Wildfire-Induced Losses of Soil Particulate and Mineral-Associated Organic Carbon Persist for Over 4 Years in a Chaparral Ecosystem.

Wildfires can lower soil carbon (C) stocks directly through combustion, but also indirectly during post-fire recovery if microbial C demands outpace photosynthetic C inputs. However, how much C is respired by soil microorganisms post-fire may depend on wildfire effects on particulate organic carbon (POC; mostly plant material accessible to microbes) and/or mineral-associated organic carbon (MAOC; considered C protected by minerals from decomposers), meaning assessment of wildfire impacts on these pools is necessary to predict microbial decomposition rates and, thus, the fate of soil C. Here, we measured POC, MAOC, pyrogenic organic matter C, plant cover, extracellular enzyme activity (EEA), and microbial community abundance and composition 17 days, and 1, 3, and 4 years after the Holy Fire burned 94 km of fire-adapted chaparral. The wildfire immediately decreased POC by 50% (from 51 ± 21 to 26 ± 6 g C kg) and MAOC by 33% (from 9.3 ± 0.9 to 6.3 ± 0.9 g C kg), consistent with MAOC being less vulnerable to loss than POC. POC decreased by another 38% 1 year post-fire, consistent with increases in microbial abundance and EEA suggesting increased microbial decomposition. Between 1 and 4 years after the fire, cover of the dominant shrub (Arctostaphylos glandulosa) increased from 3.9% ± 1.6% to 16% ± 5.4% (compared to 58% ± 4.6% in unburned plots), marking the end of net soil C losses. Still, soil C did not increase between 1 and 4 years post-fire, suggesting plant C inputs did not outpace microbial respiration, a finding consistent with isotopically heavier C from microorganisms raising bulk soil δC values. As global changes favor increases in wildfire frequency and severity, C losses via combustion and decomposition may outpace plant C inputs during the first 4 years post-fire in chaparral, slowing the replenishment of soil C stocks.