Stability of Aquatic Nitrogen Cycle Under Dramatic Changes of Water and Sediment Inflows to the Three Gorges Reservoir

Stability of nitrogen cycle is a key indicator to aquatic health. In recent years, water and sediment inflows to the Three Gorges Reservoir (TGR) have changed significantly. To reveal the effects of such dramatic hydrological changes on aquatic nitrogen cycle, this paper at first analyzed the changing trends of water and suspended sediment discharges of TGR based on dynamic harmonic regression, and found that the intra‐year distribution of water flow was significantly homogenized between flood and dry seasons, with the seasonal variations narrowed by 43.5%–69.9% during 2007–2016, while sediment concentration sharply dropped (the non‐periodic term decreased by 1.48%–2.07%/month). Modified with the effects of sediment concentration variations on nitrification/denitrification rates, the proposed numerical model surprisingly showed that ammonia nitrogen and total nitrogen concentrations in TGR were insensitive to either water flow homogenization or sediment reduction, implying relative stability of microbial community related to nitrogen cycle, which is a positive sign for aquatic health. However, N2 emission varied more violently. The variation range of nitrogen gas (N2) emitted from TGR enlarged by 30% with the homogenization of water inflow from 2010 to 2016, while the annual total N2 emission decreased by 7% due to the reduction of sediment concentration, indicating quick response and strong adaption of the microbial N2 producing process to the environmental changes of TGR, which is beneficial for maintaining ecological functions related to nitrogen cycling. This work helps understanding nitrogen cycle of reservoirs experiencing dramatic changes in water and sediment inflows. Plain Language Summary Nitrogen cycle is a global issue related to environmental health. The hydrological conditions of water bodies are responsible for water health with nitrogenous substances as indicators. Considering the significantly changed water and sediment inflows to the Three Gorges Reservoir (TGR), the effects of such dramatic hydrological changes on nitrogen cycle were revealed. We developed a numerical model, modified with the effects of sediment concentration variations on nitrogen cycle, to estimate the response of nitrogenous substances to the changed hydrological conditions. The insensitive ammonia nitrogen and total nitrogen concentrations were obtained to either the flattened annual water flow process or the sharply reduced sediment concentration in the inflows to t