In agricultural watersheds across China, small waterbodies serve as important sites for the retention and storage of phosphorus, acting as protective barriers for rivers and lakes. However, under conditions of long-term phosphorus accumulation and specific environmental factors, there is a risk of phosphorus being released. Currently, research on the risk of phosphorus release from small water bodies, and on the dynamic changes in phosphorus retention and release and the influencing factors through long-term experiments, is lacking. This study focuses on a typical agricultural watershed, conducting sediment sampling from four types of pond alongside a six-month static incubation experiment. The study analyses the phosphorus forms in sediments and the phosphorus exchange dynamics at the sediment-water interface to assess the risk of phosphorus release from the ponds and its influencing factors. The results showed that: (1) The total phosphorus (TP) content of pond sediments was higher than that of other types of wetland, ranging from 0.55 to 3.02 g/kg. The highest phosphorus content was found in village ponds, while the lowest was observed in paddy ponds. (2) In the agricultural watersheds of the middle and lower reaches of the Yangtze River and in ponds that have existed for a long time, phosphorus exchange at the sediment-overlying water interface exhibits significant seasonal variation: sediments release phosphorus to the overlying water in summer and autumn, while phosphorus settles from the overlying water to the sediments in autumn and winter. For ponds of a similar age, the phosphorus sink capacity decreased in the following order: forest pond ＞ paddy pond = dryland pond ＞ village pond. This study revealed large accumulations of phosphorus in agricultural ponds in China, highlighting their long-term function as phosphorus sinks which transform into sources during summer and autumn. The findings also clarified the influence of surrounding land use types and provided scientific guidance for improving management strategies for different pond types. These insights are applicable to the control of non-point source pollution and the protection of aquatic environments in rivers and lakes.