The development of anoxic conditions in riparian wetland (RW) soils is widely known to release dissolved phosphorus (DP), but the respective roles of reductive dissolution of Fe-oxyhydroxide, pH changes and sediment inputs in this release remain debated. This study aimed to identify and quantify these respective roles via laboratory anaerobic/aerobic incubation of RW soils with and without the addition of sediment. The investigated soils came from two RWs with contrasting P status and organic matter (OM) content in their soils, while the added sediment came from an adjacent cultivated field. Results showed that the amount and speciation of the DP released during anaerobic/aerobic incubations were controlled by soil P status and soil OM content. During anaerobic incubation, DP release in the soil with high extractable P and low OM contents was controlled by reductive dissolution of Fe-oxyhydroxides (83%), whereas that released in the soil with low extractable P and high OM contents was controlled by an increase in pH (88%). Anaerobic incubation of a mixture of eroded sediments and RW soils increased the release of DP, dissolved organic carbon and Fe(II) (by 16%, 4% and 18%, respectively) compared to the simple addition of the amounts released during their separate incubations. Management practices should decrease soil erosion from upland fields to avoid deposition of P-rich sediments on RW soils. Management efforts should focus preferentially on RWs whose Fe:P molar ratios in the soil solution during reduction are the lowest, since they indicate a high risk that the DP released will be transferred to watercourses.