COS 283-3 - Fire effect on soil microbial nutrient acquisition activity differs among burn severity levels and substrates

Background/Question/Methods

The soil microbiome encompasses incredible biodiversity and functionality in terrestrial ecosystems. Like flora and fauna, the soil microbiome can be profoundly impacted by disturbances. Fire changes the types of carbon (C) sources available to soil microbes, increasing pyrogenic C and coarse woody debris, and decreasing C from root exudates and leaf litter. We aimed to investigate the impact of this shift on microbial processes driving nutrient transformations. We compared microbial activity in a burn complex in Blue River, Oregon, late in the first year of recovery after fire. We sampled soils from sites spanning three levels of burn severity alongside unburned sites. We compared the potential activity of a suite of extracellular enzymes capturing resource acquisition of C, nitrogen (N) and phosphorus (P) rich substrates. We contextualized the microbial resource demand using measurements of mineralizable C and mineralizable N in a 28-day incubation.

Results/Conclusions

Measurements of potential extracellular enzyme activity uncovered differential responses to fire across burn severity and enzymes in the 0 – 5 cm layer of the soil. We observed modest increases in potential oxidase activity in moderate severity sites, with a mean increase of 12% above unburned sites. Meanwhile, hydrolase activity did not significantly differ between burn and control plots, despite clear differences in the resources available to microbes. Potential net nitrogen mineralization, for example, was suppressed in burned plots. This is a characteristic response to the loss of N-rich litter, yet we did not observe commensurate shifts in enzymatic activity. Taken together, our results indicate that even high severity fire did not substantially shift microbial resource acquisition activities. This outcome suggests that limitations to microbial activity, which are independent from wildfire disturbance, may stabilize soil organic matter decomposition in the short term. Further research should clarify how long enzymatic activity remains stable after fire.