Assessment of Indirect NO Emission Factors from Agricultural River Networks Based on Long-Term Study at High Temporal Resolution.

Assessment of indirect emission factors (EF) of nitrous oxide (NO) from agricultural river networks remains challenging, and results are uncertain due to limited data availability. This study compared two methods of assessing EF using data from long-term observations at high temporal resolution in a typical agricultural catchment in subtropical central China. The concentration method (method 1) and the Intergovernmental Panel on Climate Change (IPCC) 2006 method (method 2) were employed to evaluate the emission factor. EF estimated using method 1 (i.e., EF) was 0.00077 ± 0.00025 (0.00038-0.00097). EF calculated using method 2 (i.e., EF) was lower than EF, with a mean value of 0.00004 (0.000015-0.00012). Both EF and EF were significantly lower than the IPCC 2006 default value of 0.0025, suggesting that NO emissions from China and world river networks may be grossly overestimated. A complex NO production pathway and diffusion mechanism were responsible for the transfer of NO from the sediment to river water and then to the atmosphere. These findings provide essential data for refining national greenhouse gas inventories and contribute evidence for downward revision of indirect emission factors adopted by the IPCC.