Substantial Release of Recalcitrant Soil Carbon Activated by Forest Fires

The feedback between forest fires and climate represents one of the largest uncertainties in the global carbon cycle ¹ . Current estimates of the global carbon budget exclude the legacy effects of fire, potentially causing considerable systematic errors ² . Whether there are substantial postfire carbon emissions due to changes in soil carbon dynamics, however, remains unknown. Here, through postfire field investigations and soil column experiments, we reveal that up to nearly half of soil organic carbon is utilized in the rainy season after fire, along with a reduction of iron(III) minerals. This activation and utilization of recalcitrant carbon is triggered by the leaching of pyrogenic dissolved organic matter, which serves as a sole electron donor in microbial dissimilatory iron reduction, thereby facilitating the release of iron-bound carbon. Using Earth System Models, we estimate that this pathway can emit up to 274 Tg of carbon annually after fires, significantly offsetting or surpassing the carbon accumulation of 128 Tg per year from pyrogenic carbon burial ³ . Our results demonstrate that fire can activate the recalcitrant soil carbon and shifts it from a carbon sink to a carbon source. This postfire carbon release could have a significant, but overlooked, impact on the global carbon budget, leading to positive feedback on climate.