Phosphorus Loss Potential and Phosphatase Activity under Phosphorus Fertilization in Long-Term Paddy Wetland Agroecosystems

Phosphorus fertilization may lead to increased P runoff and leaching, contributing to surface water and groundwater pollution. Plots of a 25-yr field study in subtropical China representing eight P fertilization treatments were sampled to determine the effects of P fertilization on P loss potential and correlations among soil test P, soil neutral phosphatase activity, and water P content. Phosphorus fertilization increased soil test P and neutral phosphatase activity in the plow layer compared with P-free fertilization. Total P content of paddy surface water increased with P fertilization, from <0.01 to 0.35 mg L-1. Phosphorus fertilization created P runoff loss risk, especially with the combined application of both inorganic fertilizers and organic amendments. The P runoff loss potential could be reduced or avoided by utilizing physical barriers on field margins to reduce or eliminate runoff. Soil neutral phosphatase activity, soil test P in the 0- to 5-cm layer, and surface water P content were significantly and positively correlated. Soil neutral phosphatase activity contributed to soil test P and the potential of P runoff loss. Fertilization did not affect soil test P or neutral phosphatase activity beneath the plow layer or total P of paddy groundwater at the 100-cm depth. The P content of paddy groundwater was not related to that of surface water, indicating that downward movement of P and P leaching did not occur. The results suggest that paddy wetland ecosystems can store P with proper management, especially for organic amendments.