Availability Of Soil P In A Rice-Wheat Cropping Rotation In Taihu Lake Region

The research on soil phosphorus?P?was mainly carried out by continuous chemical extraction,however it was insufficient to accurately analyze the phosphorus form,pool and transformation process in-situ,and lacked adequately capture rhizosphere processes.Therefore,an appropriate soil P grading method is very important,this method can scientifically reflect the true distribution and bioavailability of different forms of soil P.Therefore,in this study,based on two reduced P-input experimental stations in Taihu rice-wheat cropping rotation area?including Yixing experimental stations at Changshu National Agro-Ecosystem Observation and Changshu Institute of Agriculture Science?,the biologically-based phosphorus?BBP?grading method was used to evaluate the availability and influencing factors of soil P.Soil P was assessed during the rice-growing season at multiple growing stages;including the seedling stage,the booting stage and the harvest stage,and the wheat-growing season at harvest stage,in the seventh year of the rice-wheat rotation.This P fertilizer reduction field experiments have been conducted for 7 years.The results showed that,for Yixing station,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 fertilization at the wheat season only?PW?and at the rice season only?PR?,compared with soil treated at both the rice and wheat seasons?PR+W?at all three rice-growing stages.Although the application of phosphate fertilizer in one season was reduced,the soil could still provide enough available P.No P fertilization during either season?Pzero?significantly reduced the concentration of soil Citrate-P and HCl-P at the rice-seedling and harvest stages.The four BBP extraction fractions were positively and significantly correlated with soil Olsen-P concentration.The available phosphorus in Yixing was mainly from HCl-P?R²=0.823,P<0.01?,Enzyme-P?R²=0.712,P<0.01?and Citrate-P?R²=0.591,P<0.01?,and partly from CaCl₂-P?R²=0.133,P<0.05?.As for Changshu station,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 PW and PR compared with PR+W at all three rice-growing stages.Pzero treatment significantly reduced the concentration of soil Enzyme-P and HCl-P at the rice-harvest stage.The available phosphorus in Changshu was mainly from Citrate-P?R²=0.196,P<0.01?,CaCl₂-P?R²=0.153,P<0.05?and HCl-P?R²=0.133,P<0.05?.During the wheat-growing season in Yixing station,CaCl₂-P under PW treatment had no significant difference with PR+W treatment,while Citrate-P and HCl-P as well as Enzyme-P under PW treatments were significantly low?P<0.05?.The result indicated that P conversion is rapid under PW treatment,and the required P source for crop growth is still sufficient.As for Changshu station,no significant differences were shown between three P-reduction treatments and the PR+W treatment.The Pzero treatment significantly reduced the concentration of soil HCl-P indicating that the soil phosphate pool in this area is being depleted.Correlation coefficients between four BBP fractions and Olsen-P at two stations were different.The Olsen-P concentration was most from Citrate-P?R²=0.587,P<0.01?,HCl-P?R²=0.587,P<0.01?and Enzyme-P?R²=0.531,P<0.01?in Yixing station,while from HCl-P?R²=0.386,P<0.05?and Citrate-P?R²=0.280,P<0.05?in Changshu station.The concentration of soil P by four BBP fractionation in the seventh rice-wheat rotation was shown as follows:HCl-P>Citrate-P>Enzyme-P>CaCl₂-P.The application of P fertilizers mainly increased the soil inorganic phosphorus pool,while the impact of soil organic phosphorus is not significant.Furthermore,the RDA analysis showed that soil alkaline phosphatase?S-ALP?activity was the dominant factor for the changes of soil P fractionation.In general,these results may improve to characterize and understand changes in soil P status while P-input was reducing,and provide scientific basis for optimal application of phosphorus fertilizer and sustainable development of agriculture in the rice-wheat rotation area of Taihu Lake Region.