Surface nitrous oxide concentrations and fluxes from water bodies of the agricultural watershed in Eastern China

Agriculture is one of major emission sources of nitrous oxide (N2O), an important greenhouse gas dominating stratospheric ozone destruction. However, indirect N2O emissions from agriculture watershed water surfaces are poorly understood. Here, surface-dissolved N2O concentration in water bodies of the agricultural watershed in Eastern China, one of the most intensive agricultural regions, was measured over a two-year period. Results showed that the dissolved N2O concentrations varied in samples taken from different water types, and the annual mean N2O concentrations for rivers, ponds, reservoir, and ditches were 30 ± 18, 19 ± 7, 16 ± 5 and 58 ± 69 nmol L-1, respectively. The N2O concentrations can be best predicted by the NO3--N concentrations in rivers and by the NH4+-N concentrations in ponds. Heavy precipitation induced hot moments of riverine N2O emissions were observed during farming season. Upstream waters are hot spots, in which the N2O production rates were two times greater than in non-hotspot locations. The modeled watershed indirect N2O emission rates were comparable to direct emission from fertilized soil. A rough estimate suggests that indirect N2O emissions yield approximately 4% of the total N2O emissions yield from N-fertilizer at the watershed scale. Separate emission factors (EF) established for rivers, ponds, and reservoir were 0.0013, 0.0020, and 0.0012, respectively, indicating that the IPCC (Inter-governmental Panel on Climate Change) default value of 0.0025 may overestimate the indirect N2O emission from surface water in eastern China. EF was inversely correlated with N loading, highlighting the potential constraints in the IPCC methodology for water with a high anthropogenic N input.