Soil microbial ecology of the Sierra Nevada: Predictions for a warm and fiery future

Forest ecosystems of the Sierra Nevada are currently responding and are predicated to continue to respond to multiple global change stressors. In particular, increased temperatures and wildfire severity are likely to impact the health of these ecosystems and the services they provide, such as soil carbon (C) sequestration. However, the magnitude of these disturbances on microbial communities and their function as well as their resilience is still unclear. Using a combination of field, laboratory, and meta-analytical methods, I demonstrated that the soil environment, microbial communities, and their biogeochemical function are dramatically altered by these global change stressors and the resilience of these microbial communities is diminished compared to global averages. Specifically, I showed that soil fungal communities (fungal species richness and mycorrhizal colonization) respond negatively to fire, but the response is mediated by fungal guild, method of measurement, and time since fire (soil fungi recover from fire after one to two decades). I then compared this global baseline of fire recovery to soils recovering from an ecologically novel, high-severity fire in the Sierra Nevada. I found that the biogeochemistry of these soils (which fungi in part control) is still significantly altered 44 y post-fire, highlighting reduced resilience following ecologically novel disturbances. In particular, soil C in the mineral topsoil 44 y after fire was half that of the late successional control site. I also showed how another global change stressor, increase soil temperatures, affects microbial processes in a 4.5 y whole-profile warming experiment in the central Sierra Nevada. I found that 4.5 y of +4 °C warming affects microbial community composition, metabolism, and function throughout the soil profile, but that the response in the subsoils was somewhat muted. This suggests that subsoil microbial communities will take longer to acclimate to increase temperatures, possibly reducing their ability to efficiently assimilate and sequester C. Taken together, these findings have important implications for the microbial ecology and C cycling in Sierra Nevada soils such that future soil C sequestration will likely decrease if these disturbances continue to impact these ecosystems unabated