Separating natural from human enhanced methane emissions in headwater streams

Abstract

Headwater streams are natural sources of methane but are suffering severe anthropogenic disturbance, particularly land use change and climate warming. The widespread intensification of agriculture since the 1940s has increased the export of fine sediments from land to streams, but systematic assessment of their effects on stream methane is lacking. Here we show that excess fine sediment delivery is widespread in UK streams (n = 236) and, set against a pre-1940s baseline, has markedly increased streambed organic matter (23 to 100 g m−2), amplified streambed methane production and ultimately tripled methane emissions (0.2 to 0.7 mmol CH4 m−2 d−1, n = 29). While streambed methane production responds strongly to organic matter, we estimate the effect of the approximate 0.7 °C of warming since the 1940s to be comparatively modest. By separating natural from human enhanced methane emissions we highlight how catchment management targeting the delivery of excess fine sediment could mitigate stream methane emissions by some 70%.

Organic Matter Accumulation and Hydrology as Drivers of Greenhouse Gas Dynamics in Newly Developed Artificial Channels

“…The shift in hydrological conditions from river to flume promoted the settling and accumulation of suspended particulate material that would otherwise remain in suspension in the River Queich. This can be explained in terms of the specific stream energy (ω), 45 which expresses the capacity of a lotic system to transport sediment 46 as a function of discharge, channel slope, and width. If we assume a representative ω of 4.76 × 10 −3 J m −2 s −1 for the River Queich (see Table S1 for details), then our flumes would be characterized by ∼1.2% of that energy flux under the Control regime and 0.6% under the Low Flow regime (Table S2).…”

Recognizing Agricultural Headwaters as Critical Ecosystems

“…In addition, estimates of CH 4 emissions rely heavily on diffusive measurements, largely overlooking the contribution of CH 4 from ebullition, which can be substantial during episodic events. Deposition of fine sediments has consistently been reported as a key driver of CH 4 production suggesting that low-gradient and fluvially unstable agricultural headwaters prone to erosion can support methanogenesis by providing organic matter-rich material and anoxic conditions. From a management perspective, the challenge of mitigating indirect GHG emissions has to be addressed with broader approaches, that integrate traditional stream mitigation measures (e.g., buffer zones, floodplains, and channel impoundments) with in-field measures that also target the landscape source and delivery of GHG …”

A rationale for higher ratios of CH4 to CO2 production in warmer anoxic freshwater sediments and soils

“…To explore this, we compiled data from the Web of Science for CH 4 and CO 2 production in typical microcosm incubations of sediments or water‐logged soils—examples of our search terms are included as metadata with the source data at https://doi.org/10.5281/zenodo.7612326. As we knew this search to be incomplete—for example, omitting the terms “oxidation” and “marine” missed Roden and Wetzel (1996) and Shelley et al (2015) that both report CH 4 and CO 2 production, we supplemented our search using the compilations in Yvon‐Durocher et al (2014) and Wilson et al (2017) or those related to our previous publications (Zhu et al 2020, 2022). Note, we further excluded any publications where the authors had not stated explicitly that they had accounted for all of the CO 2 produced in their incubations, that is, CO 2 gas in the headspace plus that dissolved in the water phase (∑DIC: CO 2 , , ).…”

“…Whereas only measuring the incubation headspace would typically capture >95% of any CH 4 produced, the same is not true for CO 2 which is approximately 100 times more soluble than CH 4 . Our search yielded 64 publications and a total of 512 measurements of CH 4 and CO 2 production including: lakes ( n = 109), wetlands ( n = 276), rice paddies ( n = 64), a reservoir ( n = 28), rivers ( n = 4) and our own incubations of streambed sediments ( n = 31, Zhu et al 2022) for which we previously published just the CH 4 data. We extracted the data using WebPlotDigitizer (https://automeris.io/WebPlotDigitizer/) and calculated CH 4 to CO 2 production ratios.…”