Streambed organic matter controls on carbon dioxide and methane emissions from streams

Greenhouse gas (GHG) emissions of carbon dioxide (CO2) and methane (CH4) from streambeds are currently understudied. There is a paucity of research exploring organic matter (OM) controls on GHG production by microbial metabolic activity in streambeds, which is a major knowledge gap given the increased inputs of allochthonous carbon to streams, especially in agricultural catchments. This study aims to contribute to closing this knowledge gap by quantifying how contrasting OM contents in different sediments affect streambed GHG production and associated microbial metabolic activity. We demonstrate, by means of an incubation experiment, that streambed sediments have the potential to produce substantial amounts of GHG, controlled by sediment OM quantity and quality. We observed streambed CO2 production rates that can account for 35% of total stream evasion estimated in previous studies, ranging between 1.4 and 86% under optimal conditions. Methane production varied stronger than CO2 between different geologic backgrounds, suggesting OM quality controls between streambed sediments. Moreover, our results indicate that streambed sediments may produce much more CO2 than quantified to date, depending on the quantity and quality of the organic matter, which has direct implications for global estimates of C fluxes in stream ecosystems

Opening opportunities for high-resolution isotope analysis - Quantification of δ15NNO3 and δ18ONO3 in diffusive equilibrium in thin–film passive samplers

The fate of nitrate transported across groundwater-surface water interfaces has been intensively studied in recent decades. The interfaces between aquifers and rivers or lakes have been identified as biogeochemical hotspots with steep redox gradients. However, a detailed understanding of the spatial heterogeneity and potential temporal variability of these hotspots, and the consequences for nitrogen processing, is still hindered by a paucity of adequate measurement techniques. A novel methodology is presented here, using Diffusive Equilibrium in Thin-film (DET) gels as high-spatial-resolution passive-samplers of δ15NNO3 and δ18ONO3 to investigate nitrogen cycling. Fractionation of δ15NNO3 and δ18ONO3 during diffusion of nitrate through the DET gel was determined using varying equilibrium times and nitrate concentrations. This demonstrated that nitrate isotopes of δ15NNO3 and δ18ONO3 do not fractionate when sampled with a DET gel. δ15NNO3 values from the DET gels ranged between 2.3 ± 0.2 and 2.7 ± 0.3‰ for a NO3– stock solution value of 2.7 ± 0.4‰, and δ18ONO3 values ranged between 18.3 ± 1.0 and 21.5 ± 0.8‰ for a NO3– stock solution of 19.7 ± 0.9‰. Nitrate recovery and isotope values were independent of equilibrium time and nitrate concentration. Additionally, an in situ study showed that nitrate concentration and isotopes provide unique, high-resolution data that enable improved understanding of nitrogen cycling in freshwater sediments

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

Agricultural streams receive large inputs of nutrients, such as nitrate (NO3−) and ammonium (NH4+), which impact water quality and stream health. Streambed sediments are hotspots of biogeochemical reactivity, characterised by high rates of nutrient attenuation and denitrification. High concentrations of nitrous oxide (N2O) previously observed in stream sediments point to incomplete denitrification, with sediments acting as a potentially significant source of global N2O. We investigated the effect of sediment type and seasonal variation on denitrification and N2O production in the streambed of an agricultural UK stream. Denitrification was strongly controlled by sediment type, with sand-dominated sediments exhibiting potential rates of denitrification almost 10 times higher than those observed in gravel-dominated sediments (0.026 ± 0.004 N2O–N μg g−1 h−1 for sand-dominated and 0.003 ± 0.003 N2O–N μg g−1 h−1 for gravel-dominated). In-situ measurements supported this finding, with higher concentrations of NO3−, nitrite (NO2−) and N2O observed in the porewaters of gravel-dominated sediments. Denitrification varied substantially between seasons, with denitrification increasing from winter to autumn. Our results indicate highest NO3− reduction occurred in sand-dominated sediments whilst highest N2O concentrations occurred in gravel-dominated sediments. This suggests that finer-grained streambeds could play an important role in removing excess nitrogen from agricultural catchments without producing excess N2O

Reply to ‘Pseudoreplication and greenhouse-gas emissions from rivers'

Tiegs et al.1 highlight the significance and relevance of the findings of Comer-Warner et al.2 on greenhouse-gas emissions from streambed sediments but raise questions about some aspects of the experimental design. We support their call for more detailed field and laboratory-based studies on this subject. However, we believe that their concerns relate to uncertainties and limitations in the experimental design that were discussed explicitly in the original paper (and accompanying transparent peer review process—available online), or represent criticisms related to highly improbable minor anomalies that may unnecessarily dismiss experimental results as discussed below

Elevated temperature and nutrients lead to increased N2O emissions from salt marsh soils from cold and warm climates

Salt marshes can attenuate nutrient pollution and store large amounts of ‘blue carbon’ in their soils, however, the value of sequestered carbon may be partially offset by nitrous oxide (N2O) emissions. Global climate and land use changes result in higher temperatures and inputs of reactive nitrogen (Nr) into coastal zones. Here, we investigated the combined effects of elevated temperature (ambient + 5℃) and Nr (double ambient concentrations) on nitrogen processing in marsh soils from two climatic regions (Quebec, Canada and Louisiana, U.S.) with two vegetation types, Sporobolus alterniflorus (= Spartina alterniflora) and Sporobolus pumilus (= Spartina patens), using 24-h laboratory incubation experiments. Potential N2O fluxes increased from minor sinks to major sources following elevated treatments across all four marsh sites. One day of potential N2O emissions under elevated treatments (representing either long-term sea surface warming or short-term ocean heatwaves effects on coastal marsh soil temperatures alongside pulses of N loading) offset 15–60% of the potential annual ambient N2O sink, depending on marsh site and vegetation type. Rates of potential denitrification were generally higher in high latitude than in low latitude marsh soils under ambient treatments, with low ratios of N2O:N2 indicating complete denitrification in high latitude marsh soils. Under elevated temperature and Nr treatments, potential denitrification was lower in high latitude soil but higher in low latitude soil as compared to ambient conditions, with incomplete denitrification observed except in Louisiana S. pumilus. Overall, our findings suggest that a combined increase in temperature and Nr has the potential to reduce salt marsh greenhouse gas (GHG) sinks under future global change scenarios

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 (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

Thermal sensitivity of CO2 and CH4 emissions varies with streambed sediment properties

Globally, rivers and streams are important sources of carbon dioxide and methane, with small rivers contributing disproportionately relative to their size. Previous research on greenhouse gas (GHG) emissions from surface water lacks mechanistic understanding of contributions from streambed sediments. We hypothesise that streambeds, as known biogeochemical hotspots, significantly contribute to the production of GHGs. With global climate change, there is a pressing need to understand how increasing streambed temperatures will affect current and future GHG production. Current global estimates assume linear relationships between temperature and GHG emissions from surface water. Here we show non-linearity and threshold responses of streambed GHG production to warming. We reveal that temperature sensitivity varies with substrate (of variable grain size), organic matter (OM) content and geological origin. Our results confirm that streambeds, with their non-linear response to projected warming, are integral to estimating freshwater ecosystem contributions to current and future global GHG emissions

Spartina alterniflora has the highest methane emissions in a St. Lawrence estuary salt marsh

Publication associated with dataset 'Methane fluxes from four elevation zones in a St. Lawrence Estuary salt marsh' (https://doi.org/10.5281/zenodo.6500188) funded under the European Union's Marie Skłodowska–Curie Action project number 838296 MarshFlux: The effect of future global climate and land-use change on greenhouse gas fluxes and microbial processes in salt marshes. Salt marshes have the ability to store large amounts of 'blue carbon', potentially mitigating some of the effects of climate change. Salt marsh carbon storage may be partially offset by emissions of CH4, a highly potent greenhouse gas. Sea level rise and invasive vegetation may cause shifts between different elevation and vegetation zones in salt marsh ecosystems. Elevation zones have distinct soil properties, plant traits and rhizosphere characteristics, which affect CH4 fluxes. We investigated differences in CH4 emissions between four elevation zones (mudflat, Spartina alterniflora, Spartina patens and invasive Phragmites australis) typical of salt marshes in the northern Northwest Atlantic. CH4 emissions were significantly higher from the S. alterniflora zone (17.7 ± 9.7 mg C m−2h−1) compared to the other three zones, where emissions were negligible (<0.3 mg C m−2h−1). These emissions were high for salt marshes and were similar to those typically found in oligohaline marshes with lower salinities. CH4 fluxes were significantly correlated with soil properties (salinity, water table depth, bulk density and temperature), plant traits (rhizome volume and biomass, root volume and dead biomass volume all at 0–15 cm) and CO2 fluxes. The relationships between CH4 emissions, and rhizome and root volume suggest that the aerenchyma tissues in these plants may be a major transport mechanism of CH4 from anoxic soils to the atmosphere. This may have major implications for the mitigation potential carbon sink from salt marshes globally, especially as S. alterniflora is widespread. This study shows CH4 fluxes can vary over orders of magnitude from different vegetation in the same system, therefore, specific emissions factors may need to be used in future climate models and for more accurate carbon budgeting depending on vegetation type