Contrasting effects of straw and biochar on biologically-based phosphorus, phosphatase activities, and rice phosphorus uptake in two subtropical paddy soils

Rice straw and rice straw-derived biochar can alter soil phosphorus (P) pools, but their effects on biologically based phosphorus (BBP) fractions and rice P uptake in contrasting paddy soils remain unclear. A pot experiment was conducted using two subtropical paddy soils, a clay-rich soil (S1) and a sandy loam soil (S2), under six treatments: P0 (N, K without P), NPK, 0.25% S (0.25% straw + NPK), 0.5% S (0.5% straw + NPK), 1% B (1% biochar + NPK), and 2% B (2% biochar + NPK). BBP fractions ranked HCl-P > Citrate-P > Enzyme-P > CaCl 2 -P. The 2% biochar treatment increased CaCl 2 -P, Citrate-P, and HCl-P in both soils, whereas Enzyme-P responses differed between soils. Despite these increases, 1% biochar resulted in the highest total rice P uptake and P use efficiency (PUE) in both soils, with PUE increases of 309.8% in S1 and 105.4% in S2 relative to NPK. The 0.5% straw treatment increased grain P uptake in both soils. Redundancy analysis (RDA) showed that Citrate-P accounted for 27.8% of the variation in rice P indices in S1, whereas Citrate-P and CaCl 2 -P together accounted for 49.3% of the variation in S2. Citrate-P was strongly positively correlated with grain P uptake in both soils (r = 0.94 in S1 and 0.90 in S2). Acid phosphatase activity was highest under 1% biochar in both soils, whereas alkaline phosphatase responses differed between soils. Overall, BBP responses and rice P uptake varied with amendment rate and soil type, and increases in individual BBP fractions were not consistently accompanied by higher rice P uptake or PUE.