Community composition and activities of denitrifying bacteria in soils

Few studies have directly compared denitrifying community composition and activities in soils by coupling molecular-genetic techniques and traditional measures of denitrification. I investigated communities of denitrifying bacteria from adjacent meadow and forest soils in the Cascade Mountains, Oregon. A key gene in the denitrification pathway, N2O reductase ( nosZ), served as a marker for denitrifying bacteria. Denitrifying enzyme activity (DEA) was an order of magnitude higher in meadow than in forest soils. Denitrifying community composition differed between vegetation types based on multivariate analyses of nosZ T-RFLPs. Screening 225 nosZ clones yielded 47 unique denitrifier genotypes. The majority of nosZ fragments sequenced from meadow or forest soils were most similar to nosZ from Rhizobiaceae in alpha- Proteobacteria.; In a second study, I examined denitrifying bacteria from three adjacent habitats in Oregon: agricultural soil that received N-fertilizer inputs, naturally vegetated riparian soil, and creek sediment. The ratio of N2O produced as a result of denitrification in the presence of glucose (10 mM) and NO 3- (5 mM) was higher for riparian soil (0.64 +/- 0.02; mean +/- standard error, n = 12) compared to agricultural soil (0.19 +/- 0.02) or creek sediment (0.32 +/- 0.03). Mean DEA was similar among habitats, but mean N2O-reductase activity was about 70% higher in agricultural soil than in riparian soil or creek sediment. Denitrifying community composition differed among habitats based on nosZ T-RFLPs. The creek sediment community was unique. Communities in agricultural and riparian soil were more closely related but distinct. Sequences of nosZ obtained from riparian soil were closely related to nosZ from Rhizobiaceae or distantly related to nosZ from Ralstonia or Azospirillum spp.; Both studies indicated that denitrifying community composition differed among adjacent habitats. The same dominant denitrifying genotypes were found in all soils, but relative abundances of these genotypes differed among habitats. Less dominant genotypes were consistently only found in certain habitats. Denitrifying community composition and activities were correlated, but relationships between composition and activities differed between studies. Previously overlooked denitrifiers related to Rhizobiaceae may dominate in these soils.