Changes in soil phosphorus fractions in a calcareous paddy soil under intensive rice cropping

We conducted a 4-year field experiment on a calcareous paddy soil in Zhejiang province of China to measure the changes in chemically extracted soil P fractions in an irrigated double-cropping rice system. Treatments included four fertilizer combinations (unfertilized control, NK, NP, and NPK) as main-plots and two rice cultivar types (inbred vs. hybrid rice) as sub-plots. Total plant P uptake and grain yield of rice declined in all treatments over time. Severe P-deficiency and significant rice yield losses began in treatments without P application after the second rice crop. Compared to inbred rice, hybrid rice increased grain yield (+18%), N uptake (+11%) and K uptake (+27%) but there was no significant difference in total plant P uptake. Recovery efficiencies of fertilizer-P averaged 31–32% in both cultivars. In treatments without P application, the P mass balance was negative (−6 to −8 kg P ha⁻¹ crop⁻¹) and phosphorus was drawn down in all inorganic P fractions, including resin, alkali- (NaHCO₃-Pi and NaOH-Pi) and acid-soluble P fractions (dilute HCl-P, concentrated HCl-P, residual-P). Only small amounts were removed from organic P fractions, 1–3 mg P kg⁻¹ year⁻¹ from NaHCO₃-Po and none from NaOH-Po. In treatments with fertilizer-P addition, the P mass balance was positive (+8 to 10 kg P ha⁻¹ crop⁻¹), soil P declined at a slower rate in inorganic P fractions and it increased (+51%) in the residual-P fraction. Hybrid rice generally caused greater depletion of inorganic soil P fractions than inbred rice, but there was no difference among cultivars in their effect on NaHCO₃-Po and NaOH-Po. Positive correlations (r = 0.63–0.81, P < 0.001) were observed between all inorganic P fractions (except residual-P) and total P uptake by rice. Our results suggest that rice plants draw P from a continuum of chemically extracted fractions that are assumed to have widely differing plant P availability. Regular P additions are required to maintain the effective soil P supply and differences between inbred and hybrid rice should be taken into account in P management strategies.