How nitrate leaching from agricultural lands provokes phosphate eutrophication in groundwater fed wetlands: the sulphur bridge

Increased phosphorus availability may provoke serious eutrophication problems in wetlands. Strong evidence indicates that sulphate induced mobilization of phosphate (internal eutrophication) has been responsible for a strong decline of the biodiversity in wetlands during the last decades. It is currently underestimated, however, that the wide spread leaching of nitrate from agricultural lands can indirectly provoke strong internal phosphate eutrophication in wetlands, via its interference with sulphur and iron biogeochemistry in the subsoil. Nitrate can mobilize sulphate from geological pyrite deposits by the oxidation of FeS_x in the aquifer, leading to a decrease of nitrate and an increase of groundwater sulphate concentrations. Furthermore nitrate immobilizes iron in the subsoil by oxidizing reduced (dissolved) iron. Increased sulphate concentrations may provoke strong phosphate eutrophication in wetlands fed directly or indirectly (via surface water) with groundwater as sulphate strongly interferes with iron phosphorus chemistry and stimulates anaerobic decomposition of organic matter. Management of wetlands should therefore be approached at a broader scale which includes the landscape-scale management of groundwater systems. Leaching of nitrate to the groundwater, for instance, should not only receive attention for its potential effects on drinking water quality but above all because of the resulting large scale mobilization of sulphate from geological pyrite deposits and the immobilization of ferrous iron.