Straw Checkerboard Barriers Improve Soil Characteristic and Growth Performance of Winter Cover Crops and Protect Sloping Land


 Aims Straw checkerboard barrier technology, as a sustainable and environment-friendly method, is intended for erosion control, plant recovery at endangered areas. Finding how soil chemical properties and water status alters and how pilot cover plants respond are vital for extra management actions. Methods In order to study soil characteristic and growth performance of winter cover crops, an experiment was carried out in 2018. Checkerboards were setup in a 1×1 m pattern with rice residues. Seeds of Onobrychis sativa, Secale montanum and Agropyron sp were sown at the checkerboards and bare ground as the control. Detailed analyses were carried out on the physiological responses and the related soil properties during the growing season in borders and centers of the straw checkerboards and bare ground. Results Soil covered with straw checkerboard barriers was more effective for improving vegetation growth, established taller plants with higher biomass. Based on the physiological analyses, the rye grown in a straw checkerboard coped well with dry conditions. Higher proline content and more efficient osmotic adjustment indicate improvements of soil water retention status, which may alleviate drought stress damages and improve cover crop performance in straw checkerboard plots. Photosynthetic pigments also showed higher contents in these conditions for rye. Soil organic matter, total nitrogen, available phosphorus and potassium and water storage increased in checkerboards too; however, such effects vary with the crop type. Conclusions This technique could be deemed as an effective management strategy in semi-arid areas and an important method for conserving natural resources and sustaining productivity.


Introduction
Successful recovery of soil and vegetation in semi-arid regions is a challenge because water availability severely limits germination, plant establishment, growth and survival (Wilson et al. 2004; Munson and Lauenroth 2012). In bare soil, the evaporation process discharges water from the soil surface, and microclimate conditions are rarely favorable for the establishment of plants; therefore, poorly covered soil can lead to erosion (Hadas 2004). Soil erosion is one of the most important soil problems in arid and semiarid Mediterranean areas, which reduces soil fertility, plant productivity and also threatens human life (Yüksek and Yüksek 2015). Therefore, in arid and semiarid sites, with undesirable seedbeds, eld techniques are canonical to conserve water, improve seedbed microclimate and seedling establishment, increase soil fertility, reconstruct soil ecological community and reduce erosion.
Straw checkerboard barrier technique is widely used for controlling of erosion and habitat recovery in arid and semi-arid areas (Li et al. 2006). This technique, as an environmentally-friendly strategy, is cheap and easy to install (Zhang et al. 2018). Straw of wheat, rice, reeds and other plants are installed in the square shape 20 cm buried in the soil and about 20 cm exposed above the soil surface (Wang et al. 2020). The checkerboard barriers can be placed at various dimensions, including 0.5×0.5,1×1, 1.5×1.5 and so on (Qu et al. 2007; Zhang et al. 2018). Straw checkerboards reduce the wind speed which decrease water loss through evaporation (Bo et (Li et al. 2006). Li et al. (2000) and Williams et al. (1995) showed that four years after revegetation in squares, cryptogamic crusts have been developed which reduce water and wind erosion. Zhang et al. (2018) reported that application of a straw checkerboard improved soil water content and physicochemical properties; thereby, contributed to the establishment of plant and promoted ecological restoration. Dai et al. (2019) and Sun et al. (2016) showed that the soil particle size decreased after the implement of checkered barriers and the physical properties of the soil improved; thus, a better environment for the growth of plant community was provided.
Winter cover crops are deemed as one of the most effective sustainable land management techniques for reducing soil erosion rates and recovery of degraded land (Marques et al. 2016). Cover crops decrease the kinetic energy of the raindrops on the soil surface, decrease runoff velocity and interception, protect winter rainwater storage in soil and reduce evaporation (Sharma et  (2019) indicated that, in the sloping area, the cover crop exerts a positive impact on carbon budget by an increase in the C input and a restriction on the C loss.
Sainfoin (Onobrychis sativa.), rye (Secale montanum) and tall wheatgrass (Agropyron sp) were selected in this study as pilot, because of properties such as highly tolerant to drought, good source of animal feed, unique adaptation to poor soils and well-developed root system (Sancak et al. 2003 This study was designed for a steep land that is prone to features of water and wind erosions, high velocity of runoff with low in ltration and poor biodiversity in a semi-arid area, Shahrekord, Iran. We were interested in studying the in uence of straw checkerboard barriers on growth and performance of rye, sainfoin, and tall wheatgrass to alleviate the drought effects caused by de ciency of precipitation, soil chemical properties, and water storage change in order to provide a theoretical basis for the application of straw checkerboard barrier technique to stabilize the slope surface and ecologic restoration.
A Mediterranean semiarid area, with warm and dry summer and cold winters. The mean annual precipitation is approximately 316 mm and average monthly temperature is 12.12 °C . The mean annual wind velocity is 2.6 m/s. The soil is classi ed as Entisol and the slope ranges from 10 to 14%. The ground-water table depth was below 80 m, which cannot be used by the plant. At the beginning of the study, soil samples were randomly collected from 0-30 cm depth using a soil auger; the important characteristics of the soil are shown in Table 1.
Experimental Set up 1×1 m square grids of rice straw barriers were established in January, 2018 as the checkerboards pattern, covering an area of approximately 700 m 2 . The straw was buried in the soil to a depth of 20 cm and protruded 20 cm above the soil. A bare ground with the same area was chosen as a control plot in the adjacent to the straw checkerboard barriers (Fig. 1). Sainfoin (Onobrychis sativa), rye (Secale montanum), and tall wheatgrass (Agropyron sp) were seeded within each square and the bare ground manually at 2-4 cm depth in November 2018.

Plants Growth Parameters
Plant growth parameter measurements were taken at the end of the growing period. Five plants were randomly selected from the borders of barriers, centers of barriers, and the bare ground by cutting them at the ground level. Afterward, the plants' height and aboveground biomass were assessed. The aerial parts were then oven-dried at 70 °C for 48 h and weighed to determine the dry weight.

Physiological Traits
Chlorophyll a, chlorophyll b, and carotenoids contents were determined according the methodology described by Lichtenthaler and Buschman (2001). At rst, 100 mg fresh leaf sample collected from matured leaves were ground with 5 ml of 80% acetone. After ltration, extracts were adjusted to 10 ml with 80% acetone solution and the absorbance of the solution was recorded at 646.8, 663.2, and 470 nm with a spectrophotometer (HACH Company, DR 6000, Germany). Pigment contents were calculated by the following equations: Chl a (mg g -1 FW) = (12.7 × abs 663.2 ) -(2.6 × abs 646.8 ) mLacetonemg - 1 (1) Electrolyte leakage (EL) value, speci ed for assessing the degree of drought tolerance was measured according to the method of Lutts et al. (1996). To this end, cleaned and freshly collected leaf samples were put into test tubes containing 10 ml distilled water and incubated for 2 h at 25 • C. Subsequently, the electrical conductivity of the solution was measured (EC1). The samples were autoclaved at 15 psi for 15 min and the second electrical conductivity (EC2) was determined. Electrolyte leakage (EL) was estimated according to the following formula: The proline content was estimated by the method of Bates et al. (1973). The leaf fresh samples were ground with 3% salicylic acid.
After adding ninhydrin and glacial acetic acid in tubes containing portions of the ground samples, tubes were kept in a 100 °C water bath. After cooling, 4-mL toluene was added. The absorbance was measured by spectrophotometer at 520 nm using the calibration curve. where SWS (mm) is the soil water storage; hi (cm) is the soil layer depth; SWCi is the volumetric soil water content of soil layer i; n is the number of soil layers, i=1, 2,… etc.

Soil Measurements
After the plants harvest, soil samples were collected along a vertical line in the borders and also the centers of each square as well as the bare ground using a soil auger in 0-25 cm soil layer. Three points per treatment were taken to collect soil samples and then pooled together. Three replications for each mixed sample were obtained. The mixed samples were air dried and then sieved through a 2 mm sieve. The soil organic carbon (SOC) content was measured by Walkley-Black, potassium dichromate oxidation method (Lu 2000).
Total nitrogen (TN) content was determined using the Kjeldahl digestion (Bremner and Mulvaney 1982). Soil available phosphorus (AP) was measured using the method provided by Olsen et al. (1954). Soil available potassium (AK) content was extracted with 1 N ammonium acetate and using ame photometry (Lu 2000).

Data analysis
Repeated-measures analyses of variance (RM-ANOVAs) were performed on the plant traits and soil chemical properties data (con dence level of 95%). The between-subject factors were the cover crops while the within-subject factor was the distance from barriers. Moreover, for the soil water storage data, the between-subject factors included the distance from barriers while the withinsubject factor included cover crops and time. Repeated-measures analyses of variance were performed using the STATISTIX 12.0 software. The differences between the mean values were compared using the least signi cant difference (LSD) at P < 0.05.

Weather Conditions
Daily precipitation and air temperature during the growing period are given in Fig. 2. The rainfall during the growing periods amounted to 300 mm and the daily mean of air temperature varied from -5.1 to 20.4°C (Fig. 2). The air temperature rose with the progress of plants growth.

Soil Chemical Properties
Total N Soil total N was affected by cover crops (p < 0.01), distance from barrier (p < 0.01) and their interaction (p < 0.05) ( Table 2). Soil total nitrogen content was increased with the establishment of straw barriers compared with the bare ground, especially at the border of barriers ( Fig. 3). In the border of the barriers, higher levels of total N were detected in soil with sainfoin (0.118%), tall wheatgrass (0.104) and the soil with no plant (0.112). Also, lower total N content detected in the soils after rye harvest (0.086%) (Fig. 3), may relate to higher biomass of the rye produced near the borders. Straw materials may enhance N contents via higher rates of mineralization supported by higher carbon and aeration of the straw rows.
Available P Soil available P was in uenced by the cover crop type, distance from barriers and interaction between the cover crops and distance from the barrier (p< 0.01) ( Table 2). Higher concentrations of available P were recorded in the border of barriers especially for the ones with no plant and after the rye harvest ( Fig. 4). Generally, P content of the border of the barriers was averagely higher by 22.24 % than the bare ground. The P content in the soil with rye was averagely lower by 24.37% and 35.12% in the center of the barriers and bare ground, respectively, compared with the borders (Fig. 4). Higher available P content in the border of the barriers with no plant indicates that straw checkerboard incorporates to increment of available P; however, it may have been consumed with the plants in other plots.
The rye may have been more e cient in availability of P.

Available K
In terms of soil available K, there were effects only by the distance from the barriers (p < 0.01) ( Table 2). The straw checkerboards increased available K averagely 27.81% near the borders and 10.47% at the centers, respectively, compared to the bare ground ( (Fig. 5).

Organic Matter
The soil organic matter content was signi cantly affected by the cover crop type (p < 0.01), the distance from barriers (p < 0.01) and the interaction between them (p < 0.05) ( Table 2). Higher soil organic matter content was recorded for the border of the barriers (Fig. 6). In the border of the barriers, the rye has been left more organic matters in the soil being on average 8.82% and 22.70% higher than sainfoin and tall wheatgrass, respectively (Fig. 6). Higher organic matter observed for the border of barriers with no plant indicates the effect of straw materials on organic matter. The rye and sainfoin grown in the bare ground improved the soil organic matter by 49.60% and 38.58% compared to a soil without cover crops (Fig. 6).

Plant Growth Parameters
Plant height, biomass, and shoot dry weight were signi cantly affected by the cover crop type (p < 0.01), the distance from barriers (p < 0.01) and their interaction (Table 3). It was observed that the straw barriers were superior to the bare ground in terms of plant height especially for the plants near the borders (Fig. 7A). For example, the height of rye in the border and center of the barriers was on average 22.36 and 16.43% higher than the bare ground (Fig. 7A). In the border of barriers, the higher shoot fresh weight belonged to the rye, followed by the sainfoin and tall wheatgrass (Fig. 7B). Also, the border of the barriers contributed positively to higher shoot fresh weight of the rye (105.94%), sainfoin (133.38%) and tall wheatgrass (167.08%) compared to the bare ground (Fig. 7B). The rye plant grown in the border of barriers showed around 60% higher shoot dry weight compared to the bare ground (Fig. 7C). Generally, the rye showed higher amounts of biomass, shoot dry weight, and plant height than that of other cover crops.

Physiological Attributes
Leaf Photosynthetic Pigments Leaf chlorophyll a, b and carotenoids contents were signi cantly different for the cover crops (p < 0.01) and the distance from barriers (p<0.01; Table 4) while the interaction effect of the cover crops and distance from the barriers was not signi cant ( Table 4). The content of the rye chlorophyll a was signi cantly higher than sainfoin and tall wheatgrass (Table 5). Chlorophyll a was signi cantly, on average 40%, greater for the plants grown in the border of the straw barriers compared to the bare ground (Table 6). Among the different cover crops, higher chlorophyll b also was also recorded for rye plants ( Table 5). The border of the straw barriers led to a signi cant increase in the chlorophyll b of leaves with a value of 0.181 mg/g, while no signi cant differences were detected between the center of the barriers and bare ground ( Table 6). The same trend was observed for the carotenoid contents. The carotenoid contents of the cover crops in a descending order were as follows: rye>tall wheatgrass>sainfoin (Table 5). In addition, the border of the barriers increased the carotenoid content by 22.5% and 41.61% relative to the center of the barriers and bare ground, respectively (Table 6).

Relative Water Content (RWC)
In terms of relative water content, there were signi cant effects of the cover crops (p < 0.05) and distance from the barrier (p < 0.01), but not their interaction ( Table 4). The rye plants maintained the highest RWC among the crops ( Table 5). The relative water content of sainfoin was not signi cantly different from that of tall wheatgrass (Table 5). Higher RWC was recorded in the border of the barriers by increasing 17.85% as compared to the bare ground (Table 6). However, no signi cant differences were detected between the plants grown in the center of the barriers and bare ground (Table 6).
Electrolyte Leakage (EL) The effects of the cover crops and distance from the barrier were signi cant on electrolyte leakage (p<0.01; Table 4). The interaction effect of the cover crops × distance from the barrier was not signi cant on the EL value (Table 4). Among the different cover crops, the lowest EL value was obtained for the rye plant ( Table 5). The measurement of electrolyte leakage in the leaves of the plants grown in the straw checkerboard and bare ground showed that on average 13.1% and 3.76% lower EL value is in the border and center of the barriers respectively, compared to the bare ground (Table 6).

Malondialdehyde (MDA)
MDA content was signi cantly affected by the cover crops and distance from the barrier (p<0.01) but, no interaction effect was found (Table 4). Among the different cover crops, sainfoin and rye recorded the highest malondialdehyde content (Table 5). Malondialdehyde content ranged between 0.033 µmol g −1 for the plant grown in the borders to 0.037 µmol g −1 for the plant grown in the bare ground ( Table 6). The border plants had signi cantly the lowest MDA as compared to the bare ground plants. No signi cant differences were observed between the border and center of the barriers (Table 6).

Proline
Proline content was signi cantly affected by the cover crops, the distance from the barrier and their interaction (p<0.01; Table 4). The highest values of proline were obtained for the rye grown in the border and center of the barriers, while the lowest value was recorded for the rye grown in the bare ground. The tall wheatgrass also showed higher amounts of proline content in the border of the barriers but the proline content of sainfoin was the only case that was not affected by the distance from the barriers (Fig. 8).

Crude Protein
Statistical analysis carried out on crude protein content revealed a signi cant difference between the cover crops and distance from the barrier (p<0.01) but not for their interaction (Table4). The crude protein content of the shoots was signi cantly higher for sainfoin that may be due to higher N xation ( Table 5). The crude protein content of the plant grown in the straw checkerboard was also signi cantly greater than the ones on the bare ground (Table 6).

Soil Water Storage
During the growing seasons of the rye, sainfoin and tall wheatgrass, the soil water storage was monitored. Analysis of the soil water storage dynamic within a 0-25cm depth under the three mentioned crops at different distances from the barriers showed that a higher value of soil water storage was observed in the checkerboard plots (Fig. 9A). At the borders, plants consumed the water so that the plot without plants had higher soil water storage (Fig. 9A). The rye drained more water than the other cover crops that may be due to the higher biomass produced. In the center of the barriers, water status was more stable (Fig. 9A). In other words, although there was no water consumption in a plot without plants, there was no water storage, which may point to more evaporation because of naked soil.
The soil water storage dynamics showed variations in the straw barriers and bare ground during the growing season (Fig. 9B). During the growing season of crops, the border and center of the barriers invariably retained higher soil moisture than the bare ground and signi cant differences among the straw barriers and bare ground were observed. The soil water storage fell dramatically over time and changed moderately during the late growth period (Fig. 9B).

Principal Component Analysis
The analysis of the principal components of the morphophysiologic properties of plants (Chlorophyll a, chlorophyll b, carotenoids, RWC, EL, MDA, proline, protein, plant height, shoot fresh weight and shoot dry weight) showed 75.77% variation explained by PC1 and PC2 ( Table 7). The highest eigenvectors are related to the chlorophyll a, carotenoids, shoot fresh weight and shoot dry weight for PC1. Moreover, there were a positive correlation protein and MDA with PC2 and a negative correlation with EL ( Table 7). As shown in Figure  10, the points related to the rye plant are located on the left side of the PC1 and the points related to the tall wheatgrass and sainfoin plant are located on the right side of the PC1.Therefore, the rye plant has higher shoot fresh weight, shoot dry weight, chlorophyll a and carotenoids. Also, the points related to the sainfoin plant are located on the top of the PC2 and the tall wheatgrass plant is located on the low part of the PC2 (Fig. 10). The protein variable has the highest positive correlation with the PC2 and has caused the separation of sainfoin from the tall wheatgrass.
The PCA on soil characteristics (total N, available P, available K, soil organic matter and soil water storage) showed that PC1 and PC2 explained the 63.95% and 11.10% data variability, respectively (Table 8). The highest eigenvectors are related to the available P and the soil water storage, which have negative correlations with this principle component. The total nitrogen has a positive correlation and the available potassium has a negative correlation with the second principal component (Table 8). The results showed that the points related to the border of barriers are located on the left side of the PC1 and the points related to the bare ground are located on the right side of the PC1 (Fig. 11). In the present research, it can be noticed that PC1 represents the changes both in different plants and between the border of the barriers, center of the barriers and bare ground.

Discussion
In the semiarid areas, a large amount of carbon is lost through high mineralization rates and soil erosion; thus, cover crops are vital to Soil organic matter improves the ability of soil to store macronutrients such as potassium and magnesium by increasing the soil cation exchange capacity (Newman et al. 2007). In this study, establishment of straw checkerboard enhanced NPK content compared with the bare ground. However, the rye absorbed more total nitrogen from soil than the other cover crops. This trend may be stimulated near the borders due to fast rates of growth. Because of N-scavenging capacity of rye, it may assimilate and recycle substantial amounts of Soil water storage is a crucial parameter for plant establishment and growth, particularly in water-limited conditions (Ren et al. 2008). The lack of su cient precipitation during mid-to late-growth season (Fig. 2) led to a decrease in the soil water storage especially in the bare ground. Cover crops can improve the soil moisture storage through water conductivity by root channels, enhancing aggregate stability, and reducing soil compaction ( The leaf chlorophyll contents are good indicators of the photosynthesis activity, nutritional state and abiotic constraints (Marcu et al. 2013). High level of chlorophyll a, chlorophyll b and carotenoids in leaves of rye (Table 5), besides its drought tolerant characteristic, may be considered for their cultivation in this situation more than other ones (Hlavinkaet al. 2009).
At the border of barriers, signi cant higher chlorophyll contents were observed. Enhanced soil moisture storage (Fig. 9A) and supply of nutrients especially nitrogen (Fig. 3)  Evaluation of RWC content revealed better results for rye (Table 5) which may be due to its highly developed root system and can take full advantage of the soil water at the beginning of the growing season. Various studies have shown that grass species maintaining high amounts of RWC content can exhibit more cellular hydration under stress conditions (Chai et al. 2010). Maintaining RWC at 67.54% in the rye grown on borders should allow the rye to retain minimal cell turgidity and represent a high capacity of osmotic adjustment.
Electrolyte leakage has been widely used as an indicator of the oxidative stress (Trabelsi et al. 2019). The rye had the lowest electrolyte leakage among the three cover crops (Table 5). Signi cant increase in EL on the bare ground plants may be due to the fact that water de cit increases electrolyte leakage and con rms that these plants are more vulnerable to border plants.
Malondialdehyde (MDA) concentration is the nal product of membrane lipid peroxidation (Anjum et al. 2011). MDA content, as an indicator of oxidative damage, re ects the extent of the membrane injury caused by reactive oxygen species (Stajner et al. 2009). Our ndings showed that the MDA content was decreased when the rye and sainfoin were grown in the straw checkerboards (Table 6). The straw checkerboards provided better protection from oxidative damages and higher tolerance to drought was created by improving the soil water storage.
As a nitrogenous compound, proline is the main amino acid that plays a major role in plants under drought conditions and is responsible for osmotic adjustment in plants (Hazrati et al. 2017). Higher proline accumulation, occurred in the rye grown on the border of barriers, may be attributed to the increase in soil total nitrogen content on the border of the barriers (Azcón and Tobar 1998). Increased soil available potassium may induce proline accumulation. It may be attributed to starvation effect of K on amino acids and proline metabolism (Bahrami-Rad and Hajiboland 2017). In this study, the rye plants showed the highest proline concentration (Fig. 8), and the least-injuries by drought in terms of plants growth; Plants grown in the bare ground suffered more ( Fig. 7A-C). High proline contents were in line with maintaining protein structures; preserve the integrity of cell membranes and antioxidant protective roles in cover crops (Zhang et al. 2009).

Conclusion
Straw checkerboard barriers improved initial vegetation recovery in steep dry lands. As an effective management practice, this technique signi cantly has improved soil macronutrient, organic matter content and soil water storage which further creates ideal conditions for growth performance of plants. As a result, the improved plant growth leads to soil preserve and reduced erosion. Cover crops, especially the rye, improved organic matter contents of the soil, which may be important as a carbon sequestration approach. Most of the physiological parameters evaluated herein indicate that for the mentioned cover crops, rye had a better ability to survive during drought stress, can be used in such a dry condition effectively. Superior drought tolerance in rye, compared to other cover crops, was associated with increased proline content and more e cient osmotic adjustment. Based on this eld experiment, it could be concluded that implement of straw checkerboards will protect cover crops more e cient to conserve natural resources and sustaining productivity.   Daily mean air temperature and rainfall in the study area after the establishment of straw checkerboard barriers (a) and during the cover crops growing seasons (b) Figure 3 Soil total nitrogen content (%) of different cover crops under straw checkerboard barrier. Bars indicate S.E. Different letters indicate signi cant differences at P < 0.05 by LSD test  Soil available potassium content (mg.kg-1) of border of barriers, center of barriers and bare ground. Bars indicate S.E. Different letters indicate signi cant differences at P < 0.05 by LSD test Figure 6 Soil organic matter content (%) of different cover crops under straw checkerboard barrier. Bars indicate S.E. Different letters indicate signi cant differences at P < 0.05 by LSD test Figure 8 Effects of cover crops type and straw checkerboard on cover crop proline (µmol. g-1FW). Bars indicate S.E. Different letters indicate signi cant differences at P < 0.05 by LSD test Figure 9 Dynamic change in soil water storage under cover crops type and straw checkerboard (A) and under straw checkerboard during growing season (B). Bars indicate S.E. Different letters indicate signi cant differences at P < 0.05 by LSD test Figure 10 Principal component analysis (PCA) of morphophysiological characteristics of different cover crops under straw checkerboard