Varying water column stability controls the denitrification process in a subtropical reservoir, Southwest China.

Reservoirs are regarded as hotspots of nitrogen transformation and potential sources of nitrous oxide (NO). However, it remains unclear how the hydrological conditions due to dam construction control the processes of nitrogen transformation in reservoir waters. To address this issue, we examined the spatial-temporal characteristics of nitrate concentrations, δN-NO, δO-NO, δO-HO, relative water column stability (RWCS), and related environmental factors in a subtropical eutrophic reservoir (Hongfeng Reservoir, HFR), Southwest China. We found that denitrification was the most important nitrogen transformation process in the HFR and that higher denitrification intensity was associated with increased RWCS in summer, which suggested hydrological control of the denitrification process. In contrast, low RWCS conditions favored the nitrification process in the HFR in winter. Additionally, dissolved oxygen (DO; p < 0.05) and nitrate concentrations (p < 0.01) had significant impacts on the denitrification rate. We also found that the spatiotemporal RWCS variations were a prerequisite for regulating DO/nitrate stratification and the coupling/decoupling of nitrification-denitrification at the local and global scales. This study would advances our knowledge of the impacts of RWCS and thermal stratification on nitrogen transformation processes in reservoirs.