Compositional Shifts Elicit Functional Responses in Soil Microbial Communities: Implications for Biogeochemical Cycles in a Changing Environment.

Compositional Shifts Elicit Functional Responses in Soil Microbial Communities: Implications for Biogeochemical Cycles in a Changing Environment
Donald R. Zak, University of Michigan, School of Natural Resources & Environment, 440 Church St., Ann Arbor, MI  48109-1041

Microorganisms mediate an array of biochemical processes in soil, interact with one another in a phylogenetically diverse community, and, in turn, mediate biogeochemical cycles that are of global importance.  Understanding how composition and function are linked in soil microbial communities remains a contemporary challenge, and developing this understanding can reveal the molecular mechanisms underlying many biogeochemical processes.  In this presentation, I will discuss how microbial community function can be contingent on community composition and the potential implications of this relationship on soil C and N cycling.  Further, I will discuss the ways in which a changing environment can modify the ecological constraints on soil microbial communities, the compositional and functional responses that result, and identify gaps in our knowledge of soil microbial communities that limit our ability to anticipate the response of biogeochemical cycles to environmental change.  I will argue that understanding the presence, abundance, and expression of particular functional genes, as well as the identity of organisms in which they reside, can reveal the molecular mechanisms regulating the biogeochemical cycling of C and N in terrestrial ecosystems.  Using a long-term, field experiment as an example, I will provide evidence that future rates of atmospheric N deposition can alter the composition of soil microbial communities, down regulate the transcription of fungal genes encoding enzymes with lignocellulolytic function, slow plant litter decay, and increase soil C storage.  This mechanism is not represented in any biogeochemical model used to simulate ecosystem response to future rates of atmospheric N deposition, and it illustrates that changes in microbial community composition elicit functional responses that have biogeochemical consequences.