Oak-dominated savannas were once the most abundant woody vegetation type in the upper Midwest. To restore oak ecosystems, land managers have increasingly incorporated prescribed burns and overstory thinning into their management plans. However, similar management techniques often produce diverse outcomes. Soils with higher silt and clay content have a greater capacity to physio-chemically protect and stabilize organic matter than sandy soils. Thus, we hypothesized that nutrients released by burning and thinning would be taken up by the microbial community and retained in fine-textured soils but be lost from coarse-textured soils. Further, we expected these differences to be greatest in plots that were both burned and thinned compared to those that were only burned. To evaluate these hypotheses, we assessed restoration-induced changes in microbial biomass carbon, nitrogen, and phosphorus; carbon and nutrient availability and fluxes; and extracellular enzyme activities in eight unglaciated oak savanna sites across Indiana, Illinois, and Wisconsin, USA. These sites were selected to reflect soil textures commonly found in oak savannas: four of the sites had sandy soils and four had loamy soils. Each site contained three plots: an unmanaged control, a burned plot, and a burned and thinned plot.
Results/Conclusions
Overall, microbial responses to restoration treatments were mediated by soil texture. Restoration effects on soil nitrogen and phosphorus dynamics were texture dependent; burning and thinning increased N mineralization rates, ammonium concentrations, and microbial biomass nitrogen concentrations in loamy soils, but decreased nitrogen mineralization and ammonium availability in sandy soils (treatment x texture interactions: P≤0.048). Similarly, burning and thinning increased microbial biomass phosphorus in sandy soils, but not in loamy soils (treatment x texture interaction: P=0.009). Furthermore, these effects were most pronounced in the plots that were both burned and thinned; the burned-only plots exhibited limited soil responses to restoration. Based on the relative abundance of extracellular enzymes, microbes in loamy soils were more N-limited while microbes in sandy soils were more P-limited (texture: P=0.014). Thus, burning and thinning began to alleviate microbial nutrient limitation, but the nutrient dynamics that were altered varied by soil texture. While restoration evaluation has focused primarily on plant community composition, the functional capacity of soils may also be altered by burning and overstory thinning. These findings illustrate that oak savanna restoration strategies have broad-reaching impacts belowground, but these outcomes are contingent on underlying soil properties.