Phosphorus fractions and release potential of ditch sediments from different land uses in a small catchment of the upper Yangtze River

Drainage ditches act as a major conduit of phosphorus (P) from contributing land to receiving waters. The aims of this study were to characterize the phosphorus fractions of ditch sediments from different land uses and to evaluate their potential for P release to provide management strategies for ditch sediment. Sediment samples were collected from ditches within areas of different land use in a small catchment of the upper Yangtze River, China. Non-apatite inorganic P (NAIP), apatite P (AP), organic P (OP), and total P (TP) of these sediments were extracted from these samples and were analyzed. Sediment P release potential was assessed from split-line relationships between sediment test indices and soluble reactive P (SRP) content of the overlying water. Sediment P release rates were quantified through laboratory incubation of intact cores. The TP of the ditch sediment samples ranged from 427 to 717 mg kg(-1), and they mainly consisted of AP (67-81%), NAIP (4-22%), and OP (7-20%). NAIP was a potential source of P release to the water and was chosen as a new criterion for estimating P release potential from sediment, in addition to Olsen P and the degree of P saturation (DPS). A threshold of 80 mg kg(-1) for NAIP in sediment was proposed to estimate the P release potential. The ditch sediments from the residential area acted as a source of water SRP because of its high NAIP (105 mg kg(-1)) and DPS (40%) values. The dynamic pattern of P release from the residential ditch sediments to the overlying water was characterized by a linear regression equation. The P release rate per area from the residential sediment was 0.143 mg P m(-2) day(-1). NAIP is recommended as a new criterion for estimating the potential for P release from ditch sediment. Ditch sediment from residence areas should be treated carefully by cleaning and sediment removal to control the risk of P release. Ditch sediment from paddy fields and forestlands could be used for P export control in headwater catchments. Paddy fields and their ditch sediments at catchment bottoms should be utilized as the main sink for sediment and associated P in the upper Yangtze River.