Nitrogen Losses in Sediments of the East China Sea: Spatiotemporal Variations, Controlling Factors, and Environmental Implications

Global reactive nitrogen (N) has increased dramatically in coastal marine ecosystems over the past decades and caused numerous eco-environmental problems. Coastal marine sediment plays a critical role in N losses via denitrification and anaerobic ammonium oxidation (anammox) and release of nitrous oxide (N₂O). However, both the magnitude and contributions of denitrification, anammox, and N₂O production in sediments still remain unclear, causing uncertainty in defining the N budget for coastal marine ecosystems. Here potential rates of N losses, and their contributions and controlling factors, were investigated in surface sediments during six cruises from 429 sites of the East China Sea. The potential rates of denitrification, anammox, and N₂O production varied both spatially and seasonally, but the contribution of anammmox to total N₂ production (%anammox) and N₂O:N₂ ratio only varied spatially. Both organic carbon and nitrate (NO₃ ⁻) were important factors controlling N losses, N₂O:N₂ ratio, and %anammox. Our results also showed that marine organic carbon induced by eutrophication plays an important role in stimulating reactive N removal and increasing N₂O production in warm seasons. The sediment N loss caused by denitrification, anammox, and N₂O production in the study area were estimated at 2.2 × 10⁶ t N yr⁻¹, 4.6 × 10⁵ t N yr⁻¹, and 8 × 10³ t N yr⁻¹, respectively. Although sediments remove large quantities of reactive N, they act as an important source of N₂O in this region influenced by NO₃ ⁻-laden rivers.