PS 28-147 - Effects of fire frequency on soil chemistry and microbial communities in a longleaf pine savanna

Abstract: Longleaf pine savannas are among the most biologically diverse ecosystems in temperate North America. A key component to maintaining community structure and function in longleaf pine forests is the frequent, recurrent application of low-intensity surface fires. While the significance of fire on plant community composition is well known, the impacts of prescribed fire regimes on soil biotic communities remains unclear. Furthermore, a large amount of research is restricted to single burn events, making it difficult to predict soil community responses to repeated fires typical of a highly frequent fire regime. We conducted a field study to determine: What are the impacts of prescribed fire regimes on soil chemical and biological properties in a longleaf pine-wiregrass savanna? We sampled soils from two long-term fire frequency treatments (annual vs. 2–3-year fires) in ten 1m2 plots at three sites in southwestern Georgia, USA. We processed and analyzed soils for moisture content and nutrient availability (nitrogen, phosphorous, pH, and macronutrients) to assess fire-induced changes between treatments.

We found that soil phosphorus differed significantly among fire frequency treatments and site, but not in their interaction. Based on post-hoc comparisons, annual fire significantly decreased both available and total phosphorus compared to 2–3-year fires (P < 0.001), indicating that shorter fire intervals may lack the time necessary for soil P pools to recover between fires. We did not detect any significant differences in other abiotic soil characteristics or fire frequency × site interactions, suggesting that changes in nutrient availability could be neutral or resilient following low severity fires. In general, low severity fires typically produce short-term increases of C, N, P, and pH due to fuel combustion and pyric mineralization. However, fire frequency effects vary depending on the ecosystem, and pine forests tend to be highly resilient with most studies showing no changes after repeated burning. Decreased phosphorus levels we observed could be attributed to increased volatilization and may have implications on post-fire recovery of plant and soil communities. Ongoing work from this study assesses the fire-associated changes in soil biotic communities to characterize the belowground patterns that drive post-fire regeneration in a pyrophilic ecosystem.