How Fire Moves Carbon in North American Ecosystems Shaped by Fire and Grazing

ABSTRACT Fire interacts with ecosystem carbon in North American fire-grazing rangelands through processes that span chemical forms, soil depths, and temporal horizons. We synthesize empirical evidence on fire-driven carbon redistribution across the continental region where frequent fire and bison grazing historically co-occurred. Across these rangelands, temperate grasses allocate roughly 75–80% of biomass belowground, outside the combustion zone. Herbaceous aboveground biomass typically recovers within one to three growing seasons. Incomplete combustion produces pyrogenic carbon (PyC) amounting to ∼12% of the carbon emitted by fire. The stable component of that PyC persists in soil on centennial to millennial timescales. Frequent burning can reduce bulk soil organic carbon relative to fire exclusion, especially in dry systems, while shifting carbon inputs toward pyrogenic material. Across the Great Plains, shorter modeled fire intervals are associated with a larger mineral-associated fraction and a smaller particulate fraction, whereas the pyrogenic fraction changes little. Where fire is excluded, woody plant encroachment shifts a larger share of ecosystem carbon into aboveground biomass exposed to fire, drought, and dieback, while altering hydrologic function, biodiversity, and resilience to disturbance. The fire-grazing interaction distributes combustion and recovery across a shifting patch mosaic in which litter retention, grazing pressure, and belowground carbon inputs vary among patches. Carbon accounting frameworks that measure only stock size can mistake the carbon vulnerability created by fire suppression for carbon gain, inadvertently penalizing disturbance that sustains persistent soil carbon in these systems.