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

Abstract

Indirect nitrous oxide (N2O) 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 N2O emissions from agricultural drainage rivers is poorly explored, especially the uncertainty in quantifying indirect N2O emission factors (EFs) from these aquatic environments. Here, a two-year study (2014-2016) was conducted to quantify indirect N2O emissions from a river draining a rice paddy watershed in subtropical China. Indirect N2O 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 N2O concentration. Results showed that seasonal dissolved N2O concentration and N2O fluxes had a similar variation pattern, with the highest and the lowest levels in summer and winter, respectively. The annual mean of model-based N2O fluxes (20.24 ± 3.34 μmol m−2 d−1) was generally in agreement with chamber-based N2O fluxes (18.70 ± 3.56 μmol m−2 d−1). The indirect emission factor of N2O was highly dependent on the surface water NO3−-N concentration. Annual mean indirect EF of N2O from the drainage river was estimated to be 0.00051, which was significantly lower than the default EF5r 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 N2O emissions from agricultural impacted riverine systems. Our study also highlights that more extensive in-situ measurements are required for monitoring indirect N2O emissions from agricultural impacted waters with different drainage characteristics, which would benefit for refining the IPCC EF5r default value to further constrain global N2O budget.