COS 142-6 - Wildfire-induced losses of particulate and mineral associated organic carbon persist for over three years in a Chaparral ecosystem

Abstract: Wildfires are increasing in frequency and severity across the globe, decreasing plant and microbial biomass with long-term consequences on soil carbon (C) storage. To better understand how C changes in post-fire environments, we partitioned soil C into a mineral-associated organic carbon (MAOC) pool, considered less susceptible to volatilization caused by burning and to microbial decomposition, and a particulate organic carbon (POC) pool, thought to be more vulnerable to burning and microbial decomposition. We hypothesized that fire would initially decrease more POC relative to MAOC, but that over time, the lower microbial biomass in burned soils would cause soil MAOC to decrease relative to unburned soils. To test this hypothesis, we measured MAOC and POC pools four times over the course of three years following the 2018 Holy Fire in a fire-adapted Chaparral shrubland within California’s Cleveland National Forest. Soil samples were collected from burned (n = 6) and unburned plots (n = 3) starting two weeks after burning. We used density fractionation to measure POC and MAOC and qPCR to measure the abundance of bacteria and fungi at each sampling event. Consistent with the first part of our hypothesis, we found that burning initially decreased POC more than MAOC; POC decreased from 62.8 ± 40.2 g C kg soil^-1 in control plots to 20.9 ± 4.67 g C kg soil^-1 in burned plots (66.7 %; p = 0.002), whereas MAOC decreased from 10.6 ± 3.45 g C kg soil^-1 in control plots to 6.37 ± 1.45 g C kg soil^-1 in burned plots (39.7 %; p = 0.03). However, neither POC nor MAOC pools recovered within three years post fire (p > 0.25). Similarly, within two weeks of burning, bacterial abundance decreased by 84% and fungal abundance decreased by 97% (p < 0.01). Both MAOC and POC were negatively associated with ash depth (R² = 0.31, p = 0.02 for MAOC; R² = 0.22, p = 0.045 for POC), an index of burn severity, suggesting that more intense fire volatilized more MAOC and POC. Moreover, the C:N ratio of MAOM remained lower in burned (11.1 ± 4.2) than in control plots (17.2 ± 5.5; p = 0.02), suggesting greater relative contribution of microbial biomass to MAOC in burned soils. Together, these results show that both MAOC and POC can be volatilized, and that soil C pools can take longer than three years to recover after large wildfires.