Changes in Soil Organic Matter Pools and Carbon Mineralization as Influenced by Fire Severity

Abstract

Fire transforms soil organic matter (SOM) to recalcitrant forms of C. The degree to which SOM is altered is dependent on fire severity. This study investigated changes in SOM composition and mineralization by controlling the fire severity of laboratory burns on reconstructed soil profiles (O, A1 [0–1 cm], and A2 [1–2 cm] horizons). Burning simulated low‐, moderate‐, and high‐severity fires. Organic and mineral soils were incubated for 180 d and CO2 production was measured with soda lime traps. Soils were analyzed for SOM composition pre‐ and post‐incubation using an alkaline extraction. Higher severity burning resulted in lower O horizon decomposition rates on a C basis. Fulvic acid C in the control and low‐severity O horizons was reduced by 13% by incubation, which was negatively correlated with cumulative C mineralized (r = −0.564). After incubation, the SOM composition of the burned O horizons remained different from the control. Higher severity burning caused mineral soil to initially have higher C mineralization rates, which disappeared by the end of the incubation. Fulvic acid concentration was reduced by 61 and 38% during incubation of the A1 and A2 horizons, respectively, returning the SOM composition to control levels. The sum of alkaline insoluble forms (e.g., humin) of SOM accounted for 74 and 61% of total soil C and N, respectively, which was not significantly different between treatments. The fire severities examined show that the major impacts to SOM occurred to the O horizon and any fire‐related changes to the mineral soil were not persistent.