Fire return interval influences soil food web structure and stability in an oak-pine savanna

Fires are expected to become more frequent in many ecosystems due to climate change. Shifting fire regimes may affect soil food web interactions and impact soil functions including decomposition, nutrient cycling, and soil organic matter formation. We evaluated soil food web structure, function, and stability along a fire frequency gradient. We studied the effect of fire return interval on soil food webs in a long-term fire return interval experiment in an oak-pine savanna. We measured soil biota biomass of major microbial groups (gram positive bacteria; gram negative bacteria; arbuscular mycorrhizal (AM) fungi; saprotrophic fungi) using phospho- and neutral-lipid fatty acid assessments. We applied an energetic food web modeling framework to investigate soil food web structure, function, and stability. Soil biota community composition and biomass were conserved across the fire frequency gradient, but biomass of AM fungal storage compounds decreased with less frequent fires. Soil food web complexity decreased with FRI, driven by a loss of fungal consumers and predators. Soil food webs at 4-year fire frequencies were significantly less stable and less resilient than those at either low or high fire frequencies. These community shifts did not lead to meaningful changes in modeled C and N mineralization. Though soil food web biomass and function were conserved along the fire frequency gradient, food webs were structurally unique and not equally stable. This study reveals latent dynamics of soil communities that hint towards vulnerable food web architectures at the transition between forest and savanna ecosystems.