Methane (CH₄) flux data for Moidach More. Years 1997 - 1998

Methane (CH4) emission data collected from an extensive pristine portion of raised peat bog located in Moidach More, Morayshire, northeast Scotland (UK). CH4 flux was measured using static chambers, which were sampled on a weekly (late spring to early autumn) to monthly basis (late autumn to early spring), starting in May 1997

Forests and methane:looking beyond carbon for nature-based climate solutions

Forests, their preservation and expansion have been long championed as nature-based climate solutions for their capacity to remove carbon dioxide from the atmosphere and retain it as carbon within living and dead organic matter (Griscom et al 2017). However, as our understanding of forests evolves, with major forest biomes either at carbon saturation or experiencing loss (Hubau et al 2020) it becomes clear that focusing solely on their carbon dynamics overlooks their involvement in the cycling of other powerful greenhouse gases such as methane. The role of vegetation more broadly, and trees more specifically in the methane cycle is complex with attention for nearly two decades focussed on the function of lowland-wetland trees as potential sources or emission pathways for soil-produced methane (Pangala et al 2017, Bastviken et al 2023). This has clarified our understanding of flooded trees as point sources of wetland methane emissions responsible for up to half of all wetland methane emitted from the Amazon floodplain and elsewhere (Gauci et al 2022). We now need to consider methane cycling in the vast expanse of upland forest on free draining soils with low water tables. In these ecosystems, methane exchange has been more difficult to disentangle chiefly due to the difficulty of making in situ measurements of very small fluxes, in either direction, and identifying their source from a range of processes at scale and from within relatively inaccessible (for chamber measurement campaigns) tree crowns and canopies. These sources include aerobic sources that have wide uncertainties (Keppler et al 2006, Kohl et al 2023). However, in Gauci et al (2024), woody stem surfaces of upland trees were shown to be a significant locus of atmospheric methane removal (AMR), potentially as large or larger than the well-understood global soil sink (Dunfield 2007). Collectively, and in the tropics in particular, tree woody surface AMR via methanotrophy may add upwards of 10% to the climate benefit of trees via processes that are entirely independent of the carbon they contain (Gauci et al 2024). Here, I elaborate on this discovery to identify the opportunities, current uncertainties and areas for further investigation in an effort to use tree woody surface AMR as a nature-based solution to reduce growth in atmospheric methane

Acid rain links to CH₄ emissions from wetlands

A variety of approaches, spanning a range of spatial and temporal scales, were applied to the investigation of the effects of low dose SO42- deposition, at rates comparable to those experienced in acid rain impacted areas, on methane (CH4) emissions from natural wetlands.Over two years of experimental manipulation of SO42- deposition to a peatland in northeast Scotland, CH4 emissions were suppressed by around 40%. There was no significant difference in suppression of CH4 flux within the sol- deposition range of 25-100 kg-S ha-1yr-1. In a similar short-term controlled environment SO42- manipulation experiment, the suppressive effect of SO42- was found to be independent of the simulated SO42- deposition rate within a range of 15-100 kg-S ha-1yr-1. The possibility that suppression of CH4 fluxes may have been the result of a 'salt effect' was ruled out. Both temperature and water table controlled the extent of CH4 flux suppression in acid rain impacted wetlands.Sulfate reduction potential in SO42- treatments were found to be 10 times larger than in control plots, suggesting that long-term suppression of CH4 fluxes is the result of the formation of an enlarged population of competitively superior sulfate reducing bacteria. SO42- concentrations were smaller in peat pore water from SO42- treatments than from controls. This is possibly the result of a stimulated SO42- reducing community scavenging available SO42-, thereby decreasing concentrations to below ambient levels. In northern peatlands (>50°) the effect of SO42- deposition at 1990 rates may have been sufficient to reduce emissions from these systems by around 15% annually. Globally, the effect of acid rain SO42- deposition may be sufficient to reduce CH4 emissions by as much as 22-28 Tg by 2030, which places this interaction within the same size category as many other components of the global CH4 budget that have received far greater attention

Tree stem bases are sources of CH₄ and N₂O in a tropical forest on upland soil during the dry to wet season transition

Tropical forests on upland soils are assumed to be a methane (CH4) sink and a weak source of nitrous oxide (N2O), but studies of wetland forests have demonstrated that tree stems can be a substantial source of CH4,and recent evidence from temperate woodlands suggests that tree stems can also emit N2O. Here, we measured CH4 and N2O fluxes from the soil and from tree stems in a semi-evergreen tropical forest on upland soil. To examine the influence of seasonality, soil abiotic conditions, and substrate availability (litter inputs) on trace greenhouse gas (GHG) fluxes, we conducted our study during the transition from the dry to the wet season in a long-term litter manipulation experiment in Panama, Central America. Trace GHG fluxes were measured from individual stem bases of two common tree species and from soils beneath the same trees. Soil CH4 fluxes varied from uptake in the dry season to minor emissions in the wet season. Soil N2O fluxes were negligible during the dry season but increased markedly after the start of the wet season. By contrast, tree stembases emitted CH4 and N2O throughout the study. Although we observed no clear effect of litter manipulation on trace GHG fluxes, tree species and litter treatments interacted to influence CH4 fluxes from stems and N2O fluxes from stems and soil, indicating complex relationships between tree species traits and decomposition processes that can influence trace GHG dynamics. Collectively, our results show that tropical trees can act as conduits for trace GHGs that most likely originate from deeper soil horizons, even when they are growing on upland soils. Coupled with the finding that the soils may be a weaker sink for CH4 than previously thought, our research highlights the need to reappraise trace gas budgets in tropical forests

Characterisation of the semi-volatile component of Dissolved Organic Matter by Thermal Desorption – Proton Transfer Reaction – Mass Spectrometry

Proton Transfer Reaction – Mass Spectrometry (PTR-MS) is a sensitive, soft ionisation method suitable for qualitative and quantitative analysis of volatile and semi-volatile organic vapours. PTR-MS is used for various environmental applications including monitoring of volatile organic compounds (VOCs) emitted from natural and anthropogenic sources, chemical composition measurements of aerosols, etc. Here we apply thermal desorption PTR-MS for the frst time to characterise the chemical composition of dissolved organic matter (DOM). We developed a clean, low-pressure evaporation/sublimation system to remove water from samples and coupled it to a custom-made thermal desorption unit to introduce the samples to the PTR-MS. Using this system, we analysed waters from intact and degraded peat swamp forest of Kalimantan, Indonesian Borneo, and an oil palm plantation and natural forest in Sarawak, Malaysian Borneo. We detected more than 200 organic ions from these samples and principal component analysis allowed clear separation of the diferent sample origins based on the composition of organic compounds. The method is sensitive, reproducible, and provides a new and comparatively cheap tool for a rapid characterisation of water and soil DOM

A large fluvial pulse of organic carbon following the Bornean peat swamp forest fires of 2009

Tropical peatlands are high carbon density ecosystems that are currently extremely vulnerable to destabilisation through human and climate induced changes. They contain 89 Gt carbon (3% global soil carbon) but rapid land use changes (deforestation, drainage) and consequent fire are rapidly releasing this. There are approximately 27.1 million hectares (Mha) (an area approximately equivalent in size to the UK) of peatland in Southeast Asia which accounts for around 56% of all tropical peatlands worldwide. 20.7 Mha (83%) is found in Indonesia, primarily in Eastern Sumatra and Central Kalimantan. The dry season of 2009 (May-October) proved to be one of the most intense Bornean fire events since 1997. We sought to establish how fluvial carbon dynamics in tropical peat rich catchments were affected in the period following these catastrophic fires. Fluvial carbon fluxes were quantified from 8 channels draining peat swamp forest (PSF) catchments in Central Kalimantan, Borneo from weekly measurements over a 6-month period immediately following the end of the fire period. These included 5 channels draining degraded PSF catchments that were subject to fires during the dry season of 2009 and 3 channels draining intact PSF that was unaffected by the fires. Cumulative total organic carbon (TOC) fluxes for the duration of the study were 32 to 68% larger in the catchment channels that had been affected by fire, when compared to fluxes during the same interval in the previous year (which was unexceptional in terms of fire). This is in marked contrast to channels draining the intact (no fire) forest where there were no differences observed in fluxes. Concentration data demonstrates that there was an initial decrease in dissolved organic carbon (DOC) concentration and an increase in particulate organic carbon (POC) immediately following the onset of the first rains which continued for ~1 month until reaching similar levels to the corresponding period in the previous year. For DOC fluxes (the largest fraction) this decrease in concentration was more than offset by a large increase in fluvial discharge (from decreased transpiration and increased runoff through denuded vegetation cover) which contributed an overall net increase in flux. The fire-stimulated increase in fluvial carbon flux is set against a pattern of higher fluxes in the degraded catchments established during the pre-fire year where oxidation of the peat column results in high losses. Our data are the first to demonstrate a large and sustained pulse of fluvial carbon following large scale fires in carbon rich tropical PSF. They point to a previously uncharacterised pathway of carbon loss from these vast carbon stores and they underscore the vulnerability of these stores when subject to anthropogenic disturbance

Reassessing the stable isotope composition assigned to methane flux from natural wetlands in isotope-constrained budgets

Stable isotope ratios in CH4 preserve information about its origin and history, and are commonly used to constrain global CH4 budgets. Wetlands are key contributors to the atmospheric burden of CH4 and typically are assigned a stable carbon isotope composition of ~-60 permil in isotope-weighted stable isotope models despite the considerable range of δ13C(CH4) values (~ -100 to -40 permil) known to occur in these diverse ecosystems. Kinetic isotope effects (KIEs) associated with the metabolism of CH4 -producing microorganisms generate much of the natural variation but highly negative and positive δ13C(CH4) values generally result from secondary processes (e.g., diffusive transport or oxidation by soil methanotrophs). Despite these complexities, consistent patterns exist in the isotope composition of wetland CH4 that can be linked conclusively to trophic status and consequently, natural succession or human perturbations that impact nutrient levels. Another challenge for accurate representation of wetlands in carbon cycle models is parameterisation of sporadic CH4 emission events. Abrupt release of large volumes of CH4 -rich bubbles in short periods of time can account for a significant proportion of the annual CH4 flux from a wetland but such events are difficult to detect using conventional methods. New infrared spectroscopy techniques capable of high temporal resolution measurements of CH4 concentration and stable isotope composition can readily quantify short-lived CH4 pulses. Moreover, the isotope data can be used conclusively to determine shifts in the mode of CH4 transport and provide the potential to link initiation of abrupt emission events to forcing by internal or external factors

Trees are important conduits for emission of methane from temperate and tropical wetlands

Methane produced in wetland soil generally is thought to be emitted to the atmosphere primarily via diffusion through pore water, release of gas bubbles (i.e., ebullition), and gas phase diffusion through the aerenchyma of herbaceous plants. The role of trees as a conduit for methane export from soil to the atmosphere has received limited attention despite evidence from mesocosm experiments showing that seedlings and saplings of wetland trees have a significant capacity to transport soil-produced gases. Notably ~60% of global wetlands are forested. We present in situ measurements of methane flux from a temperate carr (swamp) composed of alder (Alnus glutinosa) and birch (Betula pubescens) situated in the United Kingdom and a tropical forested peat swamp located in Borneo. The in situ data are complemented by a mesocosm experiment in which methane emissions were measured from alder saplings subjected to two water-regime treatments. In both the in situ and mesocosm studies, emissions from trees are compared to methane flux from the ground surface, the latter occurring via pore water diffusion, ebullition or the aerenchyma of herbaceous plants. We show that tree stem emissions are controlled by a number of factors including tree species, soil pore-water concentration and stem lenticel density. Our results demonstrate that the omission of tree-mediated methane fluxes from measurement campaigns conducted in forested wetland can significantly underestimate total ecosystem flux of methane

Suppression of rice methane emission by sulfate deposition in simulated acid rain

Sulfate in acid rain is known to suppress methane (CH4) emissions from natural freshwater wetlands. Here we examine the possibility that CH4 emissions from rice agriculture may be similarly affected by acid rain, a major and increasing pollution problem in Asia. Our findings suggest that acid rain rates of SO2-4 deposition may help to reduce CH4 emissions from rice agriculture. Emissions from rice plants treated with simulated acid rain at levels of SO2-4 consistent with the range of deposition in Asia were reduced by 24% during the grain filling and ripening stage of the rice season which accounts for 50% of the overall CH4 that is normally emitted in a rice season. A single application of SO2-4 at a comparable level reduced CH4 emission by 43%. We hypothesize that the reduction in CH4 emission may be due to a combination of effects. The first mechanism is that the low rates of SO2-4 may be sufficient to boost yields of rice and, in so doing, may cause a reduction in root exudates to the rhizosphere, a key substrate source for methanogenesis. Decreasing a major substrate source for methanogens is also likely to intensify competition with sulfate-reducing microorganisms for whom prior SO2-4 limitation had been lifted by the simulated acid rain S deposition