Soil carbon mineralization and microbial community dynamics in response to PyOM addition

Wildfires can either negatively impact soil carbon (C) stocks through combustion or increase soil carbon stocks through the production of pyrogenic organic matter (PyOM), which is highly persistent and can affect non-pyrogenic soil organic carbon (SOC) mineralization rates. In this study, we used fine-resolution 13CO2 flux tracing to investigate PyOM-C mineralization, soil priming effects, and their impacts on soil microbial communities in a Californian mixed conifer forest Xerumbrept soil burned in the 2014 King Fire. We added PyOM produced from pine biomass at 350 {degrees}C and 550 {degrees}C to the soil and separately traced the mineralization of 13C-labeled water-extractable and non-water-extractable PyOM-C fractions in a short-term incubation. Our results indicate that the water-extractable fraction is 10-50x more mineralizable in both 350 {degrees}C and 550 {degrees}C PyOM treatments than the SOC or non-water-extractable PyOM fraction. 350 {degrees}C PyOM addition led to a short-term positive priming effect, likely due to co-metabolism of easily mineralizable PyOM-C and the SOC, whereas 550 {degrees}C PyOM addition induced negative priming, potentially due to physical protection of SOC. We observed significant shifts in bacterial community composition in response to both 350 {degrees}C and 550 {degrees}C PyOM, with positive PyOM responders belonging to the genera Noviherbaspirillum, Pseudonocardia, and Gemmatimonas. In contrast, fungal communities were less responsive to PyOM additions. Our findings expand our understanding of the post-fire cycling of PyOM and SOC, providing insights into the microbial mineralization of different PyOM-C fractions and their influence on soil C dynamics in fire-affected ecosystems.