Cavigelli, M. 1998. Ecosystem consequences and spatial variability of soil microbial community structure. Ph.D. Dissertation, Michigan State University, East Lansing, Michigan, USA.

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The influence of biodiversity on ecosystem processes has been the subject of considerable research effort and debate among plant and animal ecologists but the ecosystem consequences of microbial diversity are largely unknown. I tested the hypothesis that soil microbial diversity affects ecosystem function by evaluating the effect of denitrifier community composition on nitrous oxide (N2O) production. I used a soil enzyme assay to evaluate the effect of oxygen concentration and pH on the activity of denitrification enzymes responsible for the production and consumption of N2O. By controlling, or providing in non-limiting amounts, all known environmental regulators of denitrifier N2O production and consumption, I created conditions in which the only variable contributing to differences in denitrification rate and the relative rate of N2O production in soils from two fields in southwest Michigan was denitrifier community composition. I found that the denitrification enzymes of the denitrifying communities from the two fields differed substantially in their sensitivities to oxygen and pH, indicating that the denitrifying communities in these two soils respond to environmental regulators differently.

I also isolated denitrifying bacteria from these same soil samples and, for 31 representative isolates, measured the sensitivity of their nos enzymes, which catalyze the reduction of N2O to N2, to low levels of oxygen. Cluster analysis of the cellular fatty acid profiles of 93 denitrifying bacteria isolated from the agricultural field and 63 from the successional field showed 27 denitrifying taxa with only 12 common to both soils. In addition, I found substantial diversity in the degree of sensitivity of the isolates’ nos enzymes to oxygen, indicating that the taxonomic diversity present among denitrifiers in these two soils is functionally significant. These results demonstrate a clear potential for differences in denitrifier community composition to affect differences in N2O production among ecosystems, independent of direct environmental controls.

I also investigated the spatial distribution of microbial community structure along a transect in a conventionally-tilled agricultural field. I used fatty acid methyl ester (FAME) profile analysis, a rapid technique that allows processing of the large number of samples required for spatial analyses of microbial community structure, I applied principal components analysis to these data and showed that, while a majority of 167 soil samples had similar FAME profiles, about 20 percent of samples had relatively low and about 10 percent had relatively high bacterial:fungal ratios. Using semivariance analysis I was able to capture and describe small-scale patterns of microbial population distributions. Where autocorrelation occurred, it was generally at scakes < 0.2 m, a scale analogous to individual soil peds and rhizospheres.

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