Will changing plant community structure mediate microbial homeostasis?

Changing plant community composition has been shown to affect microbial community function, including the acquisition and immobilization of nutrients. The consistency of soil microbial biomass element ratios along gradients of soil resource supply - microbial homeostasis - is a conceptual framework that underpins ecosystem process models, yet not all ecosystems display homeostasis. To determine which microbial systems may display homeostasis and through which mechanisms, we suggest that experimental systems must also consider plant-microbial interactions. Specifically, we hypothesized that an ecosystem with strong plant community plasticity to changing resources will be more likely to have a homeostatic microbial community, with less microbial effort, because plant nutrient uptake can reduce the variance in the stoichiometry of resources that are available to microbes. We examined microbial homeostasis in two long-term nutrient addition experiments in two tundra ecosystems (Moist acidic tundra and Moist non-acidic tundra) with differing plant response to nutrient addition. We show that homeostasis is stronger in the system where plant communities responded strongly to fertilization, supporting our hypothesis. However, microbial mechanisms were also stronger in this system, including shifts in both carbon use efficiency and extracellular enzyme activity, not supporting our hypothesis. Our conclusions about the homeostasis for either system were undermined by high heterogeneity in soil resources, which we propose may be common in ecosystem-level studies. Current metrics for defining homeostasis, and also how we measure soil resource supply, both which may be exacerbated by shifting plant communities, may not be appropriate for microbial studies at the ecosystem-level. Understanding the extent to which plant community dynamics may impact microbial homeostasis is essential for understanding ecosystem processes in a changing world with concurrent shifts in anthropogenic nutrient loads and plant community structure.