Physical, Biogeochemical and Hydrological Controls of Streambed Nutrient Cycling and Greenhouse Gas Production

Abstract:

Enhanced reactivity due to elevated residence times and substrate availability in the hyporheic zone, where groundwater and surface water mixes, produces hotspots and hot moments of enhanced biogeochemical reactivity. Streambeds, therefore, play a crucial role in reducing water pollution and improving ecosystem health through increased biogeochemical cycling, which is especially important in agricultural catchments. However, key reactions involved in carbon and nitrogen cycling also produce greenhouse gases. Despite their importance the controls on streambed C and N cycling and greenhouse gases in particular, remain insufficiently understood. Here we present results from in-situ biogeochemical sampling and tracer experiments combined with laboratory incubation experiments from agricultural, lowland streams in the UK. Concentrations of nutrients, dissolved organic carbon and greenhouse gases, combined with d¹⁵N_NO3^- and d¹⁸O_NO3^- isotopes were measured seasonally in streambed sediments using multilevel piezometers, in conjunction with tracer experiments using conservative (Fluorescein) and smart (Resazurin-Resorufin) tracers to determine advective transport times, dispersion and transient storage, and in-stream metabolism in sub-reaches of the stream. Incubation experiments to determine controls on CO₂ and CH₄ emissions, rates of denitrification and microbial activity were also performed. Our results show large differences in C and N cycling between sub-reaches dominated by sandy sediments and those dominated by gravel sediments, as well as seasonally. Rates of denitrification and microbial activity were also dependent on sediment type, as well as substrate availability and carbon quality. CO₂ and CH₄ emissions were influenced by temperature and organic matter quantity and quality. Streambed concentrations were correlated with advective transport but not dispersion and transient storage. This suggests temperature, sediment type, substrate and advective transport are key controls on streambed nutrient cycling and greenhouse gas production. The results of this study have important implications for future biogeochemical cycling and greenhouse gas estimates from streams and rivers, particularly as the significance of sediment greenhouse gas production is recognised.