Abstract
Nitrogen mineralization is a key biogeochemical process transforming organic nitrogen to inorganic nitrogen in estuarine and coastal sediments. Although sedimentary nitrogen mineralization is an important internal driver for aquatic eutrophication, few studies have investigated sedimentary nitrogen mineralization in these environments. Sediment-slurry incubation experiments combined with 15N isotope dilution technique were conducted to quantify the potential rates of nitrogen mineralization in surface sediments of the Yangtze Estuary. The gross nitrogen mineralization (GNM) rates ranged from 0.02 to 5.13 mg N kg-1 d-1 in surface sediments of the study area. The GNM rates were generally higher in summer than in winter, and the relative high rates were detected mainly at sites near the north branch and frontal edge of this estuary. The spatial and temporal distributions of GNM rates were observed to depend largely on temperature, salinity, sedimentary organic carbon and nitrogen contents, and extracellular enzyme (urease and L-glutaminase) activities. The total mineralized nitrogen in the sediments of the Yangtze Estuary was estimated to be about 6.17 × 105 t N yr-1, and approximately 37% of it was retained in the estuary. Assuming the retained mineralized nitrogen is totally released from the sediments into the water column, which contributed 12–15% of total dissolved inorganic nitrogen (DIN) sources in this study area. This result indicated that the mineralization process is a significant internal nitrogen source for the overlying water of the Yangtze Estuary, and thus may contribute to the estuarine and coastal eutrophication.
Highlights
Estuarine and coastal ecosystems are important transitional zones between terrestrial and ocean ecosystems, playing a significant role in nitrogen biogeochemical cycle [1]
The lowest gross nitrogen mineralization (GNM) rate was detected at site S05, while the highest GNM rate was observed at S04
The highest GNM rate appeared at site S01, whereas the lowest GNM rate occurred at site S14
Summary
Estuarine and coastal ecosystems are important transitional zones between terrestrial and ocean ecosystems, playing a significant role in nitrogen biogeochemical cycle [1]. Sediments are important sites for organic matter accumulation and nutrient cycling in estuarine and coastal environments [6]. Numerous researches have reported that nitrogen mineralization of sediments is a potential source of dissolved inorganic nitrogen (DIN) releasing into overlying water [8, 10,11,12]. Nitrogen mineralization is likely an important nitrogen internal source and contributes to exacerbation of eutrophication in these ecosystems. Quantifying the internal nitrogen mineralized from sedimentary organic matter may have potential implications for understanding the nitrogen budgets in estuarine and coastal ecosystems
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