Groundwater dynamics in wetland soils control the production and transfer mechanisms of dissolved reactive phosphorus in an agricultural landscape
Résumé
Because of its high sorption affinity on soils solid phase, mitigation options to reduce diffuse P transfer usually
focus on trapping particulate P forms delivered via surface flowpaths. Therefore, vegetated buffer zones placed
between croplands and watercourses have been promoted worldwide, sometimes in wetland areas. To investigate
the risk of such P trapping riparian wetlands (RWs) releasing dissolved P to rivers, we monitored molybdate
reactive P (MRP) in the free soil solution of two RWs in an intensively farmed catchment. Two main mechanisms
causing MRP release were identified in light of the geochemical and hydrological conditions in the RWs, controlled
by groundwater dynamics. First, soil rewetting after the dry summer was associated with the presence of a pool
of mobile P, limited in size. Its mobilization started under conditions of water saturation caused by groundwater
uprise in RWorgano-mineral soil horizons. Second, the establishment of anoxic conditions in the end of the winter
caused reductive solubilization of Fe oxide-hydroxide, along with release of P. Comparison between sites revealed
that the first MRP release occurred only in a RW with P enriched soils, whereas the second was recorded even in
a RW with a low soil P status. Seasonal variations in MRP concentrations in the stream were synchronized with
those in RW soils. Hence, enriched and/or periodically anoxic RWs can act as a key component of the P transfer
continuum in agricultural landscapes by converting particulate P from croplands into MRP released to rivers.