The use of biologically based phosphorus fractions to evaluate soil P availability in reduced P-input paddy soils

Chemical soil phosphorus (P) extraction has been widely used to characterize and understand changes in soil P fractions; however, it does not adequately capture rhizosphere processes. In this study, we used the biologically based phosphorus (BBP) grading method to evaluate the availability and influencing factors of soil P under four P fertilizer regimes in a typical rice-wheat cropping rotation paddy field. Soil P was assessed after seven rice-growth seasons at multiple growth stages: the seedling, the booting and the harvest stage. Soil CaCl2-P, citrate-P and HCl-P (inorganic P, Pi) as well as enzyme-P (organic P, Po) were not significantly different between soil treated with P fertilizer during the wheat season only (PW) and during the rice season only (PR) compared with soil treated during both the rice and the wheat seasons (PR+W) at all three rice-growth stages. No P fertilizer application during either season (Pzero) significantly reduced the concentration of soil citrate-P and HCl-P at the rice-seedling and harvest stages. Significant correlations were observed between the HCl extraction and Olsen-P (R-2=0.823, P<0.001), followed by enzyme-P (R-2=0.712, P<0.001), citrate-P (R-2=0.591, P<0.001) and CaCl2-P (R-2=0.133, P<0.05). Further redundancy analysis (RDA) suggested that soil alkaline phosphatase (S-ALP) activity played a role in soil P speciation changes and was significantly correlated with enzyme-P, citrate-P and HCl-P. These results may improve our ability to characterize and understand changes in soil P status while minimizing the overapplication of P fertilizer.