International audienceBecause of its high sorption affinity on soils solid phase, mitigation options to reduce diffuse P transfer usuallyfocus on trapping particulate P forms delivered via surface flowpaths. Therefore, vegetated buffer zones placedbetween croplands and watercourses have been promoted worldwide, sometimes in wetland areas. To investigatethe risk of such P trapping riparian wetlands (RWs) releasing dissolved P to rivers, we monitored molybdatereactive P (MRP) in the free soil solution of two RWs in an intensively farmed catchment. Two main mechanismscausing MRP release were identified in light of the geochemical and hydrological conditions in the RWs, controlledby groundwater dynamics. First, soil rewetting after the dry summer was associated with the presence of a poolof mobile P, limited in size. Its mobilization started under conditions of water saturation caused by groundwateruprise in RWorgano-mineral soil horizons. Second, the establishment of anoxic conditions in the end of the wintercaused reductive solubilization of Fe oxide-hydroxide, along with release of P. Comparison between sites revealedthat the first MRP release occurred only in a RW with P enriched soils, whereas the second was recorded even ina RW with a low soil P status. Seasonal variations in MRP concentrations in the stream were synchronized withthose in RW soils. Hence, enriched and/or periodically anoxic RWs can act as a key component of the P transfercontinuum in agricultural landscapes by converting particulate P from croplands into MRP released to rivers
