Inorganic nitrogen release from sediment slurry of riverine and estuarine ecosystems located at different river regimes

The purpose of the present study was to compare the nature of dissolved inorganic nitrogen (DIN = ammonium (NH4+) and nitrite + nitrate (NO2+3 = (NO2– + NO3–)) release from aerobic sediment slurry at two different hydrologic flow regimes. The watershed of the Guadalupe River–Estuary system receives more freshwater inflow than does the watershed of the Nueces River–Estuary system; thus, the Nueces Estuary is more saline than is the Guadalupe Estuary. Sediment samples were collected using cores, analysed for organic matter and grain size, and used to perform laboratory experiments to measure DIN release. During the experiments, DIN concentrations in overlying water were measured for 48 h in five different salinity treatments. Ammonium concentrations were higher in the Nueces River and Estuary treatments than in similarly treated samples from the Guadalupe River and Estuary. An increase in NO2+3 concentrations along salinity gradients of the Nueces Estuary treatments indicated favourable condition for nitrification. The Guadalupe River sediments that were not exposed to salinity had an increase in NH4+ concentration at 7.5 ppt. The different DIN release among salinity treatments indicated that hydrologic forcing on organic matter deposition and salinity have an important role on the retention and release of inorganic nitrogen at the sediment–water aerobic layers in rivers and estuaries.