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

Abstract

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.