Forest wildfire and grassland prescribed fire effects on soil biogeochemical processes and microbial communities: Two case studies in the semi-arid Southwest

• Prescribed fire had immediate impacts on soil microbial communities.
• Wildfire had a more pronounced impact on nitrogen cycling than prescribed fire.
• Fire decreased microbial and enzymatic activity up to 6 months post-wildfire.

Fire is a natural disturbance that shapes many ecosystems. In semi-arid regions, where high temperatures and low soil moisture limit nutrient cycling and plant growth, fire is critical to supply nutrients and drive vegetation composition. We examined soil chemical and biological properties to assess the short-term impacts of wildfire and prescribed fires on soil functioning in semi-arid regions of Texas. Better understanding of soil organic matter transformation and nutrient cycling processes will aid land managers in predicting ecosystem recovery response post-fire. Soil samples were collected following both prescribed grassland fires in June of 2009 in Lubbock, TX and the April 2012 Livermore Ranch Complex Fire located in the Davis Mountains, TX. Prescribed fire samples (0–2.5 cm) were collected within six h prior to burning and again at 0.5, 24, 48, and 168 h post-fire to experimentally examine short-term influences of fire and fire frequency (1× vs. 2×) on soil carbon dynamics, inorganic nitrogen, and microbial community composition. Wildfire samples (0–5 cm) were collected two and six months following the wildfire. We evaluated the effects of three burn severity levels and sampled under three tree species (Juniperus deppeana, Pinus cembroides, and Quercus grisea). Within 0.5 h of the prescribed fire, CO2 flux, NH4+-N concentration and total microbial biomass (as estimated by total fatty acid methyl esters) increased. A shift in the microbial community from a predominance of fungi to Gram positive bacteria occurred immediately following the fire. Chemical shifts were short lived (decreased within 24 h), but the biotic shift to a dominance of Gram negative bacteria and actinomycetes was measured in samples collected after 168 h. Soil pH and NH4+-N concentration increased at two and six months following the wildfire. In contrast, soil organic matter content decreased at two months post wildfire which, in combination of abiotic conditions such as low moisture content (<3.3%), resulted in reduced soil microbial biomass and enzyme activity. Increased soil moisture six months post fire created more favorable conditions for nitrification resulting in increased NO3−-N concentration (0.8 to 36.1 mg NO3−-N kg−1 soil), particularly following high severity fire. Prescribed fire did not have lasting impacts on soil nutrients, but both prescribed and wildfire resulted in increased NH4+-N, shifts in microbial community structure and decreased in microbial biomass. While the increase in nitrogen maybe be beneficial to the plant growth and revegetation, the loss of microbial biomass may have far reaching implications to the overall sustainability of the soils in these systems.