Simultaneous quantification of greenhouse gas and nitric oxide emissions from subtropical conventional vegetable systems: a 2-site field case study in Sichuan Basin

Quantification of greenhouse gases nitrous oxide (N₂O) and methane (CH₄) and nitric oxide (NO) emissions from subtropical conventional vegetable systems through multi-site field measurements are needed to obtain accurate regional and global estimates. N₂O, NO and CH₄ emissions from subtropical conventional vegetable systems were simultaneously measured at two different sites with hilly topography in the Sichuan basin, southwest China by using the static chamber gas chromatography technique. Results showed that annual soil N₂O and NO fluxes for the treatment receiving N fertilizer ranged from 6.34-7.71 kg N ha^-1 yr^-1 and 0.69-0.85 kg N ha^-1 yr^-1, respectively, while decreased soil CH₄ uptakes by 26.4% as compared with no N fertilizer addition across our two sites of experiment. Overall, the average direct N₂O and NO emission factor (EF_d) were 0.71% and 0.12%, respectively, which were both lower than the available EF_d for subtropical conventional vegetable systems. This finding indicates that current regional and global estimates of N₂O and NO emissions from vegetable fields are likely overestimated. Background N₂O emissions (3.42-3.62 kg N ha^-1 yr^-1) from the subtropical conventional vegetable systems were relatively high as compared with available field measurements worldwide, suggesting that background N₂O emissions cannot be ignored for regional estimate of N₂O emissions in subtropical region. Nevertheless, the significantly intra- and inter-annual variations in N₂O, CH₄ and NO emissions were also observed in the present study, which could be explained by temporal variations of environmental variables (i.e. soil temperature and moisture). The differences in N₂O and NO EF_d and CH₄ emissions between various vegetable systems in particular under subtropical conditions should be taken into account when compiling regional or global inventories and proposing mitigation practices.