Vegetation Type and Fire Severity Mediate Short-Term Post Fire Soil Microbial Responses

Background Wildfire severity mediates key dynamics, such as nutrient pulses, that regulate the recovery of ecosystem functioning. Large shifts in vegetation communities associated with plant invasions are often coupled with changes in soil communities; thus, it’s critical to understand how fire severity may interact with vegetation type and soil communities to mediate ecosystem recovery. Methods Following a 2017 wildfire in southern California, soils from areas dominated by native coastal sage scrub or exotic annual grasses that experienced a low or high severity fire event were analyzed for nutrient concentrations and two proxies for ecosystem function—microbial respiration and enzymatic activity potentials over the first-year post-fire. Aims We predicted that increasing fire severity would positively correlate with soil nutrient concentrations. Thus, higher severity burned soil would experience a greater downregulation of enzyme activity as potential microbial nutrient limitation was alleviated, a relationship that would be stronger in shrub dominated soil. Results We observed a strong soil nitrogen (N) pulse post-fire, which was greatest in shrub dominated soil; however, dominant vegetation had a variable effect on microbial responses. Enzyme activities were downregulated in CSS soil, but the grass dominated soil response was inconsistent. After 1 year, soil N remained elevated in burned soil, suggesting that basal soil N concentrations were altered. Conclusions Persistent, residual soil N concentrations are of particular concern in high fire risk ecosystems, which will likely experience increasing fire frequency associated with environmental change; thus, encouraging the regrowth of opportunistic vegetation in subsequent growing seasons will be key to minimize long-term changes to these ecosystems