Rainfall deficit and excess constrain phosphorus acquisition and maize yield through contrasting mechanisms

Context Climate-driven abnormal precipitation threatens rainfed maize systems, yet its impact on phosphorus (P) limitation and nutrient synergy remains poorly understood. Objective This study investigated P limitation mechanisms and evaluated whether nitrogen (N) rates and planting density can restore P dynamics under abnormal precipitation. Methods A six-year field experiment (2018–2023) in a Black soil region compared P application (90 kg P 2 O 5 ha −1 ) against a no-P control across four N levels and four densities under deficit, normal, and excessive precipitation. Results Abnormal precipitation induced P limitation, increasing the plant N:P ratio to 18.90 (deficit) and 27.5 (excess); P application mitigated yield losses by 8.71–12.68%. Under deficit, P diffusion was the primary constraint; high N (240 kg ha −1 ) recovered P uptake (explaining 61.67% of variation) by stimulating rhizosphere acidification and acid phosphatase activity. Under excess, reductive release of NaHCO 3 -Pi increased soil Olsen-P, but rhizosphere hypoxia (<2 mg L −1 ) inhibited root H + -ATPase activity by 34.83%, restricting active P transport. High planting density exacerbated this hypoxia, dominating P depletion (65.89% variation), while reducing density to 60,000 plants ha −1 optimized N-P synergy (Nutrient Synergism Index = 1.34). Conclusions Abnormal precipitation disrupts P homeostasis via physical diffusion barriers or metabolic inhibition. Increasing N alleviates deficit-induced P limitation, while reducing density prevents hypoxia-induced P starvation during excess precipitation. Significance Physiological uptake capacity, rather than soil supply alone, determines nutrient availability under precipitation anomalies. These findings advocate for N-P synergy strategies to enhance the resilience of similar rainfed Phaeozem agroecosystems.