Paddy-field areas are among the major non-point sources of phosphorus (P). Drainage canals can influence P concentration in drainage water because the sediments can absorb P and have large P buffering capacity. Assessment of the P buffering capacity and management of drainage canal sediments are important to reduce P effluent loadings from paddy-field areas. In this study, we investigated three paddy-field districts around Lake Biwa. Using adsorption and incubation experiments, we clarified the P adsorption and release characteristics of the sediments. The concentrations of total sedimentary P were highest in the surface layer of the sediments. The zero-equilibrium P concentrations of the sediments, estimated from the P adsorption experiment, varied widely among sites, and were similar to the peak concentrations of P released from the incubated sediments and the mean P concentrations in the drainage water (0.021 to 0.068 mgP/L). These results suggest that the sediments determine the concentrations of P in the overlying drainage water. Adsorption isotherms for dry sediments were described by a linear model with higher slope than wet sediments, indicating that drying drainage canals might enhance the P buffering capacity of sediments.
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