Legume-based crop diversification reinforces soil health and carbon storage driven by microbial biomass and aggregates

Diversified cropping is a crucial management practice for increasing carbon (C) and nitrogen (N) sequestration in agroecosystems. However, knowledge gaps regarding the mechanisms by which diversified cropping affects C sequestration and soil quality remain unclear. Herein, a field experiment across six-year was performed to explore the effect of three contrasting cropping systems (i.e., winter wheat/summer maize, winter wheat/summer maize-early soybean, and nature fallow) on soil quality and C sequestration, as well as their main drivers at both 0–20 cm and 20–40 cm soil depths. Diversified cropping increased soil organic C (SOC) stock by 9% and the majority of the soil biochemical metrics in the topsoil despite a 40% reduction in the fertilizer application relative to wheat/maize. This was attributed to increased SOC content in large macroaggregates and enhanced microbial turnover due to diverse fresh residue inputs under diversified cropping. Alternatively, the soil organic C, microbial biomass C, C-acquisition enzyme activity, and N-acquisition enzyme activity in large macroaggregates (> 2 mm) were increased by 15%, 15%, 32%, and 16% in the topsoil under diversified cropping versus wheat/maize, respectively. Partial least squares path model displayed that increased C sequestration was mainly driven by microbial biomass C irrespective of the bulk- and aggregate scale. Furthermore, diversified cropping increased the soil quality index by 1- to 2-fold relative to maize/wheat since increased aggregate stability benefited soil structure and nutrient cycling regardless of soil depth. Overall, increased SOC stock is dominantly driven by microbial biomass C, and the improved soil quality is mainly impelled by soil organic C and aggregate stability in responding to diversified cropping were expected to create win-win scenarios for agroecosystems.