Shift from physico-chemical properties to micro-organisms in driving phosphorus dynamics in paddy-maize transition...

While biochar is recognized for enhancing microbial-mediated phosphorus (P) cycling, its decadal-scale impacts on soil P dynamics during crop transitions remain poorly understood. Here, we report findings from a 9-year field experiment in alkaline alluvial soils of northwestern China, analyzing continuous rice cultivation followed by maize planting in the tenth year under three biochar rates (0, 4500, 13,500 kg ha^-1 year^-1) and two nitrogen (N) levels (0, 300 kg N ha^-1 year^-1). Long-term biochar application enhanced labile P pools (organic P by 25-51 % in rice and 35-43 % in maize; secondary mineral P by 16-50 % across crops) while mitigating N-induced P limitation (34-40 % reduction under N-only treatments). During rice cultivation, physicochemical properties-dominated by soil C:N ratio and cation exchange capacity (CEC)-governed 34.5 % of available P variability. Transitioning to maize shifted regulatory dominance to microbially mediated processes, where alkaline phosphatase activity emerged as a key predictor, with physicochemical-microbial interactions accounting for 53.8 % of P availability variation. Biochar amplified Proteobacteria-driven P mineralization networks in maize soils but maintained stable α-diversity of P-cycling communities across systems. N fertilization exhibited minimal effects on microbial richness despite altering functional gene abundance. This work establishes that long-term biochar-N synergy enhances P sustainability in alkaline soils, with redox-dependent regulatory hierarchies: physicochemical drivers dominate in flooded rice, while microbial-physicochemical interplay governs aerobic maize systems. Our findings provide a mechanistic basis for optimizing biochar use in crop transition zones to alleviate legacy P scarcity and improve nutrient use efficiency.