Effects of algal-derived organic matter on sediment nitrogen mineralization and immobilization in a eutrophic estuary

Nitrogen (N) mineralization and immobilization are two important N transformation processes in estuarine sediments. However, the biotic and abiotic mechanisms regulating those two processes in sediments of a eutrophic estuary remains poorly understood. Here, we used N-15 stable isotope dilution technique to investigate the spatial and temporal variations of gross N mineralization (GNM) and gross NH4+ immobilization (GAI) in surface sediment (0-5 cm) along the Pearl River Estuary (PRE) in 4 cruises. Results showed that GNM rates varied spatially and increased with salinity, ranging from 0.15 to 1.99 mu g N g(-1) d(-1), while GAI varied more temporally and increased with temperature, ranging from 0 to 1.96 mu g N g(-1) d(-1). The net N mineralization (NNM) rates ranged from-0.24 to 1.84 mu g N g(-1) d(-1), NNM was higher in lower estuary mainly due to higher chlorophyll-induced higher GNM, while NNM was higher in low temperature season mainly due to the inhibition of GAI. As high turbidity in upper estuary reduced light penetration and hence primary production, the decrease of settling labile organic matter from the water column weaken benthic bacterial abundances, GNM and NNM rates, whereas inorganic N (especially NH4+) had greater effect on GAI than organic matter. As an NH4+ source, the contribution of sediment daily NNM to NH4+ and dissolved inorganic N (DIN) in the overlying water column increased from upstream to downstream (-5.81 to 110.74% and-0.09 to 1.27%, respectively), suggesting an increased importance of NNM for supporting primary production in the lower estuary. Our study reveals the dynamics of net NH4+ production by distinguishing N mineralization and immobilization processes, and enhances the understanding of benthic-pelagic coupling influence on N-cycling in a eutrophic estuary.