Nitrite Cycling in Freshwater Ecosystems: A Case Study of an Artificial Reservoir in Eastern China Using Nitrite Dual Isotopes Combined with a Geochemical Model

Reservoirs are hotspots for emissions of the greenhouse gas nitrous oxide; however, the nitrite cycling processes associated with nitrous oxide production therein remain poorly understood, limiting a better assessment of the potential for reservoirs to emit nitrous oxide. Accordingly, this study presents the application of the natural abundance isotope technique combined with a geochemical model to elucidate the nitrite cycling in the freshwater aquaculture and non-aquaculture zones of a large artificial reservoir in eastern China. We employed nitrite dual isotopes to identify nitrite transformation processes. Additionally, a steady-state model was used to estimate the rates of these processes as well as the residence time of nitrite. Our findings indicate that nitrite production in this reservoir may be primarily driven by ammonia oxidation. However, the pathways of nitrite removal differ notably between the aquaculture and non-aquaculture zones, suggesting a significant impact of the aquaculture activities. The steady-state model calculations revealed that nitrification may be more pronounced in the aquaculture zones compared to the non-aquaculture zones, which may be related to the altered balance of competition for substrates between phytoplankton and microbes induced by aquaculture activities. Moreover, we observed a latitude-dependent increase in the significance of nitrite oxidation in natural environments, highlighting potential implications for regional and global nitrogen cycling. Our study highlights the complexity of the nitrite cycle and emphasizes the roles of both natural and anthropogenic factors in shaping nitrogen dynamics within freshwater reservoirs. This understanding contributes to a more accurate assessment of the greenhouse gas emission potential of reservoirs, offering valuable implications for the adoption of sustainable aquaculture practices to mitigate climate impacts and support global sustainable development goals.