Combined Effects of Biochar and Milk Vetch with Reduced Nitrogen Application on Rice Yield, Soil Aggregation, and Carbon and Nitrogen Distribution in Reddish Paddy Soil

Although rice straw biochar (RSB) and milk vetch (MV) facilitate rice production and soil carbon and nitrogen (N) storage, their combined effects under N-reduction remain unclear, particularly at the aggregate-scale in reddish paddy soils. A two-year field experiment with four treatments: 100% N (N100), 20% N-reduction + RSB (N80B), 20% N-reduction + MV (N80M), and 20% N-reduction + RSB + MV (N80BM), was conducted in reddish paddy soil. Rice yield, soil organic carbon (SOC) and total N (TN) stocks, water-stable aggregate distribution and stability, and aggregate-associated C and N allocation were measured. Over the two-year study, N80BM significantly increased grain yield by 11.2%, SOCs by 5.42%, and TNs by 6.92% compared with N100 (p < 0.05). RSB and MV combined with N-reduction increased macroaggregate proportion by 7.92–11.8% and aggregate stability by 7.14–58.1%, while decreasing silt–clay fraction by 39.6–62.9%. Soil C and N enrichment occurred predominantly in macroaggregates. Pearson correlation and random forest analyses jointly identified aggregate distribution and stability as primary predictors of rice yield, SOCs, and TNs. Therefore, RSB + MV combined with 20% N-reduction sustained rice yield while enhancing soil aggregation and C, N sequestration, providing a rational strategy for N management in sustainable rice production.