Humans have a large impact on ecosystems around us, especially with regard to disturbance regimes, climate change, and nitrogen deposition. The extent to which these human impacts interact to affect nutrient cycling and environmental quality depends on the response of microbial communities, important mediators of soil carbon and nutrient cycling. Further, it may be important to understand how microbial responses fluctuate temporally, especially at the landscape relevant time-scale of inter-annual variability. In this study we analyzed microbial community structure (lipid analysis) and function (extra-cellular enzyme activities) during three years following wildfire at the Jasper Ridge Global Change Experiment (JRGCE). The JRGCE is a full factorial manipulation of four global change factors: nitrogen deposition, elevated CO₂, elevated temperature, and elevated precipitation, all of which have been in continuation since 1998. Further, we examine relationships between our microbial data and plant community dynamics after burning; and use our study as an example within a growing body of literature on microbial communities and wildfire.
Results/Conclusions

We found that while microbial community biomass and structure responded most strongly to burning one year following the burn, enzyme activity response was strongest two years after burning. Our results, together with others from the literature, suggest that the microbial community may have a pulse-response after burning that varies in duration and intensity based on the particular ecosystem and burn severity. Specifically from our study, burning increased mycorrhizal fungal indicator biomass and decreased bacterial indicator biomass, leading to a significant increase in the fungal to bacterial ratio 1 year after burning. Burning also decreased extra-cellular enzyme activity. For both microbial community structure and function, we also found interactive effects between burning and elevated CO₂, where elevated CO₂ mitigated the effects of burning on community structure and enzyme activity. We also found that when elevated CO₂ was combined with nitrogen addition and burning, bacterial indicator biomass and enzyme activity both increased, but this trend only occurred in years where burning alone affected microbial composition or activity. In summary, the temporal nature of microbial responses to burning and global change may be important in that microbial biomass and structure may recover at different time-scales, mediating different aspects of soil function over the course of recovery.