Changes in carbon dynamics following wildfire from soils in interior Alaska

Boreal forests contain large amounts of soil carbon and are susceptible to periodic wildfires. Predicting the response of soil carbon dynamics to fire disturbance requires understanding: (1) the environmental factors governing CO₂ efflux; (2) the extent to which fire alters these factors; and, (3) the length of time over which these perturbations persist.; In interior Alaska, seasonal patterns of CO₂ efflux, soil temperature, and soil moisture potential were measured in burned and control pairs of aspen, white spruce, and black spruce stands. Averaged over the growing season, mean CO₂ efflux from burned stands (0.51 ± 0.26 g CO₂ m−2 hr−1) was two-thirds that of control stands (0.77 ± 0.44 g CO₂ m−2 hr−1) Soil temperature explained 85 to 90% of the seasonal variability in the control, whereas moisture was a more important determinant in burned stands.; Laboratory incubations of recently burned and control humic material indicate that changes in substrate chemistry and increased temperature may enhance rates of decomposition by a factor of 2.2 to 2.8 in the first decade after fire, resulting in a release of 6.3 to 13.4 Mg C ha−1 to the atmosphere. Under saturated moisture conditions, respiration from mosses may contribute 16 to 50% of total soil CO₂ emissions.; In a 140-year age-sequence of burned black spruce stands, CO₂ efflux increased at an average rate of 8.3 kg C ha−1 yr−1 up to a maximum of 1.83 Mg C ha−1 yr−1. During this same time, accumulation of carbon in organic horizons ranges from 0.34 to 0.50 Mg C ha−1 yr −1 and the ratio of microbial to root respiration decreased from 76:24 to 13:87. Numerical modeling of carbon accumulation suggests that these soils functioned as a net source of carbon for the first 7 to 15 years after fire and released 1.8 to 11.0 Mg C ha−1 to the atmosphere. Although conservative, these estimates of post-fire biogenic emissions are on the same order of magnitude as carbon losses during combustion itself, suggesting that current models may underestimate the impact of fire in northern latitudes by as much as a factor of two.