Replacing time with space: Using laboratory fires to explore the effects of repeated burning on black carbon degradation

Soil organic matter plays a key role in the global carbon cycle, representing three to four times the total carbon stored in plant or atmospheric pools. Although fires convert a portion of the faster cycling organic matter to slower cycling black carbon (BC), abiotic and biotic degradation processes can significantly shorten BC residence times. Repeated fires may also reduce residence times, but this mechanism has received less attention. Here we show that BC exposed to repeated experimental burns is exponentially reduced through four subsequent fires, by 37.0, 82.5, 98.6 and 99.0% of BC mass. Repeated burning can thus be a significant BC loss mechanism, particularly in ecosystems where fire return rates are high, relative to BC soil incorporation rates. We further consider loss rates in the context of simulated BC budgets, where 0-100% of BC is protected from subsequent fires, implicitly representing ecosystems with varying fire regimes and BC transport and incorporation rates. After five burns, net BC storage was reduced by as much as 68% by accounting for degradation from repeated burning. These results illustrate the importance of accounting for BC loss from repeated burning, further highlighting the potential conflict between managing forests for increasing soil carbon storage vs maintaining historic fire regimes.