Nitrous oxide (N2O) emissions at the air-water-sediment interfaces of cascade reservoirs in the Yunnan-Guizhou Plateau: Spatial patterns and environmental controls.

ABSTRACT

The construction of cascade reservoirs can interfere with the natural hydrologic cycles of basins, causing negative environmental effects such as altering the emission patterns of the Nitrous oxide (N2O), a potent greenhouse gas. To elucidate the impact of cascade reservoirs construction on river N2O emissions, we utilized the thin boundary model and the incubation experiments to estimate the N2O fluxes at the air-water interface and at the water-sediment interface of cascade reservoirs on the Yunnan-Guizhou Plateau, respectively. Additionally, we explored the influence of various factors, with particular emphasis on damming, on N2O emissions and production. Moreover, we identified the main pathways of N2O production and proposed management strategies to mitigate N2O emissions from cascade reservoirs. The findings revealed that N2O fluxes at the air-water interface and the water-sediment interface were 4.73 ± 1.32 µmol m-2 · d-1 and 15.56 ± 1.98 µmol m-2 · d-1, respectively. Influenced by temperature, dissolved oxygen (DO), resource substances (active nitrogen substrates and dissolved organic carbon (DOC)) and reservoir properties (scale, hydraulic retention time (HRT), reservoir age, etc.), the N2O concentration and flux exhibited notable spatial heterogeneity, gradually increasing downstream. Temperature has a significant direct impact on N2O flux, as well as indirect effects through DO and resource chemicals. Furthermore, the correlation between dissolved oxygen utilization rate (AOU) and net N2O flux (ΔN2O) indicated that N2O emissions at the water-air interface were primarily attributable to nitrification, whereas those at the water-sediment interface were predominantly driven by denitrification. These findings not only enhance our comprehension of N2O emissions at various interfaces of cascade reservoirs but also offer theoretical backing for the formulation of management strategies aimed at efficiently mitigating N2O emissions from continuously dammed rivers.