Water environment safety is the focus of engineering measures to eliminate liquid digestate in farmland. It is of great significance to study the aging characteristics of soil absorbing and fate of liquid digestate ammonium nitrogen (NH₄⁺-N) to realize safe and efficient disposal. In this paper, simulation experiments of digesting NH₄⁺-N (with application of 0, 120, 180, and 300 kg/hm²) by static soil column are carried out to study disposal efficiency, migration and transformation characteristics, and fate proportion of NH₄⁺-N in saturated water content soil. The result showed that after 3 days of application, the overlying water NH₄⁺-N concentration decreased by 63.5–80.7%, and the reduction rate of total NH₄⁺-N was 65.8–82.3%. After 4 days, the NH₄⁺-N concentration of pore water in the 0–10 cm soil layer reached the peak value. After 7 days, the NH₄⁺-N concentration adsorbed by the 0–10 cm soil layer reached the peak value. After 15 days, the overlying water NH₄⁺-N concentration decreased by 97.0–98.7%, the reduction rate was 97.9–99.2%, and the proportion of NH₄⁺-N absorbed in the 0–10 cm soil layer accounted for 63.5–76.3%. The disposal is mainly based on soil sorption and pore water migration. A duration of 0–3 days is the rapid disposal period, and 15 days is the completion period of safe digestion.

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 (12.31×10 ³ kg/ha) was obtained under Cw-W1N2, and the highest wheat yield (2.80×10 ³ 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.