Salinization lowers nutrient availability in formerly brackish freshwater wetlands; unexpected results from a long-term field experiment

Worldwide, coastal freshwater wetlands are facing salinization at an increasing rate due to large-scale land use change, freshwater extraction, climate-driven sea level rise, droughts and land subsidence. Although it is known that increased surface water salinity does influence wetland functioning, effects on nutrient dynamics reported in literature are contradictory and evidence from controlled, long-term field experiments is scarce. We therefore tested the effects of 4 levels of increased surface water salinity, from oligohaline to mesohaline conditions (0.9, 2.25, 4.5, 9 PSU), on biogeochemical and physicochemical processes in the sediment of a formerly brackish freshwater wetland. For this, we used 16 enclosures in a controlled, 5-year field experiment. Salinization unexpectedly led to a dose dependent decreased availability of nitrogen and phosphorus in the sediment, both in the short and in the long term, even though sulfate reduction rates increased. Decreased phosphorus availability was probably caused by co-precipitation with calcium that was mobilized from sediment adsorption sites. Mobilization of ammonium from the sediment and coupled nitrification–denitrification most probably explained decreased nitrogen availability. Increasing sulfate concentrations associated with increased salinity shifted the dominant mineralization process from methanogenesis to sulfate reduction, also in the long term. We show surface water salinization to have major short-term and long-term consequences for the ecological and biogeochemical functioning of coastal freshwater wetlands.