Changes in soil ecological function after fire and insect disturbance

Background/Question/Methods

Ecosystem disturbance has emerged as a critical and poorly understood driver of ecosystem function. While disturbance history influences forest productivity and biogeochemical cycling, the soil ecological processes which produce these patterns are not well understood. We took advantage of contrasting disturbance histories within the burn area of the High Park Fire (Colorado, 2012) to study the effects of insect and wildfire disturbance on the microbial controls of soil biogeochemical cycling. We established 56 research plots within the High Park burned Area across three primary study locations; Stove Prairie Ranch (SPR), United Methodist Church (UMC) and Lory State Park (LPS). Plots spanned locations that ranged from no fire disturbance through a continuum of burn severity, which was estimated subjectively from in situ observations and objectively from hyperspectral reflectance measurements from the NEON Airborne Observation Platform. At each location we measured soil CO₂ efflux, vegetation and soil characteristics and collected soils to allow quantification of microbial biomass, potential exoenzyme activity, biogeochemical pools (PO₄, NO₃, NH₄) and the constituents of soil organic matter using pyrolysis.

Results/Conclusions

The range in soil respiration (Rs; soil efflux) varied along a pH gradient from more acidic unburned soils (1-9 mmol m^-2 s^-1) to more alkaline burned soils (1-5 mmol m^-2 s^-1). Potential exoenzyme activity showed a sharp transition across the pH gradient with a shift from carbohydrate processing in more acidic soils to protein processing in more alkaline soils. This transition was consistent with the relative abundance of lignin, polysaccharides and protein-derived constituents of soil organic matter. Changes in microbial community function likely influence post-disturbance biogeochemical cycling. It is unclear whether the shift in ecological function is caused by changes in the microbial community or in plastic responses of a persistent microbial community.  The patchwork of historical disturbance legacies in the West likely contributes to the high variability of soil CO₂ efflux and biogeochemical cycling observed in the seemingly ‘undisturbed’ locations.