|Robertson, G. - MSU|
Submitted to: Soil Biology and Biochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 16, 2000
Publication Date: N/A
Interpretive Summary: Soil denitrifiers are an important group of bacteria that convert soil nitrogen in the form of nitrate to gaseous forms including dinitrogen and nitrous oxide. Nitrous oxide, the atmospheric concentration of which is increasing, is both a greenhouse gas and a catalyst of stratospheric ozone degradation. Because of its importance in global change scenarios, scientists have built models to predict nitrous oxide production from soils. These models include a number of environmental variables known to affect nitrous oxide production from soils. Unfortunately, these models do not work very well, possible because nitrous oxide production may be regulated by the types of denitrifier communities in a soil, and not just by the environmental factors influencing denitrifiers. We tested this hypothesis by measuring nitrous oxide consumption by denitrifying bacteria isolated from two soils. There were few similarities between the bacteria isolated from the two soils, suggesting that the denitrifier communities were different in each soil. In addition, the different denitrifiers consumed nitrous oxide at different rates when all environmental variables known to affect denitrifier activity were tightly controlled. Thus, denitrifier community composition may influence nitrous oxide flux from soils.
Technical Abstract: 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) flux 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 levels. 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 in press) and indicate that differences in denitrifier community composition alone can potentially influence in situ N2O production.