In an unstable environment, microbial physiological response to environmental change (i.e. microbial ecophysiology) is the bottleneck determining the flow and fate of elements in an ecosystem. Similarly to studies involving plant ecophysiology, prediction of process rates in an ecosystem undergoing change requires an understanding of how microbial community physiological parameters will be altered as the microbial community adjusts, acclimates, and adapts to a new environment. Here I address the question of how microbial response to long-term exposure to environmental stressors (such as temperature or nitrogen levels) can potentially constrain ecosystem functioning. I present results from our work at the Jasper Ridge Global Change Experiment (JRGCE, Stanford, California). The JRGCE began in November 1998 to test the effects of four global change factors (elevated carbon dioxide, nitrogen deposition, elevated temperature and elevated precipitation) on grassland functioning in a full factorial experiment with eight replicates of each treatment. Samples for microbial analysis were taken annually in the spring beginning in May 2001 and continuing until 2006. We used microbial lipid analysis to characterize and fingerprint the California grassland microbial community subjected to long-term multiple global change treatments. In addition, we asked whether there were changes in physiologic responses to nutrient additions after 8 years exposure to global change manipulations. We performed a short-term incubation experiment to test microbial physiological plasticity, with the hypothesis that long-term exposure to prior global change treatments would poise the community to respond differently to nutrients as compared to communities not previously exposed to the global change manipulations.  

Results/Conclusions

We found that after 8 years of exposure to environmental changes both general microbial community structure and physiological sensitivity to manipulated factors was altered. Nitrogen addition had the largest effect, followed by temperature. In particular, the relative abundance of mycorrhizal fungi declined under elevated N, while bacterial abundance increased. When either CO2 or temperature was elevated together with nitrogen, response to nitrogen diminished. The interactive effects of treatment were critical in predicting microbial response. In addition, communities previously exposed to elevated temperature, elevated precipitation, or nitrogen addition all had different responses to carbon, nitrogen, or phosphorus addition than soils with no prior manipulation. This work demonstrates that not only are there long-term changes in community biomass and function over time, but also that the underlying capacity of the microbial community to respond to environmental conditions changes after exposure to long-term global changes over time.