Risks of phosphorus runoff losses from five Chinese paddy soils under conventional management practices

Phosphorus (P) runoff from arable land is a major cause for eutrophication of many surface waters. However, relatively little research has been conducted on managing P in rice (Oryza sativa L.) production systems, where fanning practices differ from those of upland cropping systems due to water ponding on the soil surface (field ponding water; FPW). Because FPW is a direct source of surface runoff, identifying the main source of P and the critical period of high P concentrations in the FPW provide important insights to mitigating P runoff losses. In this study, field monitoring and laboratory incubation experiments were combined to evaluate how soil P content and conventional P fertilizer application affected FPW P concentrations in rice-wheat (Triticum aestivum L.) rotation systems of five Chinese rice producing regions. All soils had Olsen-P concentrations (10.1-20.5 mg kg(-1)) well below the critical levels (30-172 mg kg(-1)) for promoted risks of P loss. However, conventional P application rate significantly elevated FPW P concentrations compared to no P application, and P fertilizer contributed 47-92% of total P (TP) and 59-97% of total dissolved P (TDP) in the FPW. Temporarily, both TP and TDP concentrations peaked one day after P application (0.15-8.90 mg TP L-1 and 0.16-4.49 mg TDP L-1), then decreased rapidly and stabilized five days later. We conclude that fertilizer is the major source of P loss in Chinese rice production systems, and that P fertilizer rate should be optimized to reduce P concentrations in the effluent water in the first week following P application.