Role of denitrifier diversity in rates of nitrous oxide consumption in a terrestrial ecosystem

The ecosystem consequences of microbial diversity are largely unknown. We tested the hypothesis that soil microbial diversity influences ecosystem function by quantifying denitrification enzyme activity among denitrifying bacteria isolated from two geomorphically similar soils with significantly different in situ nitrous oxide (N2O) emission rates. We sampled soil from two southwest Michigan sites on the same soil series that differed in plant community composition and disturbance regime — a conventionally-tilled agricultural field and a never-tilled successional field. We isolated denitrifying bacteria from these soils, characterized them based on their fatty acid profiles, and compared denitrifier community composition for the two fields. For 31 representative isolates, we measured the sensitivity of nitrous oxide reductase (Nos) — which catalyzes the reduction of N2O to N2 — to low oxygen concentrations. Of the 93 denitrifying bacteria isolated from the agricultural field and 63 from the successional field, fatty acid profiles suggested the presence of 27 denitrifying taxa with only 12 common to both soils. In each field type the four numerically dominant taxa were either rare or absent in the other field. In addition, we found substantial diversity in the sensitivity of isolate Nos enzymes to oxygen, indicating that the taxonomic diversity present among denitrifiers in these two soils is functionally significant. These results demonstrate a clear physiological basis for differences in denitrifier community function previously described (Cavigelli and Robertson, 2000. The functional significance of denitrifier community composition in a terrestrial ecosystem. Ecology 81, 229–241.) and indicate that differences in denitrifier community composition alone can potentially influence in situ N2O production.