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 can serve as effective practices to improve the soil quality for crop growth. But it is still lacking in the influencing mechanism, especially on the soil structure and wheat plant traits. In this study, an 8-year field experiment was conducted to explore the effects of the grain-legume rotation and straw return on the soil structure and stem strength. A study area was taken as the North China Plain. Three patterns of the crop rotation were set as the wheat-maize rotation (WM), wheat-soybean rotation (WS), and wheat-maize/wheat-soybean rotation (WM/S). Each rotation pattern included two straw treatments: straw return (SR) and straw removal (CK). Six treatments were then carried out in total. The soil samples from 0-60 cm were collected after wheat harvest. Some parameters were determined on the bulk density, soil organic matter content, penetration resistance, aggregate distribution, hydraulic properties, and plant traits. In addition, a structural equation model was established to explore the effect pathway of the grain-legume rotation and straw return on the stem strength. The results showed that the grain-legume rotation (WS and WM/S) significantly changed the soil structure. Specifically, the grain-legume rotation was achieved in the lower bulk density of the surface layer (by 6.7%-12.5%) and penetration resistance (by 6.8%-17.1%), while the higher content of the soil organic matter (by 11.6%-19.6%, P < 0.05), compared with the wheat-maize rotation. The total porosity of the soil was ranked in descending order of theWM/S > WS > WM among the crop rotation patterns. Moreover, the water permeability and holding capacity of all treatments decreased with the increase in the soil depth. However, the grain-legume rotation significantly improved these properties, with the saturated hydraulic conductivity, saturated water content, and field capacity averagely increased by 92.7%-214.5%, 7.1%-11.7%, and 9.0%-21.5% (P < 0.05), respectively, compared with the wheat-maize rotation. Furthermore, 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%, 6.3%-32.5%, 6.4%-20.2% (P < 0.05) higher for the grain-legume rotation thanWM, respectively. Additionally, the large aggregate content, saturated hydraulic conductivity, field capacity, and stem strength of the straw return averagely increased by 3.4%-5.8%, 23.5%-83.1%, 24.8%-29.8%, and 3.5%-25.1% (P < 0.05), respectively, compared with the straw removal under the same crop rotation pattern. Pearson correlation coefficients indicated that the soil physicochmical properties (such as, soil bulk density, organic matter content, and total porosity) and saturated hydraulic conductivity were well correlated with the plant traits (such as, the cellulose, hemicellulose, and lignin content of stem, and wheat yield; Absolute value of correlation coefficient |r| ≥ 0.38, P < 0.05) of all treatments at the depth of 0-60 cm. The structural equation demonstrated that the grain-legume rotation and straw return indirectly enhanced the stem strength to modify the 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 the wheat stem strength (P < 0.05). In conclusion, the grain-legume rotation and straw return significantly improved the soil structure, soil hydraulic properties, and stem strength. Subsequently, there was an increase in microbial activity, nutrient cycling, and root development. The finding can also provide favorable conditions for the crop quality and yield in sustainable agriculture.