Relationships between bacterial productivity and organic carbon at a soil-stream interface

Microbial communities at soil-stream interfaces may be particularly importa nt in regulating amounts and forms of nutrients that leave upland soils and enter stream ecosystems. While microbial communities are thought to be res ponsible for key nutrient transformations within near-stream sediments, the re is relatively little mechanistic information on factors that control mic robial activities in these areas. In this study, we examine the roles of di ssolved organic carbon (DOC) vs. particulate organic carbon (POC) as potent ial controls on rates of bacterial productivity (measured as incorporation of [H-3]thymidine into bacterial DNA) and amounts of bacterial biomass (mea sured as fatty acid yield) in sediments along a transect perpendicular to a soil-stream interface. We hypothesized that spatial patterns in bacterial productivity would vary in response to strong and persistent patterns in po re-water concentrations of DOC that were observed along a soil-stream trans ect throughout a 2-year period, Our results did not support the existence o f such a link between pore-water DOC and bacterial productivity. In contras t, we found bacterial productivity and biomass were related to small-scale spatial variations in sediment POC on 3 of 4 sample dates. While our result s indicate that total bacterial productivity in near-stream sediments is no t consistently linked to spatial variations in pore-water DOG, it is likely that DOC and POC are not mutually exclusive and the relative contribution of DOC and POC to sedimentary microbes varies temporally and spatially in d ifferent riparian habitats.