Atmospheric methane concentrations reached record highs in 2021, emphasizing the need to address all sources. Rivers and streams, despite covering a small land area, significantly contribute to the global methane budget. These waterways transform organic matter in bed sediments into methane, emitting approximately 31 Tg annually. Anthropogenic disturbances, particularly intensified agriculture, threaten water resources and biodiversity. Intensified agriculture increases excess fine sediment (<2 mm) delivery to streams, impacting water quality. While some sediment delivery is natural, excess fine sediment from agriculture can reduce bed permeability and oxygen flow, creating conditions favorable for methane production. Although elevated methane concentrations in agricultural catchments are known, a systematic separation of natural and human-enhanced emissions has been lacking. Future climate change is projected to exacerbate the issue, with increased sediment and nutrient loss requiring significant land cover changes for mitigation. Headwater streams, comprising 88% of global stream length, are particularly vulnerable due to their shallowness and slow flow, facilitating sediment deposition. This study aims to partition the effects of warming and excess fine sediment on methane emissions in UK headwater streams, using pre-1940s sediment yields as a baseline to quantify human-enhanced changes since the intensification of agriculture.

Previous research has established rivers and streams as significant contributors to global methane emissions, highlighting the importance of understanding the drivers of methane production in these ecosystems. Studies have shown the active transformation of organic matter into methane in streambed sediments, and the contribution of catchment-derived methane. The negative impacts of intensified agriculture, a global threat, on water quality and stream ecosystems have been well-documented. The role of excess fine sediment delivery in degrading water quality and altering streambed environments is also recognized. While correlations between methane concentrations (or production) and organic matter or temperature are noted, a systematic analysis separating the impacts of excess sediment and warming on stream methane emissions is lacking. Prior work has shown that increased organic matter in lake and reservoir sediments can significantly enhance methane production. However, the extent to which excess fine sediment driven by agriculture influences stream methane emissions compared to warming has not been systematically studied.

This study used data from 236 UK streams, mostly small headwaters, to assess the impact of excess fine sediment delivery on methane emissions. Pre-1940s sediment yields were used to establish a natural baseline, reflecting rates sustainable for a healthy aquatic habitat. Streams were categorized into three groups based on excess fine sediment delivery relative to the baseline: natural, mild pressure, and severe pressure. Streambed organic matter (ash-free-dry weight per m²) was measured using a disturbance technique. To standardize for the stream's capacity to transport and flush out sediment, excess fine sediment delivery was standardized to specific stream power. Separate laboratory incubations were conducted with sediments from 14 streams to determine the temperature sensitivity and capacity of streambed methane production. The relationship between methane production and organic matter, and the interaction with temperature, were analyzed using linear mixed-effects models. To assess the effect of substrates on methane production, experiments were conducted with additional methanogenic substrates (acetate, hydrogen, betaine, TMA, propionate). Finally, methane emissions from a subset of 29 streams were measured and standardized to specific stream power to quantify the relationship between methane emissions and excess fine sediment delivery. Statistical analyses included linear models and mixed-effects models to account for variations in the data.

The study found that 80% of the sampled streams experienced mild to severe excess fine-sediment pressure compared to the pre-1940s baseline. Streambed organic matter showed a positive correlation with standardized excess fine-sediment delivery. The intensification of agriculture since the 1940s increased streambed organic matter fourfold (from a median of 23 g m⁻² to 100 g m⁻²). Laboratory incubations revealed a conserved temperature sensitivity of methane production (1.1 eV), consistent across all streams, while the methane production capacity varied widely (10,000-fold), strongly correlating with organic matter content. Experimental addition of methanogenic substrates significantly enhanced the capacity of sediments to produce methane, highlighting substrate limitation. Standardized methane emissions increased with excess fine sediment delivery. Prior to agricultural intensification, median methane emissions were 0.2 mmol CH₄ m⁻² d⁻¹, while present-day emissions reached 0.7 mmol CH₄ m⁻² d⁻¹, a 3.5-fold increase. Streams under severe sediment pressure showed a sevenfold increase in methane emissions. The analysis revealed that the increase in streambed organic matter due to excess fine sediment was considerably greater than the effects of warming since the 1940s.

The study's findings clearly demonstrate the significant role of excess fine sediment delivery from intensified agriculture in enhancing methane emissions from headwater streams. The large variation in methane production capacity highlights the dominant control exerted by organic matter, while the conserved temperature sensitivity emphasizes the importance of substrate availability. By establishing a pre-1940s baseline, the study effectively separated natural from human-enhanced methane emissions. The results support the conclusion that targeting excess fine sediment management could significantly reduce stream methane emissions. This is particularly relevant given the projected increases in sediment loss due to climate change. The comparable sensitivity of methane production to organic matter observed in stream sediments and wetland soils suggests a widespread phenomenon. While future warming will contribute to increased methane emissions, managing excess organic matter is considered more readily achievable than mitigating warming effects.

This research quantifies the substantial contribution of human-enhanced fine sediment delivery to increased methane emissions from headwater streams. The findings highlight the potential for significant methane emission reductions through effective catchment management focused on mitigating excess fine sediment. Future research should investigate the long-term impacts of climate change on stream methane emissions, considering the synergistic effects of warming and increased organic matter. Further research is needed to assess the feasibility and effectiveness of various management strategies aimed at reducing agricultural sediment runoff and their impact on stream ecosystems.

The study's focus on UK streams might limit the generalizability of the findings to other regions with different climatic conditions, agricultural practices, or stream characteristics. The use of pre-1940s sediment yields as a natural baseline assumes that these yields accurately represent natural conditions, which might not perfectly capture the variability or changes in natural sediment delivery. The study primarily focused on diffusive methane emissions, and further investigation into ebullitive emissions might provide a more comprehensive picture. The sampling of different streams at different times might have influenced the results.