Seasonal variability of sediment controls of carbon cycling in an agricultural stream

Sophie A Comer-Warner,Daren C Gooddy,Sami Ullah,Luke Glover,Aishling Percival,Nicholas Kettridge,Stefan Krause

doi:10.1016/j.scitotenv.2019.06.317

Sophie A Comer-Warner, Daren C Gooddy + Show 5 more

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https://doi.org/10.1016/j.scitotenv.2019.06.317

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Abstract

Streams and rivers are ‘active pipelines’ where high rates of carbon (C) turnover can lead to globally important emissions of carbon dioxide (CO2) and methane (CH4) from surface waters to the atmosphere. Streambed sediments are particularly important in affecting stream chemistry, with rates of biogeochemical activity, and CO2 and CH4 concentrations far exceeding those in surface waters. Despite an increase in research on CO2 and CH4 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 CO2 and CH4 concentrations, was investigated. We found high concentrations of CO2 and CH4 in the streambed of a small agricultural stream. Sand-dominated sediments were characterised by higher microbial activity and CO2 and CH4 concentrations than gravel-dominated sediments, with CH4:CO2 ratios higher in sand-dominated sediments but rates of recalcitrant C uptake highest in gravel-dominated sediments. CO2 and CH4 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, CO2 and CH4 concentrations, and mitigating greenhouse gas (GHG) production.

Highlights

We found high CO2 and CH4 concentrations in the streambed sediments of an agricultural river controlled by sediment type, with sanddominated sediments characterised by significantly higher CO2 and CH4 concentrations than gravel-dominated sediments, despite both sediment types having relatively low organic matter (OM) content (b3%)
This enhanced cycling resulted in high CH4:CO2 ratios in the sand-dominated sediments, suggesting that CH4 production is higher in sand-dominated than gravel-dominated sediments but that CH4 oxidation is higher in gravel-dominated sediments
The highest concentrations of CO2 and CH4 both found in the sand-dominated sediments suggests that CO2 and CH4 production may have been co-located

Summary

Introduction

Since streams and rivers have been recognised as ‘active pipelines’, where biogeochemical processes alter solutes transported in their water, there has been an increased interest in their role in the carbon (C) cycle (Battin et al, 2009; Cole et al, 2007; Comer-Warner et al, 2018; Raymond et al, 2013; Trimmer et al, 2012). Streams and rivers are recognised as globally important with respect to C emissions, contributing 1.8 Pg CO2-C yr−1 (Raymond et al, 2013), and 26.8 Tg CH4-C yr−1 (Stanley et al, 2016) into the atmosphere. Small streams have been found to be of disproportionate importance, and are estimated to contribute ~15% of the annual CO2 flux from streams and rivers (Raymond et al, 2013)

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