Microbial ecology in the high elevation, mixed-conifer critical zone

Background/Question/Methods

Fire and insect outbreak disturbances are increasing in frequency and severity across the western U.S. with enormous impacts on regional carbon cycling. The resiliency of soil microbial communities to these heterogeneous disturbances determines rates of nutrient transformations as well as forest structure and recovery. Disturbance alters organic substrate quality, microbial biomass, community structure, and activity, which together influence coupled biogeochemical cycles in ecosystems. Yet despite their fundamental importance, the distributions of microbial genotypes are largely unknown, their interactions with plants are cryptic, and the complexity of microbial communities is too often represented as simplified emergent processes. The 2013 Thompson Ridge Fire in the Jemez River Basin, NM CZO and the 2012 High Park Fire in the Colorado Rocky Mountains provide an opportunity to examine whether different disturbance regimes cause contrasting successional changes in soil microbiota that alter post-disturbance biogeochemical cycling in these forest ecosystems. Comparing microbial and biogeochemical datasets from forests recovering from fire and mountain pine beetle outbreaks of varying severity, we ask – what combination of tools and perspectives are required to understand soil microbial ecology of the high elevation, mixed-conifer critical zone?

Results/Conclusions

At landscape scales, disturbances lead to fine-scale variation in biogeochemical states and microbial population dynamics that alter soil respiration and nutrient availability. Wildfires change soil physical, chemical, and biological properties, causing wholesale transformations of organic compound substrates available for microbial metabolism. For example, fires increase soil pH, which is commonly found as an explanatory variable describing bacterial community structure. Soil microbes excrete exoenzymes to decompose polymers and acquire nutrients, and these activities can indicate changing microbial function or soil quality. We identified shifts from carbon to nitrogen-dominated exoenzyme activities in the alkaline soils of burned mixed-conifer forests, suggesting turnover of microbial taxa and function that correspond to recovering soil microbial biomass. Our results highlight the importance of incorporating a standard suite of microbial activity and community assays with soil biogeochemical and respiration measurements to enable comparisons across the broader CZO network. Characterizing local and regional microbial function and biodiversity in a standardized framework could enable more effective management and valuation of critical zone services and would better inform projections under global change scenarios.