Aside from changes in global weather patterns, climate change is likely driving important alterations in natural fire regimes. Most climate models predict an increase in fires, so understanding the influence of altered fire regimes on the components of ecosystems is crucial. Plant-soil interactions represent one such component, and are known to be able to modify fine, plant litter fuels in systems with recurrent fire. Members of the soil microbial communities driving these interactions are known to vary in their ability to survive fires, therefore we hypothesized that short term fire regimes may shape their modification of fuel loads. We tested this hypothesis in a field experiment measuring microbial decomposition of plant litter with replicated 30 µm mesh bags among sites experiencing different fire regimes over a three year period (2014 -2016). Sites were established in the Wade Tract, an old-growth pine savanna, in locations both near and away from pines. Different fire regimes were applied to the sites, followed by measurements of litter decomposition at 2, 4, and 6 months after the final 2016 burns. In addition, litter and soil abiotic variables were measured as well as soil fungal communities using Illumina Mi-seq to analyze differences in fungal assemblages among fire regimes.
Burning at least two times during the three year period caused significant differences in microbial litter decomposition. Sites that burnt at least two times saw lower litter mass loss and microbial decomposition rates than unburnt sites or sites burned only once. These differences appeared stable over time, continuing through six months. In addition, by six months litter collected away from pines had a higher decomposition rate than litter from underneath pines. Our experiment showed that increasing fire frequency suppresses microbial decomposition of fine fuels, leading to greater fuel accumulation six months after fire. This effect was greater in litter underneath pines, likely caused by increased burn frequency and the antimicrobial properties of pine needles. We will present forthcoming sequencing data in relation to these decomposition differences. Based on 2014 data between burned and unburned sites, we expect fungal communities to differ among fire regimes, with more frequent fires promoting similar communities and greater differences in the litter than in the soil. These differences may be predictably related to soil factors like C:N ratio, pH, and soil microbes and their fuel effects, potentially leading to a positive feedback on fire frequency through increased fine fuel accumulation.