Mechanism of nitrous oxide (N2O) production during thermal stratification of a karst, deep-water reservoir in southwestern China

Reservoirs are hotspots of greenhouse gas (GHG) emission, and the global warming potential of nitrous oxide (N2O) is higher than that of CO2 and CH4. To promote our understanding of the relationship of biotic and abiotic environments with the mechanism of N2O production in a quintessential karst reservoir, water samples were collected in a weak thermal stratification period (WTSP) and a strong thermal stratification period (STSP) in 2019 in the Longtan (LT) Reservoir. In the LT Reservoir, higher N2O concentrations and functional gene abundances (nitrification and denitrification genes) were found in the STSP than in the WTSP. In detail, the N2O concentrations were 10.45 ± 6.99 nmol L−1 and 23.77 ± 7.06 nmol L−1 in the WTSP and STSP, respectively; the functional gene abundances were higher in the STSP than in the WTSP in each water layer. During the STSP, the nitrification gene abundances were higher at a 30 m depth than at a 100 m depth, while denitrification gene abundances were higher at 100 m than at 30 m. Along the river, N2O flux was significantly higher in the LT Reservoir's released water than in upstream water in the STSP, while it exhibited no significant difference in whole river-reservoir system in the WTSP. In conclusion, stratification and nitrogen (N) loading facilitated higher N2O production in STSP than in WTSP; different N transformation processes and sources of N2O were present between the water layers above and below the thermocline (in deep water); finally, the river-reservoir system here acted as a source of N2O release to the atmosphere in the STSP, while it exhibited different emission patterns in the WTSP. This work highlights the mechanism of N2O production and functional gene abundances in water layers during different thermal stratification periods, which will help to elucidate the biotic and abiotic environment in a karst deep-water reservoir and will help to inform N2O production mechanisms in other reservoirs worldwide. More sampling campaigns and the denitrification potential of sediment were worthy of conducting to comprehend the mechanisms of N2O production from stratification to turnover period in the reservoir.