Magnitude and Edaphic Controls of Nitrous Oxide Fluxes in Natural Forests at Different Scales

Forest nitrous oxide (N2O) emission plays an important role in the greenhouse gas budget of forest ecosystems. However, spatial variability in N2O fluxes complicates the determination of key factors of N2O fluxes at different scales. Based on an updated database of N2O fluxes and the main edaphic factors of global forests, the magnitude of N2O fluxes from forests and the relationships between edaphic factors and N2O fluxes at different scales were analyzed. According to the results, the average annual N2O flux of the global forest was 142.91 +/- 14.1 mg N m(-2) year(-1). The range of total forest estimated N2O emission was 4.45-4.69 Tg N in 2000. N2O fluxes from forests with different leaf traits (broadleaved and coniferous) have significant differences in magnitude, whereas the leaf habit (evergreen and deciduous) was an important characteristic reflecting different patterns of N2O seasonal variations. The main factors affecting N2O fluxes on the global scale were ammonium (NH4+) and nitrate (NO3-) concentrations. With an increasing scale (from the site scale to the regional scale to the global scale), the explanatory power of the five edaphic factors to N2O flux decreased gradually. In addition, the response curves of N2O flux to edaphic factors were diversified among different scales. At both the global and regional scales, soil hydrothermal condition (water filled pore space (WFPS) and soil temperature) might not be the main spatial regulation for N2O fluxes, whereas soil nutrient factors (particularly NO3- concentration) could contribute more on N2O flux spatial variations. The results of site-control analysis demonstrated that there were high spatial heterogeneity of the main N2O controls, showing N2O fluxes from low latitude forests being more likely associated with soil WFPS and temperature. Thus, our findings provide valuable insights into the regulatory edaphic factors underlying the variability in N2O emissions, when modeling at different scales.