Nitrogen uptake and nitrification in the Changjiang estuary

Anthropogenic activities have collectively resulted in high loads of nutrients exported into the estuaries, significantly affecting the nitrogen (N) cycling. Yet nitrification and N uptake, the central processes of the N cycle, have rarely been reported simultaneously in the estuaries. Here we report on pelagic nitrification and uptake rates of ammonia (NH₄⁺) and nitrate (NO₃^-) based on ¹⁵N labeling techniques in the Changjiang estuary (CJE). N uptake rates in the surface layer were higher than in the bottom layer, while nitrification rates were the opposite. Light inhibited nitrification from 67.7 % to 100.0 %, while dark depressed NH₄⁺ uptake and NO₃^- uptake from 29.2 % to 79.7 % and 50.9 %-100.0 %, respectively. Phytoplankton and nitrifiers competed for the substrate NH₄⁺, with uptake being more competitive in the surface layer and nitrification being more competitive in the bottom layer. Across the CJE, the rates of nitrification and N uptake were higher at the phytoplankton bloom zone and turbidity maximum zone (TMZ) in intermediate-salinity. This suggests that high NO₃^- concentrations support high productivity, while high productivity and high turbidity enhance the NH₄⁺ regeneration process. The autochthonous NH₄⁺ played a key role in both nitrification and NH₄⁺ uptake processes, and exhibited a higher preference for phytoplankton and a shorter turnover time compared to NO₃^-. Moreover, in-situ nitrification could have contributed between 0.1 % and 50.5 % of the daily phytoplankton NO₃^- requirements in the surface layer. Our study provides a basic knowledge of a central N-cycle process, which were essential for understanding the role of such estuarine ecosystems in the N cycle and their responses to human activities.