Soil C/N ratio is the dominant control of annual N2O fluxes from organic soils of natural and semi-natural ecosystems

Globally, organic soils of natural and semi-natural ecosystems have been considered as an important source of atmospheric nitrous oxide (N2O), a powerful greenhouse gas. However, we have little understanding of how and to what extent the magnitude of N2O emissions from such soils is mediated by variation in environmental controls. This knowledge is critical for scaling up the results of measurements from local to regional or global scales. Here we report on two-year field measurements of N2O fluxes from various alpine land use/cover types at 24 sites across an altitudinal gradient in a catchment on the eastern Tibetan Plateau. We found that annual N2O emissions varied between 0.05 and 1.39 kg N ha(-1) yr(-1), with non-growing season contributing 11-60% to these annual budgets. Our field results, together with published data from 43 studies on organic soils globally (natural and semi-natural ecosystems) showed that across all datasets, annual N2O fluxes were more closely related to the soil C/N ratio. Weaker relationships were found to the mean water table depth (WTD) and soil total N content. In contrast to the general assumption that soil N2O emissions may consistently increase with a narrowing of the organic soil C/N ratio, we found strong indications that the relationship of annual N2O fluxes to soil C/N ratio followed an optimum Gaussian curve, with a threshold at a C/N ratio of about 18-19. Moreover, there was a tendency towards high N2O emissions for sites with a mean annual WTD > 0.15 m, indicating a potential risk for increased N2O emissions from global organic soils under water table drawdown driven by climate drying or peatland drainage. Overall, our results suggest that the soil C/N ratio could be used as an indicator to better constrain the contribution of organic soils to N2O emissions from landscape at global scales.