Biochar co-application with nitrogen and phosphorus fertilizers modulates maize growth and physiological performance by reshaping soil P pools

Nutrient limitation, particularly of phosphorus (P) and nitrogen (N), remains a major constraint on maize (Zea mays L.) productivity in tropical and subtropical soils due to high P fixation and N losses. This study investigated the interactive effects of biochar (BC), N, and P fertilization on maize growth, biochemical responses, and soil P fractionations. Seven treatments (control, BC, N, P, BC-N, BC-P, and BC-NP) were established in a randomized complete block design. Biochar application significantly enhanced plant height and grain yield compared with the control. Combined treatments, particularly BC-P and BC-NP, markedly increased chlorophyll, carbohydrates, and soluble sugars, indicating improved photosynthetic performance. Antioxidant enzyme activities also rose substantially, with catalase (CAT), peroxidase (POD), and superoxide dismutase (SOD) increased under BC-NP. Proline accumulation doubled compared to the control, demonstrating enhanced osmotic adjustment and redox stability. Nutrient uptake analysis showed the highest P assimilation in BC-P (15.5 mg plant⁻¹) and balanced N-P use efficiency in BC-NP. Sequential fractionation revealed that BC-P and BC-NP enriched labile and stable P pools: Ca₂-P and Ca₈-P increased by up to 165% under P fertilization, while Ca₁₀-P, Fe-P, and Al-P were most abundant in BC-NP, reflecting strengthened mineral-bound P stabilization. Total inorganic and total P levels peaked in integrated treatments (786.9 and 986.0 mg kg⁻¹, respectively), whereas BC alone promoted organic P stabilization (306.3 mg kg⁻¹). Correlation and random forest analyses identified chlorophyll, carbohydrate, and peroxidase activity as dominant predictors of total soil P (R² = 0.49), underscoring the strong physiological coupling between nutrient assimilation and P cycling. Overall, biochar co-applied with N and P fertilizers improved maize productivity, nutrient retention, and biochemical resilience, offering an efficient and sustainable strategy for optimizing P management and soil fertility in maize-based systems.