Spatial-temporal variability of indirect nitrous oxide emissions and emission factors from a subtropical river draining a rice paddy watershed in China.

• IndirectN2Oemissions from rivers draining agricultural watersheds are of increasing concerns. • Seasonal dissolved N2O concentrationsand N2O fluxes showed a similar variation pattern. • Annual mean model-based N2O fluxes werecomparable with chamber-based N2O fluxes. • The indirect riverine EFof N2O dependedlargely on surface water NO3–N concentrations. • Annual mean riverine indirect EF of N2O was significant lower than the IPCC default value. Indirect nitrous oxide (N 2 O) emissions from rivers draining agricultural watersheds are of increasing concerns due to riverine abundant sources of nitrogen loaded through leaching and runoff. However, the seasonal variation of N 2 O emissions from agricultural drainage rivers is poorly explored, especially the uncertainty in quantifying indirect N 2 O emission factors (EFs) from these aquatic environments. Here, a two-year study (2014-2016) was conducted to quantify indirect N 2 O emissions from a river draining a rice paddy watershed in subtropical China. Indirect N 2 O fluxes were simultaneously determined using the floating chamber method (chamber-based) and the gas exchange modeling approach (model-based) based on the measurement of dissolved N 2 O concentration. Results showed that seasonal dissolved N 2 O concentration and N 2 O fluxes had a similar variation pattern, with the highest and the lowest levels in summer and winter, respectively. The annual mean of model-based N 2 O fluxes (20.24 ± 3.34 μmol m−2 d−1) was generally in agreement with chamber-based N 2 O fluxes (18.70 ± 3.56 μmol m−2 d−1). The indirect emission factor of N 2 O was highly dependent on the surface water NO 3 −-N concentration. Annual mean indirect EF of N 2 O from the drainage river was estimated to be 0.00051, which was significantly lower than the default EF 5r value (0.0025) proposed by the Intergovernmental Panel on Climate Change (IPCC). These results suggest that the use of IPCC default value might have overestimated indirect N 2 O emissions from agricultural impacted riverine systems. Our study also highlights that more extensive in-situ measurements are required for monitoring indirect N 2 O emissions from agricultural impacted waters with different drainage characteristics, which would benefit for refining the IPCC EF 5r default value to further constrain global N 2 O budget. [ABSTRACT FROM AUTHOR]