Effects of grain-legume rotation and straw return on soil structure and wheat plant traits in the North China Plain

Grain-legume rotation and straw return, as well as their combination, are effective practices for improving soil quality and promoting crop growth. But the influencing mechanism is still lacking, especially on soil structure and wheat plant traits. Taking the North China Plain as a study area, an 8-year field experiment was conducted to explore the effects of grain-legume rotation and straw return on the characteristics of soil structure and stem strength. There were three crop rotation patterns, i.e., wheat-maize rotation (WM), wheat-soybean rotation (WS), and wheat-maize/wheat-soybean rotation (WM/S). Each rotation pattern included two straw management treatments: straw return (SR) and straw removal (CK), resulting in a total of 6 treatment combinations. After wheat harvest, soil samples from 0～60 cm were collected to determine bulk density, soil organic matter content, penetration resistance, aggregate distribution, hydraulic properties, and plant traits. In addition, a structural equation model was established to analyze the effect pathway of grain-legume rotation and straw return on stem strength. The results showed that grain-legume rotation (WS and WM/S) significantly changed soil structure. Specifically, in comparison with wheat-maize rotation, the grain-legume rotation had lower surface layer bulk density (by 7.4%-10.2%, P < 0.05) and penetration resistance (by 3.3%-12.0%, P < 0.05), higher soil organic matter content (by 11.6%-19.6%, P < 0.05) and large aggregates content (by 0.4%-4.1%). The soil total porosity of all crop rotation patterns ranked in order of WM/S > WS > WM. Moreover, the water permeability and water holding capacity of all treatments decreased with the increase of soil depth, however, grain-legume rotation significantly improved these properties, with saturated hydraulic conductivity, saturated water content, and field capacity averagely increased by 92.7%-214.5% (P < 0.05), 7.1%-11.7% (P < 0.05), and 9.0%-21.5% (P < 0.05), respectively, as compared to wheat-maize rotation. Furthermore, for grain-legume rotation, the stem traits (cellulose, hemicellulose, lignin, and total potassium content), stem mechanical strength (bending, pressure, and shear resistance), and wheat yield were 5.7%-33.9% (P < 0.05), 4.2%-34.5% (P < 0.05), 5.4%-18.7% (P < 0.05) higher than wheat-maize rotation, respectively. Additionally, when compared to straw removal under the same crop rotation pattern, the large aggregate content, saturated hydraulic conductivity, field capacity, and stem strength of straw return were averagely increased by 2.2%-4.1%, 23.5%-83.1% (P < 0.05), 24.8%-29.8% (P < 0.05), and 3.5%-25.1% (P < 0.05), respectively. Pearson correlation coefficients indicated that soil physicochemical properties (such as soil bulk density, organic matter content, and total porosity) and saturated hydraulic conductivity were well correlated with plant traits (such as the cellulose, hemicellulose, and lignin content of stem, and wheat yield; |r| ≥ 0.38, P < 0.05) of all treatments at depth of 0～60 cm. The structural equation model demonstrated that grain-legume rotation and straw return indirectly enhanced stem strength by modifying soil bulk density, water holding capacity, stem cellulose, lignin, and total potassium contents. Furthermore, the interaction between grain-legume rotation and straw return significantly influenced wheat stem strength (P < 0.05). In conclusion, grain-legume rotation and straw return significantly improved soil structure, enhanced soil hydraulic properties and stem strength, and subsequently increased microbial activity, nutrient cycling, and root development, thus playing a vital role in crop quality and yield, which subsequently may provide favorable conditions for sustainable agricultural development.