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 influenced 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 efficient 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 significantly 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 significantly 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 significantly 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 significant (Table 4). The content of the rye chlorophyll a was significantly higher than sainfoin and tall wheatgrass (Table 5). Chlorophyll a was significantly, 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 significant increase in the chlorophyll b of leaves with a value of 0.181 mg/g, while no significant 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 significant 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 significantly 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 significant 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 significant on electrolyte leakage (p<0.01; Table 4). The interaction effect of the cover crops × distance from the barrier was not significant 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 significantly 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 significantly the lowest MDA as compared to the bare ground plants. No significant differences were observed between the border and center of the barriers (Table 6).
Proline
Proline content was significantly 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).
3.4.6. Crude Protein
Statistical analysis carried out on crude protein content revealed a significant 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 significantly higher for sainfoin that may be due to higher N fixation (Table 5). The crude protein content of the plant grown in the straw checkerboard was also significantly 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 significant 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.