Fire history as a key determinant of grassland soil CO 2 flux

Aims Fire regimes are key drivers of ecosystem dynamics and are changing worldwide. Uncertainty about how fire history affects responses to individual fires hampers predictions of fire impacts on important ecosystem functions such as C cycling. Thus, we assessed how fire and fire history affect soil CO 2 flux and aboveground net primary production (ANPP). Methods We utilized a 35-year fire frequency experiment in a mesic grassland to quantify how two aspects of fire history, long-term fire frequency (fire every one, two, or four years, or no fire) and number of years elapsed since the most recent fire, affect soil CO 2 flux. We used long-term annual records from the same grassland to compare this to the effect of fire history on ANPP. Results Historic fire frequency altered the soil CO 2 flux response to fire, with greater post-fire stimulation in grassland burned annually than in grassland burned less frequently (~100% vs. ~44% increase over long-term unburned grassland). The flux increase persisted for up to two years after fire. Though fire also stimulated ANPP, this increase did not vary by long-term fire frequency and did not persist into later years. Conclusions Fire history modifies the soil CO 2 flux response to individual fires in this grassland. Predicting the dynamics of this important C flux will require considering not only the presence vs. absence of fire, but also fire history.