Phosphorus (P) release is key to improving P availability in paddy soil, but the effects of water-saving irrigation and biochar on paddy P release remain unclear. To address this gap, we used high-resolution techniques (diffusive gradients in thin films (DGT), high-resolution dialysis (HR-Peeper), zymography, planar optoelectronic technology) and two models (DGT-induced fluxes in soil (DIFS), sediment P release risk index (SPRRI)) to evaluate P release kinetics and its mechanisms under different treatments: biochar application rates (0, 20, 40 t ha−1) and irrigation regimes (water-saving irrigation, flooding irrigation). We also assessed the risk grade of P release. Results showed that flooding irrigation caused significantly more P release than water-saving irrigation, due to the strong reducing effect of Fe (hydr) oxides. This was supported by higher average concentrations of soluble P/Fe and labile P/Fe, higher desorption rates, a higher ratio of labile P to soluble P, and strong correlations between soluble P and soluble Fe, as well as labile P and labile Fe in flooded paddies. However, this intense P release led to a moderate P release risk grade, increasing the risk of P loss. Under water-saving irrigation, 40 t ha−1 biochar increased labile P by 10.24%–34.48 % and soluble P by 31.28 %–55.43 %, while reducing P release risk compared to no biochar addition. In conclusion, combining 40 t ha−1 biochar with water-saving irrigation is an effective strategy to simultaneously "enhance P availability and reduce P loss risk". This study provides new insights into the biogeochemical dynamics of P in paddies under combined biochar and water-saving irrigation.
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