Isotopic evidence for axial tree stem methane oxidation within subtropical lowland forests

Abstract

Summary Knowledge regarding mechanisms moderating methane (CH4) sink/source behaviour along the soil–tree stem–atmosphere continuum remains incomplete. Here, we applied stable isotope analysis (δ13C‐CH4) to gain insights into axial CH4 transport and oxidation in two globally distributed subtropical lowland species (Melaleuca quinquenervia and Casuarina glauca). We found consistent trends in CH4 flux (decreasing with height) and δ13C‐CH4 enrichment (increasing with height) in relation to stem height from ground. The average lower tree stem δ13C‐CH4 (0–40 cm) of Melaleuca and Casuarina (−53.96‰ and −65.89‰) were similar to adjacent flooded soil CH4 ebullition (−52.87‰ and −62.98‰), suggesting that stem CH4 is derived mainly by soil sources. Upper stems (81–200 cm) displayed distinct δ13C‐CH4 enrichment (Melaleuca −44.6‰ and Casuarina −46.5‰, respectively). Coupled 3D‐photogrammetry with novel 3D‐stem measurements revealed distinct hotspots of CH4 flux and isotopic fractionation on Melaleuca, which were likely due to bark anomalies in which preferential pathways of gas efflux were enhanced. Diel experiments revealed greater δ13C‐CH4 enrichment and higher oxidation rates in the afternoon, compared with the morning. Overall, we estimated that c. 33% of the methane was oxidised between lower and upper stems during axial transport, therefore potentially representing a globally significant, yet previously unaccounted for, methane sink.

Identifying Sources and Oxidation of Methane in Standing Dead Trees in Freshwater Forested Wetlands

“…CH 4 oxidation occurred more in samples from snag chambers and were closest to ambient air stable isotopes (−46‰ and −114‰, δ 13 C and δ 2 H respectively), providing further evidence that is consistent with CH 4 continuing to be oxidized as it moves through the snags (Figures 3, 4). A previous study also showed δ 13 C-CH 4 enrichment with increasing stem height in a flooded forested ecosystem, suggesting oxidation during tree stem transport (Jeffrey et al, 2021). This same study also found methane oxidizing bacteria (primarily Methylomonas) to be abundant within bark representing an important sink (Jeffrey et al, 2021).…”

“…Evaporation will lead to observable fractionation, yielding relatively light vapor and δ 2 H-CH 4 rich water left behind (more positive) (Michener and Lajtha, 2007). A recent study showed an enrichment of δ 13 C-CH 4 with increasing tree stem height, estimating that 33% of the CH 4 was oxidized from the lower stem towards the upper stem (Jeffrey et al, 2021). Stable isotopes have not been used to examine sources and potential transformations of CH 4 in snags.…”

Methane emissions may be driven by hydrogenotrophic methanogens inhabiting the stem tissues of poplar

“…Methanobacterium and Methanobrevibacter have been reported as the dominant methanogenic genera in the heartwood of P. canadensis (Yip et al, 2019;Li et al, 2020) and P. deltoides (Zeikus & Ward, 1974;Zeikus & Henning, 1975;Balch et al, 1979), respectively. Unexpectedly, Jeffrey et al (2021) recently found that Methanobacterium also exits in the bark of Australian lowland trees (Melaleuca). Interestingly, we identified several generasuch as Methanoculleus, Methanosarcina, Methanomassiliicoccus, and Rice Cluster Ithat accounted for c. 10% of the relative abundance of methanogens in the heartwood and sapwood (Fig.…”

“…Methanotrophy is a ubiquitous CH 4 -consuming process that affects the magnitude and direction of CH 4 fluxes across the tree surface; however, there is little evidence of this process in living trees (Covey & Megonigal, 2019). In a recent study, Jeffrey et al (2021) reported that tree bark might provide a habitat for previously uncharacterized microbiomes and unique methaneoxidizing bacteria (MOB) communities (comprising up to 25% of the total microbial community), substantially mitigating tree stem CH 4 emissions. Communities of MOB, that is, Methylocystis, Methylobacterium, and Paracoccus, were identified within both the heartwood and sapwood in our study (Fig.…”

Restoration impacts on rates of denitrification and greenhouse gas fluxes from tropical coastal wetlands

“…There has, however, been significantly less research on carbon and N cycling in Melaleuca forest soils. The effect of land-use change and restoration on GHG fluxes or N removal in Melaleucas remains unquantified (Adame et al, 2019a;Adame et al, 2019b;Adame et al, 2019c;Greenway and Jenkins, 2004;Jeffrey et al, 2020;Jeffrey et al, 2021;Tran and Dargusch, 2016;Tran et al, 2015). The current lack in understanding of carbon and N cycling across land-use and restoration gradients in coastal ecosystems prevents the responses of nutrient attenuation and GHG fluxes to global change from being assessed and consequently, prevents maximisation of ecosystem services through effective management (Allen et al, 2011).…”