The Effects of Salinization and de-Salinization on Soil Nutrients and Organic Matter in Tidal Freshwater Wetlands

Abstract

Future sea level rise and changes to river discharge due to climate change will periodically alter the tidal freshwater wetland water quality. This will lead to exposure of tidal freshwater wetlands to alternating saline and fresher conditions. The effects of increased and decreased salinity on fresh and saline wetland soils nutrients and organic matter has been measured using a manipulative field-experiment displacing soil samples from saline tidal wetlands to tidal freshwater wetlands and vice versa. Pore water analysis and loss-on-ignition tests of the extracted soil cores have been used to observe changes in nitrate, ammonium, phosphate and organic matter. The soil LOI-analysis did not show a significant change of organic matter when exposed to increased or decreased salinity on a natural site. The fresh conditions on a restored site lowered the amount of organic matter in saline soils. Increased salinity decreases the pore water phosphate and nitrate concentration and increases the soil nitrate concentration. For decreased salinity the soil conditions seem an important factor: the restored wetland conditions reduce phosphates in the pore water, whilst they increase in the natural wetland. On both sites reduced salinity leads to an increase in the ammonium concentration and a reduction in the nitrate concentration. More research is required to pinpoint the main active biological and chemical processes.