Gross Nitrogen Mineralization in Surface Sediments of the Yangtze Estuary

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 × 10(5) 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.

Fig 1. Study area.

This figure shows the location of the Yangtze Estuary and the sampling sites.

Fig 2. GNM rates and percentages of NH4+-N mineralized per day in surface sediments of the Yangtze Estuary.

Vertical bars denote standard error of triplicate samples.

Fig 3. Activities of urease and L-glutaminase in surface sediments of the Yangtze Estuary.

Vertical bars denote standard error of triplicate samples.

Fig 4. Relationships between the extracellular enzymes activities (urease and L-glutaminase) and GNM rates in surface sediments of the Yangtze Estuary.

Fig 5. Relationships of GNM rates with soil TOC and TN concentrations in different ecosystems.

Fig 6. A schematic illustration of different DIN fluxes in the Yangtze Estuary.

Data on F1, F2 and F3 are transformed via changing area according to from Huang et al. (2006), Li (2010), Li et al. (2011), and Kim et al. (2011) [16, 58, 68, 69].