Composition and stability of soil aggregates in maize farmlands under different green manure utilization patterns in an oasis irrigation area

Long-term continuous cultivation of gramineous crops can lead to soil quality degradation, thereby decreasing crop productivity. Optimization of the agronomic measures is a powerful way to improve soil physical and chemical properties, increase soil fertility, and enhance the sustainable development of agricultural production. The effects of different green manure utilization on the composition and stability of soil aggregates and soil bulk density were studied in this study to provide a theoretical and practical basis for improving the quality of farmlands in oasis irrigation areas. A field experiment was conducted in the oasis irrigation area in Northwest China to determine the composition and stability of soil aggregates and soil bulk density at 0-30 cm soil depth in maize farmlands in response to different green manure returning patterns. The green manure utilization patterns included: no tillage with the full quantity of green manure mulched on the soil surface (NTG), conventional tillage with the full quantity of green manure incorporated into the soil (TG), no tillage with root returning of green manure after removing the above-ground part of green manure (NT), conventional tillage with root returning of green manure incorporated into the soil after removing the above-ground part of green manure (T), and conventional tillage without green manure (CT) as the control. The results showed that the four green manure utilization patterns increased the soil macro-aggregate content, mean weight diameter, geometric mean diameter, and mean weight-specific surface area (P < 0.05) at 0-30 cm soil depth, compared to CT. Among the four green manure treatments, NTG had the most significant effects. However, different green manure utilization patterns significantly decreased the fractal dimension of soil aggregates (P < 0.05). Among the four green manure treatments, NTG and NT significantly decreased the fractal dimension of soil aggregates at 0-30 cm soil depth in comparison to TG and T. There was no significant difference in the fractal dimension of soil aggregates at 0-20 cm soil depth between NTG and NT treatments; however, NTG had a lower fractal dimension of soil aggregates at 20-30 cm soil depth than that of NT (P < 0.05). Compared to CT, TG and T significantly reduced the soil bulk density at 0-30 cm soil depth, but NT significantly increased the soil bulk density at each soil depth in the 0-30 cm soil layer (P < 0.05). NTG had significantly lower soil bulk density than CT at 0-10 cm and 20-30 cm soil depth in 2019, but the difference was not significant in 2018. Compared to the other green manure utilization patterns (TG, NT, and T) and conventional tillage without green manure treatment (CT), no tillage with the full quantity of green manure mulched on the soil surface treatment (NTG) increased the macro-aggregate content and stability of soil aggregates and reduced the soil bulk density at 0-30 cm soil depth.