Fire Severity Effects on Soil Carbon and Nutrients and Microbial Processes in a Siberian Larch Forest

Fire frequency and severity are increasing in tundra and boreal regions as climate warms, which can directly affect climate feedbacks by increasing carbon (C) emissions from combustion of the large soil C pool and indirectly via changes in vegetation, permafrost thaw, hydrology, and nutrient availability. To better understand the direct and indirect effects of changing fire regimes in northern ecosystems, we examined how differences in soil burn severity (i.e., extent of soil organic matter combustion) affect soil C, nitrogen (N), and phosphorus (P) availability and microbial processes over time. We created experimental burns of three fire severities (low, moderate, high) in a larch forest in the northeastern Siberian Arctic and analyzed soils at 1-day, 8-days, and 1-year post-fire. Labile dissolved C and N increased with increasing soil burn severity immediately (1-day) post-fire by up to an order of magnitude, but declined significantly 1-week later; both variables were comparable or lower than unburned soils by 1-year post-fire. Soil burn severity had no effect on P in the organic layer, but P increased with increasing severity in mineral soil horizons. Most extracellular enzyme activities decreased by up to 70% with increasing soil burn severity. Increasing soil burn severity reduced soil respiration 1-year post-fire by 50%. However, increasing soil burn severity increased net N mineralization rates 1-year post-fire, which were 10-fold higher in the highest burn severity. While fires of high severity consumed approximately five times more soil C than those of low severity, soil C pools will also be driven by indirect effects of fire on soil processes. Our data suggest that despite an initial increase in labile C and nutrients with soil burn severity, soil respiration and extracellular activities related to the turnover of organic matter were greatly reduced, which may mitigate future C losses following fire.