N_(2)O emission and its influencing factors in subtropical streams, China

Background:Rivers and streams are one of the primary sources of nitrous oxide(N_(2)O)which is an important greenhouse gas with great global warming potential.Yet,over the past century,human activities have dramatically increased reactive nitrogen loadings into and consequently led to increased N_(2)O emission from the river ecosystems.Here,we carried out a study in two subtropical rivers,i.e.,Jinshui River and Qi River with slight and intense human disturbance in their respective catchments in China.The study intended to explore spatial variability and seasonality in N_(2)O emissions,and the relative importance of physicochemical variables,nitrification and denitrification potentials,and functional genes abundance influencing N_(2)O emissions.Results:N_(2)O concentration,N_(2)O saturation,and N_(2)O flux of Jinshui River peaked in high flow season.N_(2)O concentration,N_(2)O saturations,and N_(2)O flux in Qi River and downstream of Jinshui River were significantly higher than that in other areas in normal and low flow seasons.N_(2)O concentration was positively correlated with water temperature,water NO_(3)^(−),and DOC,negatively correlated with water NH4+and DOC/NO_(3)^(−)(the ratio of dissolved organic carbon to NO_(3)^(−)in water),and positively correlated with potential nitrification rate in high flow season,but not correlated with functional genes abundance.Both rivers had lower N_(2)O saturation and flux than many freshwater systems,and their EFr-5(N_(2)O emission factor for river)was lower than the recommended values of IPCC.Conclusions:While the two rivers were moderate sources of N_(2)O and N_(2)O emissions in river systems were normally elevated in the summer,areas with intense human disturbance had higher N_(2)O concentration,N_(2)O saturations,and N_(2)O flux than those with slight human disturbance.Physicochemical variables were good indicators of N_(2)O emissions in the river ecosystems.