Denitrification and Bacterial Numbers in Riparian Soils of a Wyoming Mountain Watershed

The presence and activity of denitrifying bacteria as well as bacteria capable of reducing sulfate in 1 upland and 5 riparian soils of a mountain watershed in Wyoming were studied. Bacteria were enumerated from soil samples collected during summer along transects placed perpendicular to stream flow. Samples were taken at 3 depths within each plant community. Subsamples were frozen and later utilized to determine denitrification potential. Higher counts of total heterotrophic aerobic bacteria, sulfatereducing bacteria, denitrifying bacteria, and denitrification potential existed in the upper 5 to 15 cm of soil than at 30 cm. Soils located close to the stream's edge tended to have more bacterial activity than those further from the stream, indicating that these soils may be important areas for nitrate and sulfate reduction. Soil organic matter and water content decreased with depth in all plant communities, and those closer to the stream contained more organic matter and water than those further from the stream. Riparian ecosystems are ecologically important vegetative communities (Odum 1978). Compared to upland vegetation types, riparian zones in the Rocky Mountain States are relatively limited in area and often receive extensive user pressure by livestock, wildlife, and man (Busby 1978, Jahn 1978, Johnson 1978). These users are often accused of causing excessive damage to associated aquatic habitat, vegetation, and stream channel stability, which often causes a reduction of soil moisture, plant production, and species composition (Settergren 1977, Platts 1978, Haugen and Duff 1982). The riparian zones' location between upland and aquatic ecosystems represents a transition zone and consequently may be of particular importance in understanding the contribution of nutrients to nearby waters from the uplands (Kirby 1978). Numerous studies using vegetation, channel morphology, and sediment deposition show changes occurring in riparian zones due to livestock grazing (Duff and Cooper 1978, and Platts et al. 1983). Other authors have used bacteria indicative of fecal pollution for assessing ungulate grazing contribution to stream pollution (Morrison and Fair 1966, Jawson et al. 1982, Skinner et al. 1984a). Bacteria other than those used to determine fecal contamination in streams may be useful to predict user impact on riparian zones. Data from Skinner et al. (1984b) suggested denitrifying bacteria may be associated with stream bottom and bank areas in a mountain drainage basin. These bacteria were the only population out of several monitored to show significant response downstream even with settling of organisms because of depression storage. Bacteria capable of reducing sulfate were also present. It is possible these organisms may be entering stream flow from riparian zones through bank-stream interflow (Morrison and Fair 1966). The purpose of this study was to document: (1) denitrifyingand sulfatereducing bacterial numbers present in riparian plant communities, (2) differences in numbers of denitrifying and sulfate-reducing bac-