Carbon and nutrients regulate greenhouse gas fluxes from oxic stream sediments

Streams play a significant role in global biogeochemical cycles and are usually sources of greenhouse gasses to the atmosphere. To better understand controls on greenhouse gas production from stream sediments and their contribution to whole stream greenhouse gas evasion, we estimated fluxes of CO2, CH4, and N2O from 48 streams in southern Ontario, Canada that represented a gradient in both the amount and composition of dissolved organic carbon in addition to dissolved nutrients. Greenhouse gas fluxes from oxic sediment incubations were far lower than the fluxes estimated from water column greenhouse gas concentrations at the whole stream scale. Moreover, variation in sediment greenhouse gas fluxes was not positively associated with variation in those estimated from the whole stream or with water column greenhouse gas concentrations. However, dissolved organic carbon and nutrient concentrations were associated with increased sediment CO2 and N2O fluxes but consistent variation in dissolved organic matter composition did not appear to regulate these fluxes. Sediment physicochemical properties were unable to explain any variation in sediment CH4 fluxes. Although whole stream greenhouse gas fluxes integrate heterogeneity in biogeochemical processes within streams, our findings suggest that changes to streams and/or their watersheds resulting in the overall enrichment of nutrients and carbon (e.g., land use change) will increase the production of CO2 and N2O from sediments.