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 important emissions of carbon dioxide (CO) and methane (CH) from surface waters to the atmosphere. Streambed sediments are particularly important in affecting stream chemistry, with rates of biogeochemical activity, and CO and CH concentrations far exceeding those in surface waters. Despite an increase in research on CO and CH in streambed sediments there is a lack of knowledge and insight on seasonal dynamics. In this study the seasonally variable effect of sediment type (sand-dominated versus gravel-dominated) on porewater C cycling, including CO and CH concentrations, was investigated. We found high concentrations of CO and CH in the streambed of a small agricultural stream. Sand-dominated sediments were characterised by higher microbial activity and CO and CH concentrations than gravel-dominated sediments, with CH:CO ratios higher in sand-dominated sediments but rates of recalcitrant C uptake highest in gravel-dominated sediments. CO and CH concentrations were unexpectedly high year-round, with little variation in concentrations among seasons. Our results indicate that small, agricultural streams, which generally receive large amounts of fine sediment and organic matter (OM), may contribute greatly to annual C cycling in freshwater systems. These results should be considered in future stream management plans where the removal of sandy sediments may perform valuable ecosystem services, reducing C turnover, CO and CH concentrations, and mitigating greenhouse gas (GHG) production.