Fuel accumulation shapes post-fire fuel decomposition through soil heating effects on plants, fungi, and soil chemistry

Forty percent of terrestrial ecosystems require recurrent fires driven by feedbacks between fire and plant fuels. The accumulation of fine fuels in these ecosystems play a key role in fire intensity, which alters soil nutrients and shapes soil microbial and plant community responses to fire. Changes to post-fire plant fuel production are well known to feed back to future fires, but post-fire decomposition of new fuels is poorly understood. Our study sought to quantify how pre-fire fuel loading influenced post-fire fuel decomposition through soil abiotic properties, as well as plant and soil fungal communities. Prior to spring prescribed burns, we manipulated fine fuel loads in plots, both near (<10 m) and away (>10 m) from overstory pines, to modify soil heating in an old-growth longleaf pine savanna. We then assessed how fuel load and soil heating influenced post-fire plant fuel decomposition through changes to soil chemistry, vegetation, and fungi. Burning larger fuel loads made fires hotter, burn longer, and more completely combusted fuels. In these plots, decomposition of newly deposited fine fuels was slower in the eight months following fire. Decomposition changes from greater soil heating were mediated by greater shifts to postfire plant (2 and 4 months postfire) and fungal communities (4 and 6 months postfire). Soil properties (C: N ratios, soil pH, and P) controlled postfire decomposition throughout the year, but weakly responded to soil heating differences from fuels. Since the mechanisms for fuel effects on decomposition change over time, fire timing may be a future target for understanding fire feedbacks to fuel decomposition. Integrating these feedbacks with fuel production responses across fire-dependent ecosystems can help managers better set prescribed fire intervals and predict responses to reintroducing burning in fire-suppressed ecosystems.