Soil carbon mineralization and microbial community dynamics in response to pyrogenic organic matter 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 com-munities 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 30-day incubation. Our results indicate that at the end of the incubation period, the water-extractable fraction was 10-50x more mineralizable in both 350 degrees C and 550 degrees C PyOM treatments than the SOC or non-water-extractable PyOM fraction. The addition of 350 degrees C PyOM 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, and identified positive PyOM responders that increased in relative abundance 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