Edward R. C. Hornibrook

Trees are major conduits for methane egress from tropical forested wetlands

Wetlands are the largest source of methane to the atmosphere, with tropical wetlands comprising the most significant global wetland source component. The stems of some wetland-adapted tree species are known to facilitate egress of methane from anoxic soil, but current ground-based flux chamber methods for determining methane inventories in forested wetlands neglect this emission pathway, and consequently, the contribution of tree-mediated emissions to total ecosystem methane flux remains unknown. In this study, we quantify in situ methane emissions from tree stems, peatland surfaces (ponded hollows and hummocks) and root-aerating pneumatophores in a tropical forested peatland in Southeast Asia. We show that tree stems emit substantially more methane than peat surfaces, accounting for 62-87% of total ecosystem methane flux. Tree stem flux strength was controlled by the stem diameter, wood specific density and the amount of methane dissolved in pore water. Our findings highlight the need to integrate this emission pathway in both field studies and models if wetland methane fluxes are to be characterized accurately in global methane budgets, and the discrepancies that exist between field-based flux inventories and top-down estimates of methane emissions from tropical areas are to be reconciled.

The contribution of trees to ecosystem methane emissions in a temperate forested wetland

Wetland-adapted trees are known to transport soil-produced methane (CH4 ), an important greenhouse gas to the atmosphere, yet seasonal variations and controls on the magnitude of tree-mediated CH4 emissions remain unknown for mature forests. We examined the spatial and temporal variability in stem CH4 emissions in situ and their controls in two wetland-adapted tree species (Alnus glutinosa and Betula pubescens) located in a temperate forested wetland. Soil and herbaceous plant-mediated CH4 emissions from hollows and hummocks also were measured, thus enabling an estimate of contributions from each pathway to total ecosystem flux. Stem CH4 emissions varied significantly between the two tree species, with Alnus glutinosa displaying minimal seasonal variations, while substantial seasonal variations were observed in Betula pubescens. Trees from each species emitted similar quantities of CH4 from their stems regardless of whether they were situated in hollows or hummocks. Soil temperature and pore-water CH4 concentrations best explained annual variability in stem emissions, while wood-specific density and pore-water CH4 concentrations best accounted for between-species variations in stem CH4 emission. Our study demonstrates that tree-mediated CH4 emissions contribute up to 27% of seasonal ecosystem CH4 flux in temperate forested wetland, with the largest relative contributions occurring in spring and winter. Tree-mediated CH4 emissions currently are not included in trace gas budgets of forested wetland. Further work is required to quantify and integrate this transport pathway into CH4 inventories and process-based models.

Stable isotope switching (SIS): a new stable isotope probing (SIP) approach to determine carbon flow in the soil food web and dynamics in organic matter pools

RATIONALE Recent advances in stable isotope probing (SIP) have allowed direct linkage of microbial population structure and function. This paper details a new development of SIP, Stable Isotope Switching (SIS), which allows the simultaneous assessment of carbon (C) uptake, turnover and decay, and the elucidation of soil food webs within complex soils or sedimentary matrices. METHODS SIS utilises a stable isotope labelling approach whereby the (13)C-labelled substrate is switched part way through the incubation to a natural abundance substrate. A (13)CH(4) SIS study of landfill cover soils from Odcombe (Somerset, UK) was conducted. Carbon assimilation and dissimilation processes were monitored through bulk elemental analysis isotope ratio mass spectrometry and compound-specific gas chromatography/combustion/isotope ratio mass spectrometry, targeting a wide range of biomolecular components including: lipids, proteins and carbohydrates. RESULTS Carbon assimilation by primary consumers (methanotrophs) and sequential assimilation into secondary (Gram-negative and -positive bacteria) and tertiary consumers (Eukaryotes) was observed. Up to 45% of the bacterial membrane lipid C was determined to be directly derived from CH(4) and at the conclusion of the experiment ca. 50% of the bulk soil C derived directly from CH(4) was retained within the soil. CONCLUSIONS This is the first estimate of soil organic carbon derived from CH(4) and it is comparable with levels observed in lakes that have high levels of benthic methanogenesis. SIS opens the way for a new generation of SIP studies aimed at elucidating total C dynamics (incorporation, turnover and decay) at the molecular level in a wide range of complex environmental and biological matrices.

The Stable Carbon Isotope Composition of Methane Produced and Emitted from Northern Peatlands

Stable carbon isotope values, pore water concentration, and flux data for methane (CH 4 ) were compiled for 26 peatlands situated in the northern hemisphere to explore relationships between trophic status and CH 4 cycling. Methane produced in ombrotrophic bogs has δ 13 C values that are significantly more negative than CH 4 formed in fens apparently because of poor dissociation of acetic acid or an absence of methanogenic archaea capable of metabolizing acetic acid under low pH conditions. The δ 13 C values of CH 4 in pore water of ombrotrophic and minerotrophic peatlands exhibit the opposite trend: δ 13 C(CH 4 ) values become more positive with depth in rain-fed bogs and more negative with depth in fens. The key zone for methanogenesis occurs at shallow depths in both types of peatland and consequently, δ 13 C values of CH 4 emitted from ombrotrophic bogs (-74.9 ± 9.8‰; n = 42) are more negative than from fens (-64.8 ± 4.0‰; n = 38). An abundance of graminoids in fens contributes to more positive δ 13 C(CH 4 ) values in pore water through (1) release of root exudates which promotes aceticlastic methanogenesis, (2) rhizosphere oxidization of CH 4 causing localized enrichment of 13 CH 4 , and (3) preferential export of 12 CH 4 through aerenchyma, which also enriches pore water in 13 CH 4 . Emissions from blanket bogs and raised bogs should be attributed more negative δ 13 C(CH 4 ) values relative to fens in isotope-weighted mass balance budgets. Further study is needed of bogs that have an apparently low nutrient status but exhibit a pore water distribution of δ 13 C(CH 4 ) values similar to fens.

Archaeol: An Indicator of Methanogenesis in Water-Saturated Soils

Oxic soils typically are a sink for methane due to the presence of high-affinity methanotrophic Bacteria capable of oxidising methane. However, soils experiencing water saturation are able to host significant methanogenic archaeal communities, potentially affecting the capacity of the soil to act as a methane sink. In order to provide insight into methanogenic populations in such soils, the distribution of archaeol in free and conjugated forms was investigated as an indicator of fossilised and living methanogenic biomass using gas chromatography-mass spectrometry with selected ion monitoring. Of three soils studied, only one organic matter-rich site contained archaeol in quantifiable amounts. Assessment of the subsurface profile revealed a dominance of archaeol bound by glycosidic headgroups over phospholipids implying derivation from fossilised biomass. Moisture content, through control of organic carbon and anoxia, seemed to govern trends in methanogen biomass. Archaeol and crenarchaeol profiles differed, implying the former was not of thaumarcheotal origin. Based on these results, we propose the use of intact archaeol as a useful biomarker for methanogen biomass in soil and to track changes in moisture status and aeration related to climate change.

Large emissions from floodplain trees close the Amazon methane budget

Wetlands are the largest global source of atmospheric methane (CH4), a potent greenhouse gas. However, methane emission inventories from the Amazon floodplain, the largest natural geographic source of CH4 in the tropics, consistently underestimate the atmospheric burden of CH4 determined via remote sensing and inversion modelling, pointing to a major gap in our understanding of the contribution of these ecosystems to CH4 emissions. Here we report CH4 fluxes from the stems of 2,357 individual Amazonian floodplain trees from 13 locations across the central Amazon basin. We find that escape of soil gas through wetland trees is the dominant source of regional CH4 emissions. Methane fluxes from Amazon tree stems were up to 200 times larger than emissions reported for temperate wet forests and tropical peat swamp forests, representing the largest non-ebullitive wetland fluxes observed. Emissions from trees had an average stable carbon isotope value (δ13C) of −66.2 ± 6.4 per mil, consistent with a soil biogenic origin. We estimate that floodplain trees emit 15.1 ± 1.8 to 21.2 ± 2.5 teragrams of CH4 a year, in addition to the 20.5 ± 5.3 teragrams a year emitted regionally from other sources. Furthermore, we provide a ‘top-down’ regional estimate of CH4 emissions of 42.7 ± 5.6 teragrams of CH4 a year for the Amazon basin, based on regular vertical lower-troposphere CH4 profiles covering the period 2010–2013. We find close agreement between our ‘top-down’ and combined ‘bottom-up’ estimates, indicating that large CH4 emissions from trees adapted to permanent or seasonal inundation can account for the emission source that is required to close the Amazon CH4 budget. Our findings demonstrate the importance of tree stem surfaces in mediating approximately half of all wetland CH4 emissions in the Amazon floodplain, a region that represents up to one-third of the global wetland CH4 source when trees are combined with other emission sources.

The role of natural wetlands in the global methane cycle

Methane (CH4) is the most important greenhouse gas after water vapor and carbon dioxide (CO2), and wetlands represent its largest natural source. But the high spatial and temporal variability of CH4 emissions from natural wetlands, combined with patchy and incomplete information on global wetland distribution, makes them especially difficult to quantify CH4 from natural wetlands still has the largest uncertainty of any CH4 source. This is of concern because projections for the future suggest a rise in CH4 emissions and thus a positive feedback in climate change. We also know from ice cores that atmospheric CH4 concentration has varied by a factor of 2, at orbital and suborbital frequencies, during the past 400,000 years. The causes of these variations are largely unknown, but it is likely that variations in wetland extent and productivity due to changes in glaciation, sea level, atmospheric CO4 concentration, and climate played a part. Other mechanisms, including changes in the oxidation capacity of the atmosphere and pulsed releases from marine and terrestrial gas hydrates, may have contributed as well. None of these mechanisms is adequately quantified.

Erratum: Large emissions from floodplain trees close the Amazon methane budget

This corrects the article DOI: 10.1038/nature24639