THE FUNCTIONAL SIGNIFICANCE OF DENITRIFIER COMMUNITY COMPOSITION IN A TERRESTRIAL ECOSYSTEM

Abstract

We tested the hypothesis that soil microbial diversity affects ecosystem func- tion by evaluating the effect of denitrifier community composition on nitrous oxide (N20) production. Denitrification is a major source of atmospheric N20, an important greenhouse gas and a natural catalyst of stratospheric ozone decay. The major environmental controls on denitrification rate and the mole ratio of N20 produced during denitrification have been incorporated into mechanistic models, but these models are, in general, poor predictors of in situ N20 flux rates. We sampled two geomorphically similar soils from fields in southwest Michigan that differed in plant community composition and disturbance regime: a con- ventionally tilled agricultural field and a never-tilled successional field. We tested whether denitrifier community composition influences denitrification rate and the relative rate of N20 production (AN20/A(N20 + N2)), or rN20, using a soil enzyme assay designed to evaluate the effect of oxygen concentration and pH on the activity of denitrification enzymes responsible for the production and consumption of N20. By controlling, or providing in nonlimiting amounts, all known environmental regulators of denitrifier N20 production and consumption, we created conditions in which the only variable contributing to differences in denitrification rate and rN2O in the two soils was denitrifier community composition. We found that both denitrification rate and rN2O differed for the two soils under controlled incubation conditions. Oxygen inhibited the activity of enzymes involved in N20 production (nitrate reductase, Nar; nitrite reductase, Nir; and nitric oxide reductase, Nor) to a greater extent in the denitrifying community from the agricultural field than in the community from the successional field. The Nar, Nir, and Nor enzymes of the denitrifying community from the successional field, on the other hand, were more sensitive to pH than were those in the denitrifying community from the agricultural field. Moreover, the denitrifying community in the soil from the successional field had relatively more active nitrous oxide reductase (Nos) enzymes, which reduce N20 to N2, than the denitrifying community in the agricultural field. Also, the shape of the rN20 curve with increasing oxygen was different for each denitrifying community. Each of these differences suggests that the denitrifying commu- nities in these two soils are different and that they do not respond to environmental regulators in the same manner. We thus conclude that native microbial community composition reg- ulates an important ecosystem function in these soils.