Green manure incorporation with 20 % nitrogen reduction enhances soil organic carbon storage by improving aggregate stability and functional group composition

Balancing soil carbon sequestration with crop productivity is a pivotal challenge in sustainable agriculture. Leguminous green manure incorporation offers a potential pathway, yet the mechanisms underlying its synergy with reduced nitrogen fertilization in achieving co-benefits for soil carbon sequestration (SCS) and yield stability require further elucidation. A field experiment was conducted from 2023 to 2025 in an oasis irrigated area of Northwest China to evaluate five nitrogen application rates (100 % (N100, traditional nitrogen application), 90 % (N90), 80 % (N80), 70 % (N70), and 60 % (N60)) under leguminous green manure incorporation on maize yield, soil organic carbon storage (SOCs), and the underlying mechanisms. The results indicated that the N80 treatment effectively maintained maize grain yield while significantly enhancing soil organic carbon storage. This synergistic effect was attributed to the improvement of soil aggregate stability and the enhancement of soil organic carbon chemical stability, which was specifically manifested in the N80 treatment exhibiting the highest aromaticity index and the lowest proportion of labile functional groups. Specifically, the N80 treatment increased the proportion of macro-aggregates (>2 mm) and mean weight diameter (MWD), geometric mean diameter (GMD), and the proportion of aggregates > 0.25 mm (DR 0.25 ). Concurrently, scanning electron microscopy observations indicated that the N80 treatment promoted the formation of abundant honeycomb-like pores and ridge-shaped surface structures in the soil. These unique microstructures effectively enhanced the accumulation of organic carbon within aggregates. Fourier transform infrared spectroscopy (FTIR) revealed that the N80 treatment enhanced the aromaticity index (Aromatic C/Aliphatic C) of SOC and reduced the proportion of labile functional groups (specifically, O–H/N–H and C–O groups indicative of polysaccharides and amides). Structural equation modeling confirmed that green manure incorporation with 20 % nitrogen reduction primarily facilitated SOC sequestration by improving aggregate stability and optimizing organic carbon composition. Regression analysis based on a multi-objective comprehensive evaluation model revealed that a nitrogen reduction of 18.74 % is optimal for systems with green manure incorporation. In conclusion, reducing nitrogen application by 20 % combined with leguminous green manure incorporation is an optimal management strategy for synergistically enhancing SOC sequestration, improving soil structure, and securing crop yield in oasis agroecosystems. • N80 treatment increased SOC storage while maintaining a stable yield. • N80 treatment improved soil aggregate stability and enriched organic carbon within them. • N80 treatment increased organic carbon aromaticity but lowered labile N-H group. • N80 treatment promoted SOC sequestration by improving aggregate stability and chemical composition. • A multi-model evaluation pinpointed 18.74 % as the optimal N reduction rate.