Post-fire effects of soil heating intensity and pyrogenic organic matter on microbial anabolism

Wildfires cause direct and indirect CO2 emissions due to combustion and post-fire decomposition. Approximately half of temperate forest carbon (C) is stored in soil, so post-fire soil C cycling likely impacts forest C sink strength. Soil C sink strength is partly determined by soil microbial anabolism versus catabolism, which dictates the amount of C respired versus stored in microbial biomass. Fires affect soil C availability and composition, changes that could alter carbon use efficiency (CUE) and microbial biomass production, potentially influencing C sink recovery. Wildfire intensity is forecast to increase in forests of the western United States, and understanding the impacts of fire intensity on microbial anabolism is necessary for predicting fire-climate feedbacks. Our objective was to determine the influence of soil heating intensity and pyrogenic organic matter (PyOM) on microbial anabolism. We simulated the effects of fire intensity by heating soils to 100 or 200 °C for 30 min in a muffle furnace, and we amended the soils with charred or uncharred organic matter. Higher intensity soil heating (200 °C) led to lower microbial biomass carbon (MBC) accumulation, greater C respiration, and lower CUE proxies compared to unheated soils. Conversely, lower intensity heating (100 °C) yielded MBC accumulation and estimated CUE that was similar to unheated soils. Soils amended with PyOM exhibited similar MBC accumulation compared to uncharred organic matter, but lower CO2 emissions. These results indicate that high intensity soil heating decreases soil C-sink strength over the short-term by decreasing the amount of microbial anabolism relative to catabolism.