How well ecosystem processes respond to environmental perturbation may depend on the diversity of species within the community and the level of functional redundancy among those species.  However, species responses may be context-dependent, with some perturbations affecting entire communities of organisms, while others affect only a subset of the community.  Characterizing the relationship between environmental change and communities therefore is vital given vast species loss predicted under current and future global change scenarios.  In a microcosm study, we investigated the relationship between microbial community structure and function under different levels of environmental change: broad change (i.e pH manipulation), and narrow change (i.e. nitrate addition).  We predicted that 1) broad change would affect whole communities of microbes but not ecosystem function while 2) narrow change would affect specific functional groups and ecosystem function. Microcosms were constructed in 500 mL glass jars using soil from a natural vernal pool and freshly fallen leaf litter and pH and nitrate levels were manipulated in the jars.  The pH ranged from 5-8, and microcosm received either10 mg NO₃-N, 10 mg NH₄-N, or sterile distilled water.  TRFLP was used to profile the community structure of bacteria, fungi, and denitrifiers and the population size of denitrifiers was determined with QPCR. 

Results/Conclusions

The community structure of bacteria and fungi was altered by pH, which corresponded with significant differences in microbial respiration at different pH values.  This suggests that broad environmental change can affect both microbial community structure and function.  There were no differences in bacterial or fungal community structure between nitrogen treatments.  The community structure of denitrifying organisms were unaffected by pH or nitrogen treatment, nor were any differences in denitrifier population size observed between the treatments, even with nitrate addition.  However, with the addition of nitrate, the denitrification rate ranged from 1251 μgN*kg^-1*d^-1 to 99,079 μgN*kg^-1*d^-1, while denitrification was not detected in the ammonium and distilled water treatments.  This indicates that narrow change such as nitrate addition, may not affect community structure or population size, even of those functional groups that utilize nitrate.  Rather, gene expression in communities may up-regulate, resulting in changes in ecosystem function without altered microbial community structure.  Overall, our results suggest that community stability and effects on ecosystem function are highly context-dependent, driven by the nature of the environmental perturbation and the physiological plasticity of the communities affected.