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).
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 2). It was observed that border of barrier with value of 40.11 cm and center of barrier with value of 36.27 cm were superior to the bare ground in terms of plant height (Table 2). For example, the height of rye in the border and center of the barriers was on average 22 and 16% higher than the bare ground respectively (Table 3). In the border of barriers, the higher shoot fresh weight belonged to the rye, followed by the sainfoin and tall wheatgrass (Table 3). Also, the border of the barriers contributed positively to higher shoot fresh weight of the rye (105%), sainfoin (133%) and tall wheatgrass (167%) compared to the bare ground (Table 3). The rye plant grown in the border of barriers showed around 60% higher shoot dry weight compared to the bare ground (2.7 vs. 1.7 g) (Table 3). Generally, the rye showed higher amounts of biomass, shoot dry weight, and plant height than that of other cover crops.
Table 2
Effects of cover crop types and distance from barrier on growth characteristics, concentrations of leaf photosynthetic pigments, RWC, Electrolyte leakage, MDA, Proline and Crude Protein and their average contents.
Source of variation
|
Plant height
(cm)
|
Shoot fresh weight /plant (g)
|
Shoot dry weight /plant (g)
|
Chlorophyll a
(mg g− 1 FW)
|
Chlorophyll b
(mg g− 1 FW)
|
Carotenoids
(mg g− 1 FW)
|
RWC
(%)
|
Electrolyte leakage (%)
|
Malondialdehyde (µmol. g− 1FW)
|
Proline
(µmol. g− 1FW)
|
Crude Protein
(%)
|
Cover crop (C)
|
∗∗
|
∗∗
|
∗∗
|
∗∗
|
∗∗
|
∗∗
|
∗
|
∗∗
|
∗∗
|
∗∗
|
∗∗
|
Rye
|
82.44 a
|
4.40 a
|
2.17 a
|
0.626 a
|
0.191 a
|
0.280 a
|
63.18 a
|
70.71 c
|
0.038 a
|
3.14 a
|
9.18 b
|
Sainfoin
|
11.16 b
|
0.98 b
|
0.37 b
|
0.311 b
|
0.109 c
|
0.154 c
|
55.56 b
|
72.72 b
|
0.039 a
|
2.095 b
|
10.77 a
|
Tall wheatgrass
|
13.66 b
|
0.68 c
|
0.25 b
|
0.334 b
|
0.151 b
|
0.182 b
|
54.44 b
|
79.32 a
|
0.029 b
|
2.30 b
|
8.44 b
|
Distance from barrier (D)
|
∗∗
|
∗∗
|
∗∗
|
∗∗
|
∗∗
|
∗∗
|
∗∗
|
∗∗
|
∗∗
|
∗∗
|
∗∗
|
Border of barriers
|
40.11 a
|
2.91 a
|
1.2 a
|
0.504 a
|
0.181 a
|
0.245 a
|
63.22 a
|
69.16 c
|
0.033 b
|
3.04 a
|
10.55 a
|
Center of barriers
|
36.27 b
|
1.81 b
|
0.9 b
|
0.408 b
|
0.141 b
|
0.200 b
|
56.49 b
|
75.38 b
|
0.036 a
|
2.67 a
|
9.43 ab
|
Bare ground
|
30.88 c
|
1.34 c
|
0.68 c
|
0.360 c
|
0.128 b
|
0.173 c
|
53.46 b
|
78.22 a
|
0.037 a
|
1.82 b
|
8.41 b
|
C × D
|
∗
|
∗∗
|
∗
|
NS
|
NS
|
NS
|
NS
|
NS
|
NS
|
∗∗
|
NS
|
p (Sephericity test)
|
0.06
|
0.038
|
0.007
|
0.362
|
0.162
|
0.451
|
0.224
|
0.869
|
0.975
|
0.329
|
0.008
|
Huynh-Feldt Epsilon
|
0.909
|
0.868
|
0.780
|
1
|
1
|
1
|
1
|
1
|
1
|
1
|
0.784
|
** and * indicate, respectively, differences at P ≤ 0.01 and P ≤ 0.05 probability level, ns indicates not significant difference. Means with same letter in each column are not significantly different according to the LSD test (P < 0.05). |
Table 3
Average contents of plant height, shoot fresh weight, shoot dry weight and proline affected by interactions of cover crop types and distance from barrier.
Distance from barrier
|
Plant height (cm)
|
|
Shoot fresh weight /plant (g)
|
|
Shoot dry weight /plant (g)
|
|
Proline (µmol. g− 1FW)
|
Rye
|
Sainfoin
|
Tall wheatgrass
|
|
Rye
|
Sainfoin
|
Tall wheatgrass
|
|
Rye
|
Sainfoin
|
Tall wheatgrass
|
|
Rye
|
Sainfoin
|
Tall wheatgrass
|
Border of barriers
|
89.3±3.71
a
|
14± 0.57
cd
|
17±1.52
c
|
|
6.24±0.02
a
|
1.44±0.11
d
|
1.05±0.09
de
|
|
2.7±0.17
a
|
0.51± 0.03
d
|
0.37±0.03
de
|
|
4.29±0.32
a
|
2±0.11
c
|
2.84±0.35
b
|
Center of barriers
|
85± 2.08
a
|
10.83± 1.48
d
|
13±0.57
cd
|
|
3.93±0.13
b
|
0.90±0.03
ef
|
0.59±0.03
ef
|
|
2.08±0.1
b
|
0.41±0.04
de
|
0.23±0.003
de
|
|
3.82±0.05
a
|
2.03±0.31
c
|
2.16±0.03
c
|
Bare ground
|
73±2.64
b
|
8.66±0.88
d
|
11±1.52
d
|
|
3.03±0.4
c
|
0.61±0.04
ef
|
0.39±0.05
f
|
|
1.7±0.15
c
|
0.20±0.02
de
|
0.15±0.01
e
|
|
1.32± 0.16
d
|
2.25±0.38
bc
|
1.91±0.1
cd
|
Values are mean ± SE. Means with same letters in the same column are not significantly different according to the LSD test (P < 0.05). |
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 2) while the interaction effect of the cover crops and distance from the barriers was not significant (Table 2). The content of the rye chlorophyll a was significantly higher than sainfoin and tall wheatgrass (Table 2). Plants grown in the borders (0.504 mg g− 1) indicated higher chlorophyll a contents compared to the bare ground (0.36 mg g− 1) (Table 2). Among the different cover crops, higher chlorophyll b was also recorded for rye plants (Table 2). 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 2). 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 2). In addition, the border of the barriers increased the carotenoid content by 22% and 41% relative to the center of the barriers and bare ground, respectively (Table 2).
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 2). The rye plants maintained the highest RWC among the crops (Table 2). The relative water content of sainfoin was not significantly different from that of tall wheatgrass (Table 2). Higher RWC was recorded in the border of the barriers by increasing 18% as compared to the bare ground (63.22 vs. 53.46%) (Table 2). However, no significant differences were detected between the plants grown in the center of the barriers and bare ground (Table 2).
Electrolyte Leakage (EL)
The effects of the cover crops and distance from the barrier were significant on electrolyte leakage (p < 0.01; Table 2). The interaction effect of the cover crops × distance from the barrier was not significant on the EL value (Table 2). Among the different cover crops, the lowest EL value was obtained for the rye plant (70.71%) (Table 2). The measurement of electrolyte leakage in the leaves of the plants grown in the straw checkerboard and bare ground showed that on average 13% and 3% lower EL value is in the border and center of the barriers respectively, compared to the bare ground (Table 2).
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 2). Among the different cover crops, sainfoin and rye recorded the highest malondialdehyde content (Table 2). 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 2). The border plants had significantly the lowest MDA as compared to the bare ground plants (0.033 vs. 0.037 µmol g− 1FW). No significant differences were observed between the center of the barriers and bare ground (Table 2).
Proline
Proline content was significantly affected by the cover crops, the distance from the barrier and their interaction (p < 0.01; Table 2). 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 (Table 3). 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 (Table 3).
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 (Table 2). The crude protein content of the shoots was significantly higher for sainfoin (10.77%) (Table 2). The crude protein content of the plant grown in the straw checkerboard was also significantly greater than the ones on the bare ground (Table 2).
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. 3A). At the borders, plants consumed the water so that the plot without plants had higher soil water storage (Fig. 3A). The rye consumed 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. 3A). In other words, although there was no more water consumption in a plot without plants, but there was more evaporation resulted from bare soil and may lead to lower water storage.
The soil water storage dynamics showed variations in the straw barriers and bare ground during the growing season (Fig. 3B). 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. 3B).
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 4). Soil total nitrogen content was increased with the establishment of straw barriers compared with the bare ground, especially at the border of barriers (Table 5). In the border of the barriers, higher levels of total N were detected in soil with sainfoin (1184.3 mg kg− 1), tall wheatgrass (1031.7 mg kg− 1) and the soil with no plant (1156.7 mg kg− 1). Also, lower total N content detected in the soils after rye harvest (868.3 mg kg− 1), may relate to higher biomass of the rye produced near the borders (Table 5).
Table 4
Chemical parameters of soil as influenced by cover crop types and distances from barriers at harvest stage.
Source of variation
|
TN (mg kg− 1)
|
AP (mg kg− 1)
|
AK (mg kg− 1)
|
SOM (mg kg− 1)
|
Cover crop (C)
|
∗∗
|
∗∗
|
NS
|
∗∗
|
Rye
|
879.1 b
|
17.2 ab
|
619.4 a
|
10821.2 a
|
Sainfoin
|
966.2 a
|
16.8 bc
|
630.6 a
|
9751.9 b
|
Tall wheatgrass
|
867.4 b
|
16 c
|
670.3 a
|
8290.6 d
|
Straw checkerboard and bare ground without plant
|
968 a
|
18.3 a
|
623.3 a
|
8724.3 c
|
Distance from barrier (D)
|
∗∗
|
∗∗
|
∗∗
|
∗∗
|
Border of barriers
|
1060.2 a
|
19.1 a
|
720.8 a
|
11567.7 a
|
Center of barriers
|
922.8 b
|
16.4 b
|
623.05 b
|
8888.5 b
|
Bare ground
|
777.5 c
|
15.6 c
|
563.9 c
|
7734.8 c
|
C × D
|
∗
|
∗∗
|
NS
|
∗
|
p (Sephericity test)
|
0.001
|
0.449
|
0.036
|
0.910
|
Huynh-Feldt Epsilon
|
0.798
|
1
|
0.952
|
1
|
** and * indicate, respectively, differences at P ≤ 0.01 and P ≤ 0.05 probability level, ns indicates not significant difference. Means with same letter in each column are not significantly different according to the LSD test (P < 0.05). |
Table 5
Average contents of chemical parameters of soil at harvest stages as influenced by interaction effects of cover crop types and distances from barriers.
Distance from barrier
|
TN (mg kg− 1)
|
|
|
AP (mg kg− 1)
|
|
|
SOM (mg kg− 1)
|
|
Rye
|
Sainfoin
|
Tall wheatgrass
|
WP
|
|
Rye
|
Sainfoin
|
Tall wheatgrass
|
WP
|
|
Rye
|
Sainfoin
|
Tall wheatgrass
|
WP
|
Border of barriers
|
868.3±60.3 cd
|
1184.3± 60.3
a
|
1031.6±60.38 ab
|
1156.6±60.3a
|
|
20.3±0.88
a
|
18.4±0.5
b
|
16.9±0.87
cd
|
21±0.2
a
|
|
12745.8±491.2a
|
11678.6± 491.2
b
|
10359.4±491.2 d
|
11487.3±491
c
|
Center of barriers
|
994± 38.01 bc
|
911.6± 38.01 bcd
|
875.6±38.01 bcd
|
910±38.01
bcd
|
|
16.3±0.5 cde
|
16.7±0.1
cd
|
15.1±0.31
f
|
17.5±0.
bc
|
|
10156.6±385.1e
|
8717.7±385.1
h
|
8353.9±385.1
i
|
8326±385.1
j
|
Bare ground
|
775±29.9
de
|
802.6±29.9
de
|
695±29.9
e
|
837.3±30
cde
|
|
14.9±0.01
f
|
15.3±0.38 ef
|
15.8±0.15 def
|
16.5±0.32
cde
|
|
9561.4±211.3
f
|
8859.6±211.3
g
|
6158.6±211.3
l
|
6359.7±211.3
k
|
WP: Straw checkerboard and bare ground with no plant, Values are mean ± SE. Means with same letters in the same column are not significantly different according to the LSD test (P < 0.05). |
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 4). Higher concentrations of available P were recorded in the border of barriers especially for the ones with no plant and after the rye harvest (Table 5). Generally, P content of the border of the barriers was averagely higher by 22% than the bare ground (19.1 vs 15.6 mg kg− 1). The P content in the soil with rye was averagely lower by 24% and 36% in the center of the barriers and bare ground, respectively, compared with the borders (16.3 and 14.9 vs 20.3 mg kg− 1) (Table 5).
Available K
In terms of soil available K, there were effects only by the distance from the barriers (p < 0.01) (Table 4). The straw checkerboards increased available K averagely 27% near the borders and 10 % at the centers, respectively, compared to the bare ground (Table 4).
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 4). Higher soil organic matter content was recorded for the border of the barriers (11567.7 mg kg− 1) (Table 4). The rye has been left more organic matters in the borders compare to sainfoin (9%) and tall wheatgrass (23%) (Table 5). Higher organic matter observed for the border of barriers with no plant indicates the effect of straw materials on organic matter. The rye (9561.4 mg kg− 1) and sainfoin (8859.6 mg kg− 1) grown in the bare ground improved the soil organic matter compared to a soil without cover crops (6359.7 mg kg− 1) (Table 5).
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 6). The highest eigenvectors werre related to the chlorophyll a, carotenoids, shoot fresh weight and shoot dry weight for PC1. Moreover, MDA and protein had a positive correlation with the PC2 and the EL had a negative correlation with the PC2 (Table 6). As shown in Fig. 4, the points related to the rye plant were located on the left side of the PC1 and the points related to the tall wheatgrass and sainfoin plant were located on the right side of the PC1. Therefore, the rye had higher shoot fresh weight, shoot dry weight, chlorophyll a and carotenoids compare to sainfoin (Fig. 4). The protein variable had the highest positive correlation with the PC2 and has caused the separation of sainfoin from the tall wheatgrass.
Table 6
Eigenvector of morphophysiological characteristics of cover crops and soil chemical properties, eigenvalues and variance in PCA axes
crop
|
Trait
|
Eigenvector
|
1
|
2
|
Chlorophyll a
|
-0.3673
|
-0.1006
|
Chlorophyll b
|
-0.3220
|
-0.2269
|
Carotenoids
|
-0.3573
|
-0.1548
|
Relative water content
|
-0.3165
|
0.2006
|
Electrolyte leakage
|
0.2723
|
-0.4786
|
Malondialdehyde
|
-0.0543
|
0.2846
|
Proline
|
-0.2650
|
-0.0007
|
Protein
|
-0.0531
|
0.7326
|
Plant height
|
-0.3460
|
-0.1525
|
Plant fresh weight
|
-0.3654
|
-0.0228
|
Plant dry weight
|
-0.3608
|
-0.0599
|
Eigenvalue
|
6.987**
|
1.569
|
Variance (%)
|
63.51
|
14.26
|
soil
|
Total N
|
-0.4392
|
0.4762
|
Available P
|
-0.4622
|
0.1556
|
Available K
|
-0.4118
|
-0.8381
|
Soil organic matter
|
-0.4378
|
-0.0761
|
Soil water storage
|
-0.4818
|
-0.2022
|
Eigenvalue
|
3.987**
|
1.569
|
Variance (%)
|
65.93
|
11.10
|
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 6). The highest eigenvectors were related to the available P and the soil water storage, which have negative correlations with this principle component. The total nitrogen had a positive correlation and the available potassium had a negative correlation with the second principal component (Table 6). The results showed that the points related to the border of barriers were located on the left side of the PC1 and the points related to the bare ground were located on the right side of the PC1 (Fig. 5). In the present research, it could 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.