Alternate wetting and drying irrigation with biochar-based struvite enhances phosphorus availability, reduces phosphorus loss potential, and improves yield and water use efficiency in paddy systems

In alternate wetting and drying (AWD) paddy systems, frequent redox fluctuations during the drying phase promote soil phosphorus (P) fixation through pH reduction, while the reflooding phase increases the risk of non-point source P pollution. Biochar-based struvite (BS), a novel P fertilizer with self-regulating alkalinity and slow-release properties, offers a promising solution to mitigate both P fixation and P loss in AWD systems. We conducted a two-year field experiment to evaluate the effects of BS on soil properties, available phosphorus (AP), P utilization, total P (TP) dynamics in surface water, rice yield, and water use efficiency (WUE). Notably, BS was applied only once during the first cropping season to evaluate its residual effects in the second season without additional application. Structural equation model indicated that BS mitigated soil P fixation during the drying phase of AWD irrigation (I_AWD) by increasing soil pH. Meanwhile, BS reduced the sharp increase in TP concentration in surface water during the reflooding phase of I_AWD by increasing soil macroaggregate content. However, when soil available P exceeded a threshold of 20.84 mg kg⁻¹, the TP concentration in surface water significantly increased, highlighting the need for a 25 % reduction in P fertilizer application to minimize the risk of non-point P pollution. Furthermore, BS application not only extended the rapid growth phase of rice but also effectively regulated soil pH due to its self-buffering alkalinity, thereby improving soil P availability. Path analysis revealed that this mechanism facilitated post-anthesis dry matter accumulation and enhanced P translocation to grains, ultimately increasing yield. Under the I_AWD mode, compared to conventional fertilization, the treatment with a 25 % fertilizer reduction combined with 10 t ha^–1 BS (N_3/4B₂ treatment) reduced TP in surface water by 5.95 %, increased yield by 4.82 %, and improved water savings by 6.76 % and WUE by 23.57 % (two-year average). These findings suggest that I_AWDN_3/4B₂ is a reliable strategy for optimizing soil P utilization, guiding rational fertilizer application, and mitigating agricultural P-induced non-point source pollution.