Seasonal Variability of Sediment Controls of Carbon Cycling in an Agricultural Stream

Streams and rivers are‘active pipelines’where high rates of carbon (C) turnover can lead to globally importantemissions of carbon dioxide (CO2) and methane (CH4) from surface waters to the atmosphere. Streambed sedi-ments are particularly important in affecting stream chemistry, with rates of biogeochemical activity, and CO2and CH4concentrations far exceeding those in surface waters. Despite an increase in research on CO2and CH4instreambed sediments there is a lack of knowledge and insight on seasonal dynamics. In this study the seasonallyvariable effect of sediment type (sand-dominated versus gravel-dominated) on porewater C cycling, includingCO2and CH4concentrations, was investigated. We found high concentrations of CO2and CH4in the streambed ofa small agricultural stream. Sand-dominated sediments were characterised by higher microbial activity and CO2and CH4concentrations than gravel-dominated sediments, with CH4:CO2ratios higher in sand-dominated sedi-ments but rates of recalcitrant C uptake highest in gravel-dominated sediments. CO2and CH4concentrationswere unexpectedly high year-round, with little variation in concentrations among seasons. Our results indicatethat small, agricultural streams, which generally receive large amounts offine sediment and organic matter(OM), may contribute greatly to annual C cycling in freshwater systems. These results should be considered in fu-ture stream management plans where the removal of sandy sediments may perform valuable ecosystem services,reducing C turnover, CO2and CH4concentrations, and mitigating greenhouse gas (GHG) production