Indirect nitrous oxide emission factors of shallow eutrophic lakes: the importance of dissolved organic carbon and nitrate

Abstract Lakes are an important source of nitrous oxide (N 2 O). However, the indirect N 2 O emission factor (EF) model, applied by the Intergovernmental Panel on Climate Change (IPCC) for estimating inland water N 2 O emissions, is inadequate for lakes due to uncertainties in the N 2 O production pathways and significant spatiotemporal variations in EF values. Herein, we combined in situ N 2 O emission investigations, metagenomic sequencing, and isotopic mapping techniques to determine the pathway of N 2 O production and environmental control of EF values in shallow eutrophic lakes along the middle and lower reaches of the Yangtze River. Dissolved N 2 O concentration exhibited a significant correlation with nitrate (NO 3 − -N), coupled with a positive relationship between the ( nirK + nirS )/ nosZ ratio and N 2 O fluxes. Furthermore, a δ 15 N SP - δ 18 O N2O mapping approach model verified that 94.0 ± 9.1% of the total N 2 O production was contributed by denitrification. These observations jointly demonstrated that denitrification dominated the N 2 O production in the studied lacustrine system. Based on these findings, the nitrogen source (NO 3 − -N) concentration for denitrification and the ratio of the carbon source (dissolved organic carbon (DOC)) to NO 3 − -N concentration (DOC/NO 3 − -N) were incorporated into a model, both of which showed high correlations with EF values in the studied lacustrine system and global meta-analysis dataset. This model, which provides a good fit, corrects N 2 O emission overestimations from IPCC models by using easily obtained water quality variables.