantly di er among the three soil layers. other botanical families did not significantly differ among the three soil layers. 3.3. Real Weed Flora 3.3. Real Weed Flora The results demonstrated that the real flora, in terms of cover-abundance, is quite similar between The results demonstrated that the real flora, in terms of cover-abundance, is quite similar the two cultivated plots, i.e., OCS and CCS (Figure 4). between the two cultivated plots, i.e. OCS and CCS (Figure 4). In both CCS and OCS, Brassicaceae highlighted the highest values, followed by Papaveraceae, In both CCS and OCS, Brassicaceae highlighted the highest values, followed by Papaveraceae, Euphorbiaceae, Asteraceae and Malvaceae. In UCP, together with Brassicaceae and Papaveraceae, the highest Euphorbiaceae, Asteraceae and Malvaceae. In UCP, together with Brassicaceae and Papaveraceae, the cover-abundance values were reported for Poaceae, Lamiaceae and Rubiaceae. Asteraceae and Euphorbiaceae highest cover-abundance values were reported for Poaceae, Lamiaceae and Rubiaceae. Asteraceae and showed significant di erences among the two cultivated plots and UCP (23% vs. 4%), as also reported Euphorbiaceae showed significant differences among the two cultivated plots and UCP (23% vs. 4%), for Fabaceae (15% vs. 4%). UCP showed a significantly higher percentage of cover-abundance for as also reported for Fabaceae (15% vs. 4%). UCP showed a significantly higher percentage of cover- Papaveraceae, Poaceae, Lamiaceae, Rubiaceae and Amaranthaceae than OCS and CCS. abundance for Papaveraceae, Poaceae, Lamiaceae, Rubiaceae and Amaranthaceae than OCS and CCS. Considering the normalized frequency of occurrence for each weed (Table 2), the component of species in class I was rather widespread, while only Galactites elegans (All.) Soldano, Malva rotundifolia L., Medicago polymorpha L. and P. rhoeas reached class V in all the evaluated cultivation systems. Other species such as F. convolvulus, H. echioides, S. asper and S. oleraceus were present at higher concentrations Agriculture 2019, 9, 192 9 of 14 (classes IV and V) in all the three cultivation systems. By contrast, Avena sterilis L. and Lolium rigidum Gaud. exclusively appeared in the cultivated plots. Solanum nigrum L. and Veronica cymbalaria Bodard greatly reduced their presence in UCP, passing from class V (in the cultivated plots) to classes II and I, Agriculture 2019, 9, x FOR PEER REVIEW 9 of 15 respectively. In UCP, on the contrary, Lamium amplexicaule L. is widespread (class V), but almost absent in the two cultivated plots (for both class I). Figure 4. Percentage of weed cover-abundance as affected by the ‘botanical family × cultivation Figure 4. Percentage of weed cover-abundance as a ected by the ‘botanical family  cultivation system’ interaction. system’ interaction. Considering the normalized frequency of occurrence for each weed (Table 2), the component of Table 2. Normalized frequency of occurrence [as expressed as presence classes (I: 0–20%; II: 20–40%; III: species in class I was rather widespread, while only Galactites elegans (All.) Soldano, Malva rotundifolia 40–60%; IV: 60–80%; V: 80–100%)] for each investigated species in relation to the cultivation system L., Medicago polymorpha L. and P. rhoeas reached class V in all the evaluated cultivation systems. Other (CCS: Conventional Cultivation System; OCS: Organic Cultivation System; UCP: Uncultivated Plot). species such as F. convolvulus, H. echioides, S. asper and S. oleraceus were present at higher concentrations (classes IV and V) in all the three cultivation systems. By con Cultivation trast, System Avena sterilis L. and Lolium rigidum Gaud. exclusively appeared in the cultivated plots. Solanum nigrum L. and Veronica Species CCS OCS UCP cymbalaria Bodard greatly reduced their presence in UCP, passing from class V (in the cultivated Amaranthus blitoides S. Watson I plots) to classes II and I, respectively. In UCP, on the contrary, Lamium amplexicaule L. is widespread Amaranthus retroflexus L. III III II (class V), but almost absent in the two cultivated plots (for both class I). Anagallis arvensis L. IV IV IV Avena sterilis L. V IV Capsella bursa-pastoris (L.) Medik. I Table 2. Normalized frequency of occurrence [as expressed as presence classes (I: 0–20%; II: 20–40%; Chenopodium album L. I I I III: 40–60%; IV: 60–80%; V: 80–100%)] for each investigated species in relation to the cultivation system Chenopodium murale L. I (CCS: Conventional Cultivation System; OCS: Organic Cultivation System; UCP: Uncultivated Plot). Cynodon dactylon (L.) Pers. II Daucus carota L. III Cultivation System Digitaria sanguinalis (L.) Scop. I Species CCS OCS UCP Diplotaxis erucoides (L.) DC. I II Amaranthus blitoides S. Watson I Amaranthus retroflexus L. III III II Anagallis arvensis L. IV IV IV Avena sterilis L. V IV Capsella bursa-pastoris (L.) Medik. I Chenopodium album L. I I I Chenopodium murale L. I Cynodon dactylon (L.) Pers. II Daucus carota L. III Digitaria sanguinalis (L.) Scop. I Diplotaxis erucoides (L.) DC. I II Agriculture 2019, 9, 192 10 of 14 Table 2. Cont. Cultivation System Species CCS OCS UCP Euphorbia chamaesyce L. I Euphorbia helioscopia L. I I II Euphorbia sp. I I I IV V V Fallopia convolvulus (L.) Á. Löve Fumaria ocinalis L. I II I Fumaria parviflora Lam. I I I Galactites elegans (All.) Soldano V V V Galium aparine L. III IV II Glebionis coronaria (L.) Cass. ex Spach I II I Helminthotheca echioides (L.) Holub IV V V Lamium amplexicaule L. I I V Lavatera trimestris L. I I Lolium rigidum Gaud. V IV Malva rotundifolia L. V V V Medicago arabica (L.) Huds. I I Medicago polymorpha L. V V V Orobanche crenata Forsk. IV IV . Papaver hybridum L. III II I Papaver rhoeas L. V V V Phalaris sp. I Polycarpon tetraphyllum (L.) L. I Portulaca oleracea L. II II II Raphanus raphanistrum ssp. landra (Moretti) Bonnier IV IV IV Reichardia picroides (L.) Roth I Sinapis arvensis L. I II Sisymbrium ocinale (L.) Scop. II Solanum nigrum L. IV V II Sonchus asper (L.) Hill IV V V Sonchus oleraceus L. V IV V Sonchus tenerrimus L. III II I Torilis nodosa (L.) Gaertn. I Trifolium subterraneum L. I I Veronica cymbalaria Bodard V III I Veronica polita Fr. II I I 4. Discussion A total of 32 species were detected with a prevalence of therophytes (>78%) over hemy-cryptophytes, due to the short life cycle which enables therophytes to resist the high degree of anthropogenic disturbance caused by agricultural techniques [26]. Several studies [27–29] demonstrated that di erent cultivation systems can determine changes in the richness of weed species. In our study, significant di erences in the total weed seedbank density were observed according to the following order: CCS > OCS > UCP. Indeed, the way in which each weed species may expand its habit range into agricultural environments, remaining as an invader, depends upon complex interactions [10]. Here, the high abundance of weed seeds m in CCS was related to the high density of Portulacaceae and Amaranthaceae, with both botanical families represented by only a species (P. oleracea and A. retroflexus, respectively). In addition, a higher weed competition in UCP determined the decrease of the total weed seedbank density in the superficial soil layer depth for P. oleracea and through the first 10 cm of soil layer for A. retroflexus. With respect to these two species, our results are in good agreement with some previous studies which showed that they have long been problematic in the Mediterranean Basin. Their presence under both organic and conventional cultivation systems [5,13,17] may be explained by the high number of small seeds produced per plant, a good adaptation to the highly disturbed and moist conditions of irrigated field crops [30], as well as by their need for adequate moisture levels Agriculture 2019, 9, 192 11 of 14 and good seed–soil contact to absorb moisture and germinate. Such biotic and abiotic factors may contribute to their higher seedbank density in all the cultivation systems under study. In particular, the high light intensity and high temperatures, typically experienced in the Mediterranean Basin [31] and usually found in tilled soils [32], can support the germination and establishment of P. oleracea in CCS, while A. retroflexus problems are common also in no-tillage systems with conventional herbicides, in which seeds are arranged on the soil surface and selected within herbicide-resistant populations [33]. However, high populations of such prolific species can be observed in both organic and conventional systems, in tilled and no-till systems. Both A. retroflexus and P. oleracea have specific characteristics that make them very invasive. As an instance, a single plant of A. retroflexus, which emerges, grows, flowers and forms mature seeds from late spring to autumn, can produce 100,000–600,000 seeds (1–1.2 mm in diameter), characterized by multiple dormancy mechanisms [34], depending on the environmental conditions experienced by the mother plant during seed maturation [35], allowing them to be persistent in the soil for long time periods. Because of their dimension, only seeds located within the first 10 cm of soil are able to germinate; while those buried in deeper soil layers become dormant and maintain their ability to geminate for several years when brought to the surface by minimum tillage or cultivation. P. oleracea, an r-strategist, can produce more than 10,000 seeds per plant [36], which are readily dispersed over short and long distances, and have a percentage of germination higher than 90% in 24 h under suitable conditions [36,37]. Their ability to produce a large number of small (0.5 mm in diameter) seeds in di erent environments and an excellent seed survival rate allow this weed species to enrich the soil seedbank in short time [38]. Based on our results, it is also worth mentioning that Amaranthaceae maintained a similar number of seeds m in OCS and UCP. In contrast, CCS and OCS achieved a reduction of Primulaceae and Asteraceae seeds m ; in addition both cultivated systems were characterized by a longer-term persistent seedbank [39] if compared to UCP. Probably the ecological conditions in undisturbed soils (absence of agronomic treatments), which may determine a limited soil aeration, inhibited seed germination, determining an accumulation of seeds in the soil [11]. The results on real weed flora composition and distribution showed that 45 species, belonging to 19 di erent botanical families, were present in the studied agroecosystem. In particular, both cultivated plots (OCS and CCS) showed a higher number of weed species belonging to class V than UCP. Indeed, the agricultural practices (sowing time, weed management control, fertilization, etc.) modify the physical, chemical and biological conditions of the soil, which may have influenced the emergence, growth and ability of some species to produce seeds, and thus their relative cover-abundance [1]. Moreover, as reported by many researchers [40,41], several ecological factors (i.e., predation and decay) may determine a decrease of the number of weed seeds in the soil, as well as a high percentage of weed seeds that do not germinate probably because of their dormant state. Thus, a discrepancy between the soil seedbank and the standing vegetation is possible. Some species, such as G. elegans, H. echioides, M. rotundifolia, M. polymorpha, P. rhoeas, S. asper and S. oleraceus, represent the most dangerous infesting weeds for cultivated crops. Our results showed that some dominant weed species identified through the floristic analysis, i.e., A. sterilis, L. rigidum and M. polymorpha, belonging to the class of presence V, were underrepresented or even completely absent in the soil seedbank. Conversely, some plant species [i.e., Beta vulgaris L., Conyza spp., Crepis vescicaria L. Heliotropum europium L., Lotus ornithopodioides L. and Polypogon monspeliensis (L.) Desf.] present in the soil seedbank were poorly represented or even absent in the established weed flora. Also A. retroflexus and P. oleraceae, the dominant species in the soil seedbank, were poorly represented (presence class II) in the floristic cortege. Seed size significantly influences the survival of seedlings and can dramatically change the e ect of the soil seedbank on the community composition. Moles and Westoby [42] reported that small-seeded plants were characterized by lower survival as seedlings, smaller canopies and shorter reproductive lifespans. Agriculture 2019, 9, 192 12 of 14 In addition, some soil factors, such as moisture, organic content and seed density, may decrease seed survival due to a higher fungal activity [43] and thus would probably increase the mortality of small seeds over large seeds due to the di erence in protection and nutrient reserves [42]. Amaranthus blitoides S. Watson, Chenopodium murale L., Daucus carota L., Sinapis arvensis L., Sisymbrium ocinale (L.) Scop., Torilis nodosa (L.) Gaertn. and T. subterraneum are more sensitive to the presence of crops and, for this reason, they were present only in the uncultivated plots. Conversely, A. sterilis, Capsella bursa-pastoris (L.) Medik., Digitaria sanguinalis (L.) Scop., Euphorbia chamaesyce L., Galium aparine L., Glebionis coronaria (L.) Cass., L. rigidum, Medicago arabica (L.) Huds., M. polymorpha, Papaver hybridum L., Polycarpon tetraphyllum L., Raphanus raphanistrum ssp. landra (Moretti) Bonnier and Sonchus tenerrimus L. were the only species observed in the cultivated plots. Some of them were typical ruderal helio-subnitrophilous therophytic weeds. 5. Conclusions Overall, our study provided useful information about the e ects of cultivation systems on the weed population dynamics in a Mediterranean environment. Results indicated that cultivated (organic and conventional) systems determine the abundance of weed species in the soil seedbank, although the di erences in composition are rather small. From a practical point of view, the qualitative and quantitative knowledge of the soil weed flora, as well as the spatial arrangement of weed seeds through the soil layer depths, can help in predicting the dynamics of weed emergence and interference with the cultivated crop. In particular, bio-ecological information on the behavior of weed dynamics are fundamental to develop sustainable weed control protocols, especially under organic farming. Thus, further research should be carried out to evaluate the e ects of eco-friendly weed management tools (such as soil solarization, false sowings and direct seeding) on seeds’ viability and germination. Author Contributions: All the authors equally contributed to the conceptualization, methodology and investigation, as well as to the data curation and writing—original draft preparation. Funding: This research received no external funding. Conflicts of Interest: The authors declare no conflict of interest. References 1. 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Weed seed movement and dispersal strategies in the agricultural environment. Weed Biol. Manag. 2007, 7, 141–157. [CrossRef] 42. Moles, A.T.; Westoby, M. Seed size and plant strategy across the whole life cycle. Oikos 2006, 113, 91–105. [CrossRef] 43. Pakeman, R.J.; Small, J.L.; Torvell, L. Edaphic factors influence the longevity of seeds in the soil. Plant Ecol. 2012, 213, 57–65. [CrossRef] © 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Agriculture Multidisciplinary Digital Publishing Institute

Impact of a Cultivation System upon the Weed Seedbank Size and Composition in a Mediterranean Environment

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agriculture Article Impact of a Cultivation System upon the Weed Seedbank Size and Composition in a Mediterranean Environment Alessia Restuccia , Sara Lombardo * and Giovanni Mauromicale Dipartimento di Agricoltura, Alimentazione e Ambiente (Di3A), University of Catania, via Valdisavoia 5, 95123 Catania, Italy * Correspondence: saralomb@unict.it; Tel.: +39-0954783421 Received: 16 July 2019; Accepted: 2 September 2019; Published: 5 September 2019 Abstract: The knowledge of the soil seedbank is crucial to predict the dynamics of weed communities and potential future problems in agroecosystem weed management. Therefore, the aim of this study was to evaluate the qualitative and quantitative variation of the potential and real weed flora as a function of di erent cultivation systems (namely organic, conventional and uncultivated) in a Mediterranean environment (Sicily, south Italy). The results proved that soil seedbank density was significantly di erent in superficial (0–10 cm) and deeper soil layers (10–15 cm) in both organic and conventional cultivation systems. Portulacaceae and Amaranthaceae were the dominant botanical families, although they achieved a higher total number of seeds m under a conventional cultivation system than under organic and uncultivated ones. The whole weed flora was represented by 45 taxa, but the presence of the crop reduced the qualitative and quantitative composition of real weed flora. In conclusion, the knowledge of the seedbank size and composition, as well as the variation in time and space of real flora, may contribute to predict the dynamics of weed emergence and their possible interference with crops. In particular, information on the weed dynamics is essential to develop sustainable control protocols, especially under organic farming. Keywords: seedbank; botanical family; cultivation system; soil layer depth; real weed flora 1. Introduction Despite the significant advances in weed management technologies reached in the latest years, weed flora remains one of the most detrimental factors a ecting crops’ performance, as they are able to cause significant yield and quality losses [1]. While in developing countries, manual or mechanical weed management practices are prevalent, in developed ones the use of herbicides is dominant. Weed management outcomes strictly depend upon infestation size and composition [1], so that it is often necessary to adopt an integrated control approach (based on a combination of mechanical, cultural, biological and chemical management tools) to achieve an acceptable level of weed suppression [2]. As already reported in literature, cultivation systems may influence weed flora dynamics [3], and particularly, there is a growing interest by growers in the limitation of chemical weed control. Therefore, these eco-friendly cultivation systems rely more on cultural (crop rotation diversification and cover cropping) and mechanical weed control strategies [2]. In particular, the composition and abundance of weed flora in crop fields is mainly a ected by crop rotation and soil tillage systems [4]. Crop rotation is an e ective method of controlling weeds, since this modifies the weed seedbank and above-ground weed communities. However, the choice and sequence of crops may significantly a ect weed community dynamics, because of their di erent biological cycle, end use, competition against weeds and crop management practices [1]. This should prevent the proliferation and dominance Agriculture 2019, 9, 192; doi:10.3390/agriculture9090192 www.mdpi.com/journal/agriculture Agriculture 2019, 9, 192 2 of 14 of certain weed species, by proving an unsuitable environment for them. Also tillage systems can influence the size and composition of a weed seedbank, since it may determine a vertical redistribution of seeds through the soil layers [5]. In addition, tillage modifies the soil environment, particularly its temperature, moisture, aeration and biological activity. For example, conservation tillage systems are frequently reported to increase weed infestations [5,6] and to rely upon higher herbicides use [7]. Therefore, there is a growing interest in the use of environmental sound strategies for weed suppression. In particular, the ecologically-based weed management practices (e.g., the use of cover crops, no-tillage or reduced tillage systems) are increasingly adopted in organic farming [2]. These nonchemical weed management tools may reduce the problems of herbicide resistance, environmental pollution, weed diversification and crop performance. However, the e ectiveness of such strategies is more variable than the chemical weed management practices. In particular, the size of the weed seedbank is critical to the success of this ecologically-based weed control [8], since it can have long-lasting e ects on crop yields; but it is not just the size of the seedbank, as seed germination may be impaired by allelopathic e ects due to the phytotoxic activity of allelochemicals, introduced through crop rotations or by extracts applications [9]. Keeping in view all of the above-mentioned aspects, the knowledge of potential flora composition is essential to predict the weed cover of future infestations, and to develop integrated strategies for the management and control of weed flora [4,10–12], respecting environmental and economic constraints. The distribution of the seed stocks depends, indeed, upon the response of the spontaneous plant species (in terms of germination, seedlings survival, di usion, etc.) to the di erent agricultural practices [13]. Although it is known that weeds’ infestation strongly reduces crop yields [13,14], no experimental data on the relationship between potential and real flora for organic agroecosystems, (in terms of weed seedbank and weed flora dynamics using a phytosociological approach), is reported. Therefore, the purpose of this study was to assess the qualitative and quantitative composition of the potential (seedbank) and real flora in a Mediterranean agroecosystem, as a ected by the cultivation system (conventional, organic and uncultivated). A thorough knowledge of the impact of agricultural management practices on weed flora is fundamental to simultaneously developing control protocols capable of reducing the use of herbicides and to select a weed community with little impact on the agroecosystems. 2. Materials and Methods 2.1. Experimental Site, Climate and Soil Field experiments were carried out in the coastal plain of Siracusa (Sicily—South Italy, 37 03 N, 15 18 E, 10 m a.s.l.) in a moderately deep (~0.7 m) calcixerollic xerochrepts soil [15] in the 2012 and 2013 seasons. The pre-experimental soil composition was: Clay 30%, silt 25%, sand 35%, organic matter 2.0%, pH 8.4 (slightly alkali), total nitrogen 0.18%, available phosphorus 78 ppm, exchangeable potassium 337 ppm. The local climate is semiarid-Mediterranean, with mild winters and hot, rainless summers. On average, the coolest month is January (11.2 C), while October (111 mm) and July (2 mm) are, respectively, the rainiest and the driest months. The mean temperature during the hottest month (August) is 25.1 C; the local mean annual temperature is 18 C (Figure 1). On the basis of climatic data from 1965 to 1994 and the Rivas-Martinez bioclimatic indices, the whole sampling area may be classified within the thermo-Mediterranean inferior bioclimatic belt, with upper dry ombrotype [16]. Agriculture 2019, 9, 192 3 of 14 Agriculture 2019, 9, x FOR PEER REVIEW 3 of 15 Figure 1. Climatic ombrothermic diagram at the experimental field site (average data from 1965 to Figure 1. Climatic ombrothermic diagram at the experimental field site (average data from 1965 to 1994). 1994). 2.2. Experimental Design and Management Practices 2.2. Experimental Design and Management Practices The experiments included three treatments: Organic cultivation system (OCS), conventional The experiments included three treatments: Organic cultivation system (OCS), conventional cultivation system (CCS) and uncultivated plots (UCP), as a control. Each treatment, which included cultivation system (CCS) and uncultivated plots (UCP), as a control. Each treatment, which included three replicate plots of 100 m (4 25 m), were separated by 25 m-wide rows of Trifolium subterraneum three replicate plots of 100 m (4 × 25 m), were separated by 25 m-wide rows of Trifolium subterraneum L. cover cropping. The plots used for OCS and UCP were fully converted three years before our L. cover cropping. The plots used for OCS and UCP were fully converted three years before our experimental trials by a period of crop rotation involving potato, globe artichoke and faba bean. experimental trials by a period of crop rotation involving potato, globe artichoke and faba bean. The The same crop rotation was adopted in the CCS. same crop rotation was adopted in the CCS. For both OCS and CCS, in November 2012, seeds of Vicia faba L. subsp. major cv. Aguadulce, an For both OCS and CCS, in November 2012, seeds of Vicia faba L. subsp. major cv. Aguadulce, an early-maturing Spanish commercial genotype used for both fresh and dry seeds production, were early-maturing Spanish commercial genotype used for both fresh and dry seeds production, were hand sown at a depth of 4 cm; inter-row spacing was 75 cm, and the inter-plant spacing within the row hand sown at a depth of 4 cm; inter-row spacing was 75 cm, and the inter-plant spacing within the was 33 cm. Since faba bean had been previously grown in our experimental field, no supplemental row was 33 cm. Since faba bean had been previously grown in our experimental field, no inoculation of Rhizobium spp. was given in the soil. Prior to sowing, CCS was managed according to supplemental inoculation of Rhizobium spp. was given in the soil. Prior to sowing, CCS was managed the local farming practices: tillage consisted of a 25 cm (10”) deep moldboard ploughing, followed by according to the local farming practices: tillage consisted of a 25 cm (10”) deep moldboard ploughing, tine harrowing during the late summer; fertilization with 50 kg ha of P O (as superphosphate) and 2 5 −1 followed by tine harrowing during the late summer; fertilization with 50 kg ha of P2O5 (as 20 kg ha of N (as ammonium nitrate); weeds were controlled by hand when necessary. For OCS, −1 superphosphate) and 20 kg ha of N (as ammonium nitrate); weeds were controlled by hand when in which synthetic herbicides are banned, soil was ploughed to a 10 cm (4”) depth during the late spring. necessary. For OCS, in which synthetic herbicides are banned, soil was ploughed to a 10 cm (4”) depth The crop management (i.e., crop rotation, fertilization and protection of plants) in the OCS followed during the late spring. The crop management (i.e., crop rotation, fertilization and protection of plants) current European Union (EU) regulations (Regulation CE 834/2007, 889/2008, 967/2008, 1235/2008, in the OCS followed current European Union (EU) regulations (Regulation CE 834/2007, 889/2008, 1254/2008 and their modifications). UCP had no cultivation. 967/2008, 1235/2008, 1254/2008 and their modifications). UCP had no cultivation. 2.3. Seedbank Sampling and Analysis 2.3. Seedbank Sampling and Analysis InIn order to as order to assess sess the the sp spatial atial arrangement arrangement of weed of weed se seeds eds in the in thesoi soil, l, in al in all l thr thr ee tre ee tr aeatments, tments, soil soil samples sample wer s were col e collected lected ev every ery mont month h (fr(om from November November to to September) September) at at thr th ee ree di di er fferent ent depths depths ( (0–5 0–5 cm, cm, 5–10 cm and 10–15 cm) using a 4 cm diameter steel probe. Along the diagonals of each 100 m 5–10 cm and 10–15 cm) using a 4 cm diameter steel probe. Along the diagonals of each 100 m sampling sampling area (25 m length × 4 m width), a total of 990 soil cores (10 cores × 3 soil depth layers × 11 area (25 m length 4 m width), a total of 990 soil cores (10 cores 3 soil depth layers 11 months 3 months × 3 cultivation systems) were collected. The distance between sampling sites was 2.5 m. Soil cultivation systems) were collected. The distance between sampling sites was 2.5 m. Soil samples were samples were stored for a few days at 4 °C in the dark, to prevent seed germination prior to stored for a few days at 4 C in the dark, to prevent seed germination prior to identification. identification. Soil samples from each of the three soil layer depths were well-mixed and the inert fraction (stones, pebbles, etc.) was manually removed. For the direct seed extraction, seeds were separated from the soil by washing the sample with water. In order to have a better representation of the entire soil core, four Agriculture 2019, 9, 192 4 of 14 (150 g) replicates for each soil layer were previously pre-treated with 5 g of sodium hexametaphosphate solution for 20 min to disperse the colloid matrix. To evaluate Orobanche seeds, because of their small size and specific weight, only the suspension was filtered through a series of di erent mesh sieves (from 5 to 300 m) and then, after this sieving/floatation technique as described by Benvenuti and Macchia [11], they were observed by a MS5 stereo-microscope (Leica Microsystems, Wetzlar, Germany). The remaining solutions of each of the sub-samples were placed into a metal tube with a removable cap comprising a 250 m steel mesh and then were washed through an electric adjustable pressure (20–120 bar max) washer (Kärcher, K 3500 model, Winnenden, Germany). The extracted material was transferred to Petri dishes and then air-dried for the separation, identification and counting of the weed seeds with a stereo-microscope. The extracted seeds were submitted to the “seed crushing test” through slight finger pressuring for the assessment of viable seeds [17]. The values of viable seeds in each subsample were expressed as the number of seeds m of surface area for each soil depth horizon [18]. The potential flora data were grouped as function of the botanical family, and each species was assigned to a life-form category considering the Raunkiaer system [19]. Then, with the aim of evaluating the e ects of the cultivation system and soil layer on weed dynamics, we grouped these species into main botanical families (i.e., Portulacaceae, Amaranthaceae, Asteraceae, Primulaceae, Scrophulariaceae) and included the remaining ones into the ‘others’ group. The Orobanchaceae family was considered separately because of the heavy infestation of faba bean fields by Orobanche crenata Forsk [20]. 2.4. Weed Flora Analysis For the study of the real weed flora, in each plot (OCS, CCS and UCP) 16 permanent 0.5 m squares were randomly marked and monthly surveyed according to the scale proposed by Braun-Blanquet [21]. The reported technique has been widely applied in studies on agricultural systems and arable fields [22]. Such scale provides the following classes of cover: 5 = 75–100%; 4 = 50–75%; 3 = 25–50%; 2 = 5–25%; 1 = numerous individuals, but less than 5%, or scattered, with cover up to 5%; + = few individuals, with small cover; r = rare, solitary, with small cover. The study allows the calculation of the number of plants rooted within each quadrat, as a measurement of the abundance or density of a single species. Each species was identified using di erent identification keys [23,24], and the nomenclature was given to each of them [25]. For all of the surveys, the normalized frequency of occurrence for each species was determined with the aim to assign each species to one of the five presence classes (I: 0–20%; II: 20–40%; III: 40–60%; IV: 60–80%; V: 80–100%). Frequency, expressed as the proportion of total quadrats containing at least one rooted individual of a given species, was also used to determine the biological spectrum of the whole weed community analyzed. 2.5. Statistical Analysis Levene’s test was used to test homoscedasticity (or homogeneity of variance); data were then subjected to a three-way (cultivation system  soil layer  botanical family) analysis of variance (ANOVA). Means were separated by the least significance di erence (LSD) test when the F-test was significant. Percent values were transformed to arcsin x (Bliss transformation) prior to being statistically analyzed; untransformed data were reported and discussed. All calculations and ANOVA were performed using CoStat version 6.003 (CoHort Software, Monterey, CA, USA). Agriculture 2019, 9, 192 5 of 14 3. Results 3.1. Weed Species Richness The relative density (expressed as percentage of total seeds m ) of each weed species in relation to the cultivation system and soil layer is reported in Table 1. In total, 32 taxa belonging to 18 botanical families were recorded in the three soil layer depths; 78% of which were therophytes, while the remainder were hemi-cryptophytes. No significant di erences in terms of weed seedbank composition at di erent soil layer depths were found among the evaluated cultivation systems. Indeed, the data about weed seedbank showed that one species (Lolium multiflorum Lam.) was unique to the OCS, one (Veronica polita Fr.) was exclusively found in UCP and the others were present in all the cultivation systems (Table 1). In particular, Portulaca oleracea L. was the most dominant species in all the soil layer depths and cultivation systems with mean values ranging from 1883 to 4967 seeds m , followed by Amaranthus retroflexus L. with values ranging from 1583 to 2767 seed m . Both weed species mainly contributed to the total seed m under CCS than under OCS and UCP (Table 1). For A. retroflexus, it is interesting to note that the number of seeds m increased under both CCS and OCS, going from the superficial to the deeper soil layer, while the opposite trend was observed under UCP. Other weed species, i.e., Anagallis arvensis L., Fumaria ocinalis L., Helminthotheca echioides (L.) Holub and Sonchus asper (L.) Hill were also widespread in all of the cultivation systems and soil layer depths, but showing mean values of density below 900 seeds m (data not shown). Among the sporadic weeds, Fallopia convolvulus (L.) À. Love, Papaver rhoeas L. and Verbena ocinalis L. were found in all the cultivation systems under study, despite the fact that they were absent in specific soil layer depths; while e.g., L. multiflorum and Sonchus oleraceus L. were recorded only in a few plots (Table 1). 3.2. Weed Seedbank Density and Distribution for the Main Botanical Families ANOVA results indicated that, regardless of soil layer depth and botanical family, the total weed seedbank density was significantly higher in CCS (31,650 seeds m ) than in OCS and UCP (26,200 and 23,250 seeds m , respectively) (data not shown). However, both weed seedbank density and distribution were a ected by the ‘botanical family x cultivation system’ (Figure 2) and ‘botanical family x soil layer depth’ (Figure 3) interactions. Agriculture 2019, 9, 192 6 of 14 Table 1. Weed seedbank species composition and distribution (% of total seeds) in relation to cultivation system and soil layer depth. Cultivation System CCS OCS UCP Biological Group Botanical Family Soil Layer Depth (cm) Soil Layer Depth (cm) Soil Layer Depth (cm) Species 0–5 5–10 10–15 0–5 5–10 10–15 0–5 5–10 10–15 T Amaranthaceae Amaranthus retroflexus L. 32.3 2.9 27.4 1.3 17.1 1.2 27.8 2.5 18.2 1.4 15.7 2.4 16.8 1.3 21.6 1.2 23.5 1.1 T Primulaceae Anagallis arvensis L. 5.2 0.7 4.6 0.7 - 6.8 1.0 6.1 0.8 4.1 0.5 7.8 0.9 9.9 0.9 15.4 1.0 H Chenopodiaceae Beta vulgaris L. - 2.3 0.4 - - - - 1.2 0.2 0.6 0.1 0.7 0.1 T Chenopodiaceae Chenopodium album L. 1.3 0.2 0.9 0.1 - 0.5 0.1 - - - - - T Asteraceae Conyza spp. 0.9 0.1 0.5 0.1 0.5 0.1 1.5 0.3 - - - - - H Asteraceae Crepis vescicaria L. 0.9 0.2 0.5 0.1 - 0.5 0.1 1.7 0.3 1.7 0.3 1.2 0.2 - - T Euphorbiaceae Euphorbia helioscopia L. 0.9 0.1 - 0.5 0.1 - - - 1.8 0.4 1.9 0.2 2.9 0.5 T Polygonaceae 0.9 0.1 0.9 0.1 - - 0.5 0.1 - 2.4 0.3 0.6 0.1 1.5 0.4 Fallopia convolvulus (L.) Á. Löve T Papaveraceae Fumaria ocinalis L. 0.9 0.1 1.4 0.2 1.1 0.3 1.0 0.2 3.5 0.6 4.1 0.3 4.8 0.3 3.1 0.5 2.9 0.2 T Papaveraceae Fumaria parviflora Lam. - 1.4 0.3 1.1 0.4 - - - 6.0 0.4 2.5 0.4 7.4 0.6 H Asteraceae Galactites elegans (All.) Soldano 1.3 0.2 - 0.5 0.1 - - - 1.2 0.2 3.1 0.2 0.7 0.1 T Boraginaceae Heliotropum europeum L. 0.4 0.1 - 0.5 0.1 - - 0.8 0.1 - - - T Asteraceae Helminthotheca echioides (L.) Holub 1.3 0.5 1.4 0.2 1.6 0.5 1.5 0.3 5.6 0.5 3.3 0.4 0.6 0.1 4.3 0.3 2.2 0.4 T Lamiaceae Lamium amplexicaule L. - - - - 2.0 0.1 0.8 0.1 0.6 0.1 3.1 0.4 1.5 0.1 T Malvaceae Lavatera trimestris L. 0.4 0.1 0.9 0.2 - - 0.5 0.1 - - - - T Poaceae Lolium multiflorum Lam. - - - - 0.5 0.1 - - - - T Fabaceae Lotus ornithopodioides L. 0.4 0.1 - - - - - - 0.6 0.1 - H Malvaceae Malva rotundifolia L. - - 0.5 0.1 - - - - - 0.7 0.1 H Orobanchaceae Orobanche crenata Forsk. 3.5 0.3 4.1 0.6 3.7 0.6 4.9 0.8 4.5 0.3 4.1 0.4 - - - T Papaveraceae Papaver rhoeas L. 0.9 0.2 - 1.1 0.5 1.0 0.2 - - 1.2 0.1 4.9 0.4 - T Poaceae Polypogon monspeliensis (L.) Desf. - - 0.5 0.1 0.5 0.1 - - - - - T Portulacaceae Portulaca oleracea L. 43.7 1.5 45.2 1.5 52.9 2.8 46.3 5.2 43.9 1.7 51.2 2.7 28.7 2.8 24.1 1.1 19.1 1.6 H Asteraceae Reichardia picroides (L.) Roth - 0.5 0.1 0.5 0.1 - 0.5 0.1 - - - - T Rubiaceae Galium aparine L. - 0.5 0.1 - - 0.5 0.1 - - - - T Solanaceae Solanum nigrum L. - - 0.5 0.1 - - - 3.0 0.5 1.9 0.3 2.2 0.3 T Asteraceae Sonchus asper (L.) Hill 3.5 0.9 4.6 0.4 6.4 0.8 2.4 0.5 8.1 0.8 8.3 0.9 13.8 1.0 13.0 0.9 18.4 1.1 T Asteraceae Sonchus oleraceus L. - - 3.2 0.7 - - - - 4.3 0.4 - H Verbenaceae Verbena ocinalis L. - - 1.1 0.3 0.5 0.1 1.0 0.1 - 0.6 0.1 - - T Scrophulariaceae Veronica cymbalaria Bodard - - 1.1 0.3 - - - 5.4 1.0 - 0.7 0.1 T Scrophulariaceae Veronica hederifolia L. 1.3 0.4 3.2 0.5 3.7 0.3 3.9 0.3 4.0 0.7 4.1 0.4 - - - T Scrophulariaceae Veronica persica Poiret - 1.6 0.3 1.0 0.2 - 1.7 0.2 - - - T Scrophulariaceae Veronica polita Fr. - - - - - - 3.0 0.4 0.6 0.1 - 1 2 H = hemicryptophytes; T = terophytes; CCS = conventional cultivation system; OCS = organic cultivation system; UCP = uncultivated plots. Agriculture 2019, 9, x FOR PEER REVIEW 7 of 15 3.2. Weed Seedbank Density and Distribution for the Main Botanical Families ANOVA results indicated that, regardless of soil layer depth and botanical family, the total weed −2 seedbank density was significantly higher in CCS (31,650 seeds m ) than in OCS and UCP (26,200 −2 and 23,250 seeds m , respectively) (data not shown). However, both weed seedbank density and Agricultur distribueti2019 on ,were a 9, 192 ffected by the ‘botanical family x cultivation system’ (Figure 2) and ‘botan 7 of ical 14 family x soil layer depth’ (Figure 3) interactions. Figure 2. Total weed seedbank distribution (%) and density (values in brackets and expressed as Figure 2. Total weed seedbank distribution (%) and density (values in brackets and expressed −2 2 number of seeds m ) as affected by the ‘botanical family x cultivation system’ interaction. CCS: as number of seeds m ) as a ected by the ‘botanical family x cultivation system’ interaction. Conventional Cultivation System; OCS: Organic Cultivation System; UCP: Uncultivated Plot. CCS: Conventional Cultivation System; OCS: Organic Cultivation System; UCP: Uncultivated Plot. LSDinteraction (P < 0.05) = 460. LSD (P 0.05) = 460. interaction Agriculture 2019, 9, 192 8 of 14 Agriculture 2019, 9, x FOR PEER REVIEW 8 of 15 −2 Figure 3. Total weed seedbank density (seed m ) as affected by the ‘botanical family × soil layer depth’ Figure 3. Total weed seedbank density (seed m ) as a ected by the ‘botanical family  soil layer interaction. depth’ interaction. In both CCS and OCS Portulacaceae represented the main family accounting for, respectively, In both CCS and OCS Portulacaceae represented the main family accounting for, respectively, −2 47% 47% and and 26% 26% of the of the total total seed seedss m m , f , followed ollowed b by y Amara Amaranthaceae nthaceae ((r respect espectively ively, 26 , 26% % a and nd 21 21%) %) and and Asteraceae (respectively, 9% and 11%) (Figure 2). Conversely, in UCP such botanical families were Asteraceae (respectively, 9% and 11%) (Figure 2). Conversely, in UCP such botanical families were −2 uniformly uniformly represented represented (24–20 (24–20 and and 21 21%). %). In a In addition, ddition, b both oth Por Portulacaceae tulacaceae ( (4967 4,967vs. vs. 4067 4,067 seeds seeds m m ) ) and and 2 2 −2 −2 Amaranthaceae (2767 vs. 1867 seeds m ) achieved a higher total number of seed m in CCS than in Amaranthaceae (2,767 vs. 1,867 seeds m ) achieved a higher total number of seed m in CCS than in OCS. OCS. Primulaceae Primulaceaewer wee re mo morere present in present in UCP UCP than than in the in the twotwo cultivated cultivate plots d pl (Figur ots (F eig 2). ure The 2).same The s tra end me was observed for Papaveraceae, which contributed more to the ‘others’ group than the other botanical trend was observed for Papaveraceae, which contributed more to the ‘others’ group than the other families botanical fa (i.e.,mili Chenopodiaceae es (i.e., Chen , op Euphorbiaceae odiaceae, Euphorbiace , Fabaceae a,e, Lamiaceae Fabaceae, , Malvaceae Lamiaceae, , Polygonaceae Malvaceae, Polygonacea , Solanaceae e,, Verbenaceae) (data not shown). Solanaceae, Verbenaceae) (data not shown). The The weed weed seedbank seedbank density density wa was s significantly significantly a affec ected ted by by the the interaction interaction betwe between en the the bot botanical anical family and the soil layer depth (Figure 3). In each soil layer depth, Portulacaceae was the predominant family and the soil layer depth (Figure 3). In each soil layer depth, Portulacaceae was the predominant botanical botanical fa family mily followed followed by by Amaranthaceae Amaranthaceae and and Asteraceae Asteraceae.. G Going oing f fr rom om the the sup superficial erficial ( (0–5 0–5 cm cm) ) to to the the deep (10–15 cm) soil layer, the amount of seeds per square meter was respectively reduced by 23.0% deep (10–15 cm) soil layer, the amount of seeds per square meter was respectively reduced by 23.0% and and 47 47.8% .8% for for Portulaca Portulacaceae ceae and and Amara Amaranthaceae nthaceae,, re respectively spectively. In . In contr contrast, ast, the the w weed eed seed seed d density ensity of of th the e other botanical families did not significantly di er among the three soil layers. other botanical families did not significantly differ among the three soil layers. 3.3. Real Weed Flora 3.3. Real Weed Flora The results demonstrated that the real flora, in terms of cover-abundance, is quite similar between The results demonstrated that the real flora, in terms of cover-abundance, is quite similar the two cultivated plots, i.e., OCS and CCS (Figure 4). between the two cultivated plots, i.e. OCS and CCS (Figure 4). In both CCS and OCS, Brassicaceae highlighted the highest values, followed by Papaveraceae, In both CCS and OCS, Brassicaceae highlighted the highest values, followed by Papaveraceae, Euphorbiaceae, Asteraceae and Malvaceae. In UCP, together with Brassicaceae and Papaveraceae, the highest Euphorbiaceae, Asteraceae and Malvaceae. In UCP, together with Brassicaceae and Papaveraceae, the cover-abundance values were reported for Poaceae, Lamiaceae and Rubiaceae. Asteraceae and Euphorbiaceae highest cover-abundance values were reported for Poaceae, Lamiaceae and Rubiaceae. Asteraceae and showed significant di erences among the two cultivated plots and UCP (23% vs. 4%), as also reported Euphorbiaceae showed significant differences among the two cultivated plots and UCP (23% vs. 4%), for Fabaceae (15% vs. 4%). UCP showed a significantly higher percentage of cover-abundance for as also reported for Fabaceae (15% vs. 4%). UCP showed a significantly higher percentage of cover- Papaveraceae, Poaceae, Lamiaceae, Rubiaceae and Amaranthaceae than OCS and CCS. abundance for Papaveraceae, Poaceae, Lamiaceae, Rubiaceae and Amaranthaceae than OCS and CCS. Considering the normalized frequency of occurrence for each weed (Table 2), the component of species in class I was rather widespread, while only Galactites elegans (All.) Soldano, Malva rotundifolia L., Medicago polymorpha L. and P. rhoeas reached class V in all the evaluated cultivation systems. Other species such as F. convolvulus, H. echioides, S. asper and S. oleraceus were present at higher concentrations Agriculture 2019, 9, 192 9 of 14 (classes IV and V) in all the three cultivation systems. By contrast, Avena sterilis L. and Lolium rigidum Gaud. exclusively appeared in the cultivated plots. Solanum nigrum L. and Veronica cymbalaria Bodard greatly reduced their presence in UCP, passing from class V (in the cultivated plots) to classes II and I, Agriculture 2019, 9, x FOR PEER REVIEW 9 of 15 respectively. In UCP, on the contrary, Lamium amplexicaule L. is widespread (class V), but almost absent in the two cultivated plots (for both class I). Figure 4. Percentage of weed cover-abundance as affected by the ‘botanical family × cultivation Figure 4. Percentage of weed cover-abundance as a ected by the ‘botanical family  cultivation system’ interaction. system’ interaction. Considering the normalized frequency of occurrence for each weed (Table 2), the component of Table 2. Normalized frequency of occurrence [as expressed as presence classes (I: 0–20%; II: 20–40%; III: species in class I was rather widespread, while only Galactites elegans (All.) Soldano, Malva rotundifolia 40–60%; IV: 60–80%; V: 80–100%)] for each investigated species in relation to the cultivation system L., Medicago polymorpha L. and P. rhoeas reached class V in all the evaluated cultivation systems. Other (CCS: Conventional Cultivation System; OCS: Organic Cultivation System; UCP: Uncultivated Plot). species such as F. convolvulus, H. echioides, S. asper and S. oleraceus were present at higher concentrations (classes IV and V) in all the three cultivation systems. By con Cultivation trast, System Avena sterilis L. and Lolium rigidum Gaud. exclusively appeared in the cultivated plots. Solanum nigrum L. and Veronica Species CCS OCS UCP cymbalaria Bodard greatly reduced their presence in UCP, passing from class V (in the cultivated Amaranthus blitoides S. Watson I plots) to classes II and I, respectively. In UCP, on the contrary, Lamium amplexicaule L. is widespread Amaranthus retroflexus L. III III II (class V), but almost absent in the two cultivated plots (for both class I). Anagallis arvensis L. IV IV IV Avena sterilis L. V IV Capsella bursa-pastoris (L.) Medik. I Table 2. Normalized frequency of occurrence [as expressed as presence classes (I: 0–20%; II: 20–40%; Chenopodium album L. I I I III: 40–60%; IV: 60–80%; V: 80–100%)] for each investigated species in relation to the cultivation system Chenopodium murale L. I (CCS: Conventional Cultivation System; OCS: Organic Cultivation System; UCP: Uncultivated Plot). Cynodon dactylon (L.) Pers. II Daucus carota L. III Cultivation System Digitaria sanguinalis (L.) Scop. I Species CCS OCS UCP Diplotaxis erucoides (L.) DC. I II Amaranthus blitoides S. Watson I Amaranthus retroflexus L. III III II Anagallis arvensis L. IV IV IV Avena sterilis L. V IV Capsella bursa-pastoris (L.) Medik. I Chenopodium album L. I I I Chenopodium murale L. I Cynodon dactylon (L.) Pers. II Daucus carota L. III Digitaria sanguinalis (L.) Scop. I Diplotaxis erucoides (L.) DC. I II Agriculture 2019, 9, 192 10 of 14 Table 2. Cont. Cultivation System Species CCS OCS UCP Euphorbia chamaesyce L. I Euphorbia helioscopia L. I I II Euphorbia sp. I I I IV V V Fallopia convolvulus (L.) Á. Löve Fumaria ocinalis L. I II I Fumaria parviflora Lam. I I I Galactites elegans (All.) Soldano V V V Galium aparine L. III IV II Glebionis coronaria (L.) Cass. ex Spach I II I Helminthotheca echioides (L.) Holub IV V V Lamium amplexicaule L. I I V Lavatera trimestris L. I I Lolium rigidum Gaud. V IV Malva rotundifolia L. V V V Medicago arabica (L.) Huds. I I Medicago polymorpha L. V V V Orobanche crenata Forsk. IV IV . Papaver hybridum L. III II I Papaver rhoeas L. V V V Phalaris sp. I Polycarpon tetraphyllum (L.) L. I Portulaca oleracea L. II II II Raphanus raphanistrum ssp. landra (Moretti) Bonnier IV IV IV Reichardia picroides (L.) Roth I Sinapis arvensis L. I II Sisymbrium ocinale (L.) Scop. II Solanum nigrum L. IV V II Sonchus asper (L.) Hill IV V V Sonchus oleraceus L. V IV V Sonchus tenerrimus L. III II I Torilis nodosa (L.) Gaertn. I Trifolium subterraneum L. I I Veronica cymbalaria Bodard V III I Veronica polita Fr. II I I 4. Discussion A total of 32 species were detected with a prevalence of therophytes (>78%) over hemy-cryptophytes, due to the short life cycle which enables therophytes to resist the high degree of anthropogenic disturbance caused by agricultural techniques [26]. Several studies [27–29] demonstrated that di erent cultivation systems can determine changes in the richness of weed species. In our study, significant di erences in the total weed seedbank density were observed according to the following order: CCS > OCS > UCP. Indeed, the way in which each weed species may expand its habit range into agricultural environments, remaining as an invader, depends upon complex interactions [10]. Here, the high abundance of weed seeds m in CCS was related to the high density of Portulacaceae and Amaranthaceae, with both botanical families represented by only a species (P. oleracea and A. retroflexus, respectively). In addition, a higher weed competition in UCP determined the decrease of the total weed seedbank density in the superficial soil layer depth for P. oleracea and through the first 10 cm of soil layer for A. retroflexus. With respect to these two species, our results are in good agreement with some previous studies which showed that they have long been problematic in the Mediterranean Basin. Their presence under both organic and conventional cultivation systems [5,13,17] may be explained by the high number of small seeds produced per plant, a good adaptation to the highly disturbed and moist conditions of irrigated field crops [30], as well as by their need for adequate moisture levels Agriculture 2019, 9, 192 11 of 14 and good seed–soil contact to absorb moisture and germinate. Such biotic and abiotic factors may contribute to their higher seedbank density in all the cultivation systems under study. In particular, the high light intensity and high temperatures, typically experienced in the Mediterranean Basin [31] and usually found in tilled soils [32], can support the germination and establishment of P. oleracea in CCS, while A. retroflexus problems are common also in no-tillage systems with conventional herbicides, in which seeds are arranged on the soil surface and selected within herbicide-resistant populations [33]. However, high populations of such prolific species can be observed in both organic and conventional systems, in tilled and no-till systems. Both A. retroflexus and P. oleracea have specific characteristics that make them very invasive. As an instance, a single plant of A. retroflexus, which emerges, grows, flowers and forms mature seeds from late spring to autumn, can produce 100,000–600,000 seeds (1–1.2 mm in diameter), characterized by multiple dormancy mechanisms [34], depending on the environmental conditions experienced by the mother plant during seed maturation [35], allowing them to be persistent in the soil for long time periods. Because of their dimension, only seeds located within the first 10 cm of soil are able to germinate; while those buried in deeper soil layers become dormant and maintain their ability to geminate for several years when brought to the surface by minimum tillage or cultivation. P. oleracea, an r-strategist, can produce more than 10,000 seeds per plant [36], which are readily dispersed over short and long distances, and have a percentage of germination higher than 90% in 24 h under suitable conditions [36,37]. Their ability to produce a large number of small (0.5 mm in diameter) seeds in di erent environments and an excellent seed survival rate allow this weed species to enrich the soil seedbank in short time [38]. Based on our results, it is also worth mentioning that Amaranthaceae maintained a similar number of seeds m in OCS and UCP. In contrast, CCS and OCS achieved a reduction of Primulaceae and Asteraceae seeds m ; in addition both cultivated systems were characterized by a longer-term persistent seedbank [39] if compared to UCP. Probably the ecological conditions in undisturbed soils (absence of agronomic treatments), which may determine a limited soil aeration, inhibited seed germination, determining an accumulation of seeds in the soil [11]. The results on real weed flora composition and distribution showed that 45 species, belonging to 19 di erent botanical families, were present in the studied agroecosystem. In particular, both cultivated plots (OCS and CCS) showed a higher number of weed species belonging to class V than UCP. Indeed, the agricultural practices (sowing time, weed management control, fertilization, etc.) modify the physical, chemical and biological conditions of the soil, which may have influenced the emergence, growth and ability of some species to produce seeds, and thus their relative cover-abundance [1]. Moreover, as reported by many researchers [40,41], several ecological factors (i.e., predation and decay) may determine a decrease of the number of weed seeds in the soil, as well as a high percentage of weed seeds that do not germinate probably because of their dormant state. Thus, a discrepancy between the soil seedbank and the standing vegetation is possible. Some species, such as G. elegans, H. echioides, M. rotundifolia, M. polymorpha, P. rhoeas, S. asper and S. oleraceus, represent the most dangerous infesting weeds for cultivated crops. Our results showed that some dominant weed species identified through the floristic analysis, i.e., A. sterilis, L. rigidum and M. polymorpha, belonging to the class of presence V, were underrepresented or even completely absent in the soil seedbank. Conversely, some plant species [i.e., Beta vulgaris L., Conyza spp., Crepis vescicaria L. Heliotropum europium L., Lotus ornithopodioides L. and Polypogon monspeliensis (L.) Desf.] present in the soil seedbank were poorly represented or even absent in the established weed flora. Also A. retroflexus and P. oleraceae, the dominant species in the soil seedbank, were poorly represented (presence class II) in the floristic cortege. Seed size significantly influences the survival of seedlings and can dramatically change the e ect of the soil seedbank on the community composition. Moles and Westoby [42] reported that small-seeded plants were characterized by lower survival as seedlings, smaller canopies and shorter reproductive lifespans. Agriculture 2019, 9, 192 12 of 14 In addition, some soil factors, such as moisture, organic content and seed density, may decrease seed survival due to a higher fungal activity [43] and thus would probably increase the mortality of small seeds over large seeds due to the di erence in protection and nutrient reserves [42]. Amaranthus blitoides S. Watson, Chenopodium murale L., Daucus carota L., Sinapis arvensis L., Sisymbrium ocinale (L.) Scop., Torilis nodosa (L.) Gaertn. and T. subterraneum are more sensitive to the presence of crops and, for this reason, they were present only in the uncultivated plots. Conversely, A. sterilis, Capsella bursa-pastoris (L.) Medik., Digitaria sanguinalis (L.) Scop., Euphorbia chamaesyce L., Galium aparine L., Glebionis coronaria (L.) Cass., L. rigidum, Medicago arabica (L.) Huds., M. polymorpha, Papaver hybridum L., Polycarpon tetraphyllum L., Raphanus raphanistrum ssp. landra (Moretti) Bonnier and Sonchus tenerrimus L. were the only species observed in the cultivated plots. Some of them were typical ruderal helio-subnitrophilous therophytic weeds. 5. Conclusions Overall, our study provided useful information about the e ects of cultivation systems on the weed population dynamics in a Mediterranean environment. Results indicated that cultivated (organic and conventional) systems determine the abundance of weed species in the soil seedbank, although the di erences in composition are rather small. 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Agriculture – Multidisciplinary Digital Publishing Institute

Published: Sep 5, 2019

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