Nitrous Oxide Production from KBS LTER Soils: The Role of Denitrifier Diversity

Cavigelli, M. A. and G.P. Robertson

Presented at the All Scientist Meeting (1998-07-21 to 1998-07-22 )

We tested the hypothesis that soil microbial diversity affects ecosystem function by evaluating the effect of denitrifier community composition on nitrous oxide (N2O) production. We sampled two geomorphically similar soils from fields that differed in plant community composition and disturbance regime — a conventionally-tilled agricultural field and a never-tilled successional field (KBS LTER treatments 1 and 8, respectively). We tested whether denitrifier community composition influences denitrification rate and the relative rate of N2O production [DN2O/(DN2O+N2)], or rN2O, 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 N2O. By controlling, or providing in non-limiting amounts, all known environmental regulators of denitrifier N2O 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. These differences suggest that the denitrifying communities in these two soils are different and that they do not respond to environmental regulators in the same manner. We also found evidence of differential regulation of denitrification enzymes among denitrifiers isolated from these same two soils. We isolated 93 denitrifying bacteria from treatment 1 and 63 from treatment 8 soils. Cluster analysis of cellular fatty acid profiles of these isolates revealed two levels of taxonomic clustering at which the two communities differed. For example, the four numerically dominant isolates from each soil were rare or not present in the other soil. We measured the sensitivity of nos enzyme, which is responsible for the reduction of N2O to N2, for representative isolates from 20 taxa. There was great diversity in the degree of sensitivity to oxygen among isolates. Since culturing bacterial isolates under one set of conditions tends to select for organisms with similar characteristics, this level of physiological diversity probably represents a conservative estimate of that which exists in situ. This diversity provides a physiological basis for community-level differences in nos sensitivity to oxygen that could result in ecosystem-level effects of denitrifier community composition. Understanding soil N2O flux may require a better understanding of the role of denitrifier diversity on this important ecosystem function.Return to Contents

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