Prescribed Fire and Soil Carbon Sequestration
Understanding how forest management influences carbon storage has become an important topic in the light of concerns about global warming and potential opportunities for carbon credit trading. We have continued work using the Tall Timbers Fire Ecology Plots (Stoddard Plots) long-term study to test the response of total soil carbon to 1, 2, and 3-year fire intervals as well as fire-exclusion for over forty years. We are also interested in how restoring agricultural fields to forest while using prescribed fire influences soil carbon storage. The Stoddard Plots are thought to be old-field (post-agriculture) forests, so we compared their total carbon levels with current fields that have been tilled for several decades to infer carbon accumulation with periodic burning following field abandonment. Total carbon levels in tilled fields were vertically distributed more evenly among soil depths, attributable to tilling, and had much lower carbon levels in the surface soil where most of the carbon is stored (Figure 1). These findings suggest that using frequent (1-3 year interval) prescribed burning is compatible with forest restoration on agricultural lands for the purpose of sequestering carbon.
We have also tested our hypothesis that the main reason for differences in soil carbon among fire frequencies is the production of charcoal, which might be related to forest structure and amounts of woody debris at the time of fire. We tested soil samples for charcoal by dissolving off the non-charcoal carbon. We found that charcoal makes up about 10-30 percent of total carbon, and that charcoal levels are about the same among the fire treatments and fire-excluded plots, which were burned frequently prior to fire-exclusion about four decades ago. However, tilled plots had significantly less charcoal, especially near the soil surface. So far, it appears that charcoal production is not responsible for differences in soil carbon among fire frequencies. Other possibilities include differences in annual production of woody material, which may relate to carbon turnover rate, and average amounts of groundcover during the fire-free interval, which may influence carbon loss to runoff and leaching.