Nitrogen Cycling Across a Salinity Gradient From the Pearl River Estuary to Offshore: Insight From Nitrate Dual Isotopes

Dual isotope measurements were performed on nitrate (δ15N‐NO3− and δ18O‐NO3−) in the Pearl River Estuary (PRE) and the adjacent northern South China Sea (NSCS) to investigate nitrate sources and its biogeochemical processes during rainy season. Our results indicated that in the PRE, high nitrate levels were associated with the intense human activities in PRE delta, the sources of which included the reduced nitrogen (N) fertilizer (29%), SN (nitrate derived from soil N) (18%), manure and sewage (10%), and atmospheric deposition (7%). In the nearshore area, nitrate was characterized by relatively high δ15N‐NO3− and δ18O‐NO3− values suggestive of incomplete nitrate assimilation. Besides, nitrification might be significant in the intermediate and bottom waters due to ammonia release from remineralization of sinking and sedimented organic matter (OM). In the offshore area, nitrate was depleted due to nearly complete utilization, and δ15N‐NO3− and δ18O‐NO3− showed moderate values. In addition, δ18O‐NO3− exhibited a clear vertical gradient with a decrease from the surface to the bottom. A coupled nitrification‐denitrification process in sediment porewater was supposed, which left isotope imprints in bottom waters of the offshore area. In surface waters from the nearshore to the offshore area, although assimilation was the prominent process, the δ15N‐NO3− and δ18O‐NO3− relationship deviated from the 1:1 line, suggesting significant contribution of atmospheric deposition that increased offshore. Our study suggests the external source of nitrate shifts from anthropogenic nitrogen to atmospheric source in the continuous river–estuary–ocean system, and nitrification‐denitrification become more active as it goes seaward. Plain Language Summary A comprehensive understanding in nitrogen cycle and its sources in the upper Pearl River Estuary (PRE) to the offshore Northern South China Sea (NSCS) was presented in this study. High nitrate levels were associated with the intense human activities in PRE delta, and the nitrate sources mainly originated from reduced N fertilizer (29%). Incomplete nitrate assimilation and nitrification occurred in the nearshore area. In the offshore area, nitrate was depleted due to nearly complete utilization, and a coupled nitrification‐denitrification process in sediment porewater was supposed in the bottom water. Finally, an increasing contribution of atmospheric deposition to the nitrate sources was found in the surface water from the nearsh