We investigated communities of denitrifying bacteria from adjacent meadow and forest soils. Our objectives were to explore spatial gradients in denitrifier communities from meadow to forest, examine whether community composition was related to ecological properties (such as vegetation type and process rates), and determine phylogenetic relationships among denitrifiers. nosZ, a key gene in the denitrification pathway for nitrous oxide reductase, served as a marker for denitrifying bacteria. Denitrifying enzyme activity (DEA) was measured as a proxy for function. Other variables, such as nitrification potential and soil C/N ratio, were also measured. Soil samples were taken along transects that spanned meadow-forest boundaries at two sites in the H. J. Andrews Experimental Forest in the Western Cascade Mountains of Oregon. Results indicated strong functional and structural community differences between the meadow and forest soils. Levels of DEA were an order of magnitude higher in the meadow soils. Denitrifying community composition was related to process rates and vegetation type as determined on the basis of multivariate analyses of nosZ terminal restriction fragment length polymorphism profiles. Denitrifier communities formed distinct groups according to vegetation type and site. Screening 225 nosZ clones yielded 47 unique denitrifying genotypes; the most dominant genotype occurred 31 times, and half the genotypes occurred once. Several dominant and less-dominant denitrifying genotypes were more characteristic of either meadow or forest soils. The majority of nosZ fragments sequenced from meadow or forest soils were most similar to nosZ from the Rhizobiaceae group in ␣-Proteobacteria species. Denitrifying community composition, as well as environmental factors, may contribute to the variability of denitrification rates in these systems.

and CfoI135) were found more frequently in meadow and transition zone soil samples than in forest samples at both sites. At the Lookout site the combination AluI491-CfoI135 was found primarily in meadow samples expressing the highest N mineralization rates. Four unique amoA sequences were identified among 15 isolates recovered into pure culture from various transect locations. Six isolates possessed the most common T-RFLP amoA fingerprint of the soil samples (TaqI283-AluI392-CfoI66), and their amoA sequences shared 99.8% similarity with a cultured species, Nitrosospira sp. strain Ka4 (cluster 4). The other three amoA sequences were most similar to sequences of Nitrosospira sp. strain Nsp1 and Nitrosospira briensis (cluster 3). 16S ribosomal DNA sequence analysis confirmed the affiliation of these isolates with Nitrosospira clusters 3 and 4. Two amoA clone sequences matched T-RFLP fingerprints found in soil, but they were not found among the isolates.For many years the process of nitrification was thought to play a minimal role in N cycling in coniferous forest ecosystems. Based on the physiological properties of a limited number of NH 3 -oxidizing bacteria (AOB) in pure culture, several soil factors were considered detrimental to them, including soil acidity, high C/N ratios, low N availability, and the presence of allelochemical compounds (12,13,22,38,39,49,54). With the advent of improved 15 N tracer methods and mass spectrometric technology, N cycling processes were shown to be tightly coupled in forest soils and that both nitrification and NO 3 Ϫ assimilation can occur simultaneously with mineralization and ammonification (11,15,45,53). These observations stimulated interest in learning more about the nature and physiological ecology of AOB in forest ecosystems (12,14,30,44,51).In recent years, considerable progress has been made in elucidating the composition of AOB communities in soil by taking advantage of variation in gene sequences of either 16S ribosomal DNA (rDNA) or the catalytic subunit of NH 3 monooxygenase, amoA (2,19,37,40). Most research has focused on grasslands and agroecosystems where, in general, soil populations are dominated by the genus Nitrosospira rather than by the widely studied genus Nitrosomonas (4, 8, 24-28, 36, 47, 48). Nitrosospira can be grouped into several phylogenetically distinct clusters (3, 25), with cluster 3 being widely distributed in agricultural soils with high N availability (8,18,27,28,55). Clusters 1, 2, and 4 are more prevalent in soils that are either acidic (47, 48), retired from agricultural use (27, 28), or that were never exposed to tillage and/or applications of N fertilizer (8, 55).The objective of the present study was to examine the nature of the AOB communities associated with soil along high-elevation (ϳ1,500 m) meadow-to-forest transects at two sites in the H. J. Andrews Experimental Forest of Oregon. We hypothesized that these transects would provide a gradient along which differences in vegetation composition and plant N inputs interacting with chang...