The functional significance of denitrifier community composition in a terrestrial ecosystem

We tested the hypothesis that soil microbial diversity affects ecosystem fu nction by evaluating the effect of denitrifier community composition on nit rous oxide (N2O) production. Denitrification is a major source of atmospher ic N2O, an important greenhouse gas and a natural catalyst of stratospheric ozone decay. The major environmental controls on denitrification rate and the mole ratio of N2O produced during denitrification have been incorporate d into mechanistic models, but these models are, in general, poor predictor s of, in situ N2O flux rates. We sampled two geomorphically similar soils f rom fields in southwest Michigan that differed in plant community compositi on and disturbance regime: a conventionally tilled agricultural field and a never-tilled successional field. We tested whether denitrifier community c omposition influences denitrification rate and the relative rate of N2O pro duction [Delta N2O/Delta(N2O + N-2)], or rN(2)O, using a soil enzyme assay designed to evaluate the effect of oxygen concentration and pH on the activ ity of denitrification enzymes responsible for the production and consumpti on of N2O. By controlling, or providing in nonlimiting amounts, all known e nvironmental regulators of denitrifier N2O production and consumption, we c reated conditions in which the only variable contributing to differences in denitrification rate and rN(2)O in the two soils was denitrifier community composition. We found that both denitrification rate and rN(2)O differed f or the two soils under controlled incubation conditions. Oxygen inhibited t he activity of enzymes involved in N2O production (nitrate reductase, Nar; nitrite reductase, Nir; and nitric oxide reductase, Nor) to a greater exten t in the denitrifying community from the agricultural field than in the com munity from the successional field. The Nar, Nir, and Nor enzymes of the de nitrifying community from the successional field, on the other hand, were m ore sensitive to pH than were those in the denitrifying community from the agricultural field. Moreover, the denitrifying community in the soil from t he successional field had relatively more active nitrous oxide reductase (N os) enzymes, which reduce N2O to N-2, than the denitrifying community in th e agricultural field. Also, the shape of the rN(2)O curve with increasing o xygen was different for each denitrifying community. Each of these differen ces suggests that the denitrifying communities in these two soils art: diff erent and that they do not respond to environmental regulators in the same manner. We thus conclude that native microbial community composition regula tes an important ecosystem function in these soils.