Alternate wet‑dry irrigation coupled with magnesium-biochar fertilizer improves carbon sequestration, NH₃ mitigation, and water‑N use efficiency in rice paddies

The development of low-carbon, water-efficient rice cultivation systems is crucial for sustainable agriculture. Nevertheless, although alternate wet-dry irrigation (I_AWD) can save water and promote the mineralization of soil organic nitrogen (N), it may reduce soil organic carbon (SOC) sequestration and increase ammonia (NH₃) volatilization. To address this trade-off, a magnesium–modified biochar–based fertilizer (MBF) was applied to optimize biochar’s inherently high C:N ratio while enable controlled nitrogen release. A two-year field split-plot experiment was conducted to evaluate the effects of two irrigation regimes (main plots) and five fertilization practices (subplots) on SOC, soil inorganic N, water–N use efficiency, yield, and NH₃ emissions. Results showed that I_AWD combined with N fertilizer reduction and MBF enhanced stem/leaf-to-grain N translocation through the modulation of crop growth rates, increasing yield and grain N use efficiency (NUE_g). Correlation analysis demonstrated that higher soil NH₄⁺ –N suppressed SOC priming, while reduced NH₄⁺–N during basal fertilization and increased SOC limited NH₃ emissions and improved water use efficiency (WUE). Structural equation modeling indicated NH₃ emissions directly reduced NUE and indirectly affected WUE. Compared with conventional fertilization, the treatment of 25 % N reduction combined with 10 t ha⁻¹ MBF (N_3/4B₂) increased SOC by 14.40 %, optimized NH₄⁺–N distribution (reduction during basal fertilization but enhancement during topdressing periods), reduced NH₃ emissions by 10.78 %, and increased yield by 4.82 % and WUE by 10.86 % (two–year averages). Thus, TOPSIS modeling confirmed I_AWDN_3/4B₂ as a sustainable strategy integrating water-saving, yield stability, carbon sequestration, and NH₃ mitigation.