Fire Frequency Affects Soil Pyrogenic Organic Matter Content and Food Web Dynamics in an Oak-Savannah Ecosystem.

Soil biota play a critical role in carbon (C) cycling in forest ecosystems through decomposition of organic matter and trophic interactions within the soil food web. Fire, the primary disturbance in forest ecosystems, is predicted to increase in frequency in the future, altering C cycling in forest ecosystems. Fire affects soil biota directly by causing mortality and changing soil food web structure and indirectly by changing the soil environment. Pyrogenic carbon (PyC) – organic C produced from incomplete biomass combustion during fire – is a slow-cycling C source and alters the nature of the soil organic matter with which soil biota interact. We predict that fire and PyC will influence soil food web structure by favoring bacteria and their consumers, thereby enhancing decomposition of detritus and soil organic matter as a result. We extracted, identified, and estimated biomass of soil biota functional groups across a fire return interval experiment (unburned, 1-, 2-, 4-year burn interval) at the Pushmataha Forest Habitat Research Area in Southeastern Oklahoma. We then modeled soil food web structural and functional properties across the fire frequency gradient. To determine the relationship between fire frequency, soil food web dynamics, and PyC, we used high-performance liquid chromatography to quantify benzene polycarboxylic acid (BPCA), molecular markers of PyC content. We determined the relative importance of fire frequency and PyC content as drivers of soil food web structure and function in fire-disturbed forest ecosystems. PyC content is greatest at moderate fire frequency (2-year burn interval). Consistent with our predictions, preliminary results suggest that soil food webs are dominated by fungi and their consumers in infrequently burned plots (4-year burn interval), but bacteria and their consumers dominate frequently burned plots (1-, 2-year burn interval). Increased fire frequency with climate change may promote bacterially-dominated soil food webs, accelerating decomposition and C return to the atmosphere.