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Assessment of indirect emission factors (EF5r) of nitrous oxide (N2O) from agricultural river networks remains challenging, and results are uncertain due to limited data availability. This study compared two methods of assessing EF5r 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. EF5r estimated using method 1 (i.e., EF5r1) was 0.00077 ± 0.00025 (0.00038-0.00097). EF5r calculated using method 2 (i.e., EF5r2) was lower than EF5r1, with a mean value of 0.00004 (0.000015-0.00012). Both EF5r1 and EF5r2 were significantly lower than the IPCC 2006 default value of 0.0025, suggesting that N2O emissions from China and world river networks may be grossly overestimated. A complex N2O production pathway and diffusion mechanism were responsible for the transfer of N2O 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.

Methane (CH4) emitted from river networks provides a globally significant, yet poorly constrained barrier for the refinement of national greenhouse gas (GHG) inventories and the global CH4 budget. This study employed three isotopic methodologies, i.e., a simple mixing model, apparent fractionation factor, and dual isotopes (δ13C−CH4 and δD−CH4) to identify the biogenic CH4 production pathway (BMPP) in a typical agricultural river network in subtropical China. The results indicated that the contribution of acetoclastic methanogenesis (AM) to BMPP in the Tuojia River network (TRN) ranged from 85 to 99%, with the first reach (S1) supporting the most biogenic CH4 production. Furthermore, the αC value ranged from 1.00 to 1.039 and increased spatially over the four reaches (S1 to S4), while the αD value varied from 1.393 to 1.394, with S1 having the lowest value. The dual isotopes of δ13C and δD reflected a mixed BMPP. Clearly, the isotopic composition of CH4 corroborated the predominance of AM in the BMPP in the TRN. Additionally, there was a remarkable spatiotemporal variation in δ13C−CH4 and δD−CH4, as well as αC and αD. In combination with the correlation between the two paired isotopic signatures (antipathetic and positive, p

Rice cultivar is the most influential factor affecting methane emissions from double rice fields. A two-year field experiment was conducted at Huizhou, Guangdong province, South China, to identify from among nine cultivars those cultivars with high-yield potential and lower yield-scaled methane emissions (YSMEs). Methane emissions were measured using the static chamber – gas chromatograph method. Results indicate that the cultivars Qihuazhan (QH), Yexianzhan 8 (YX8) and Yue’erzhan (YE) provide higher rice grain yield (8.69%) with lower YSME (30.27%) compared to the other six cultivars (Yexianzhan 6, Yuejingsimiao, Hefengzhan, Huangsizhan, Huangruanzhan and Huangxiuzhan) (p