Interactive effects of biochar, nitrogen fertilizer, and irrigation on SOC and crop yield in a rice-wheat rotation...

Rational management of coupled carbon (C), nitrogen (N), and water (W) inputs (C-N-W) is essential for the sustainability of rice-wheat rotation systems. Using a soil column pot experiment, we investigated the interactive effects of C-N-W on soil organic carbon (SOC) and crop grain yields. Two straw-derived wet biochars-wheat straw biochar (Cw) and rice straw biochar (Cr)-were applied only during the rice season. Three nitrogen levels were tested for both crops: N0 (no N), N1 (20% reduced N with a 3:7 ratio of quick-release to slow-release urea), and N2 (conventional N with a 4:6 ratio of quick-release to slow-release urea). Two irrigation regimes were applied: W1 (shallow flooding for rice/conventional irrigation for wheat) and W2 (alternate wetting-drying for rice/reduced irrigation for wheat). The control (CK) received no N and no biochar under W2 irrigation. All treatments significantly increased SOC in the whole system. C-N-W management enhanced SOC by 15.4%-23.9% during the rice season and by 1.6%-26.8% at wheat harvest. The optimal rotational rice/wheat SOC was achieved under Cr-W2N1 (8.03/8.11 g/kg) and Cr-W1N1 (7.86/8.22 g/kg). Wheat-season SOC was positively correlated with wheat root and shoot dry weights (p < 0.05). Compared with CK, certain C-N-W treatments increased rice grain yield by 1.3% to 136.3% and wheat grain yield by 6.2% to 14.6%. The highest rice yield (12.31×10³ kg/ha) was obtained under Cw-W1N2, and the highest wheat yield (2.80×10³ kg/ha) under Cr-W1N1. Random Forest analysis identified nitrogen management and stem biomass as variables closely associated with rice yield (p < 0.01), with Cw biochar being favorable; wheat grain yield depended on water-mediated growth modulated by preceding-crop management. Correlation analysis showed that rice dry biomass and effective panicle number were significantly correlated with wheat dry biomass and effective panicle number (p < 0.05). Comprehensive evaluation showed that Cr-W1N1 was the most favorable treatment, increasing rice-wheat yields and soil organic carbon and generating positive annual effects, thus offering an effective pathway for sustainable intensification through integrated C-N-W optimization.