Effects of topography and fire on soil CO2 and CH4 flux in boreal forest underlain by permafrost in northeast China

Regional quantification of soil CO2 and CH4 fluxes in boreal forests remains difficult because of high landscape heterogeneity and fire disturbance coupled with a sparse measurements network. Most of the work focuses on the separate effects of fire or topography, and as a result, the spatial variability in the response of soil carbon flux to fire remains unclear. A two-year field experiment was conducted in the boreal forest of the DaXing'anling Mountains to investigate the effects of topography (ridge and depression) and fire on soil CO2 and CH4 fluxes and to determine how the effects of fire vary with topography. The results showed that soil carbon flux to the atmosphere in this region was dominated by soil CO2 flux. Topography had significant effect on soil carbon fluxes, with higher soil CO2 emissions and CH4 uptakes on the ridge than in the depression, whether burned or not, and these topographical differences were amplified by fire. Fire significantly increased soil CO2 emissions and CH4 uptakes both in the depression and on the ridge. However, the factors that determined soil CO2 and CH4 fluxes and the extent of the response to fire varied with topography. Although the depression released less soil carbon to the atmosphere, the increase in Q10 of soil CO2 flux and the permafrost degradation following fire in the depression indicated stronger positive feedbacks to climate warming. To concluded, topography regulated the effects of fire on soil carbon fluxes and might control the post-fire soil carbon flux feedbacks to climate warming. Therefore, large-scale predictions of soil carbon fluxes in response to fire in the boreal region must explicitly incorporate topographic features.