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

Knowledge regarding mechanisms moderating methane (CH4) sink/source behaviour along the soil-tree stem-atmosphere continuum remains incomplete. Here, we applied stable isotope analysis (delta C-13-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 delta C-13-CH4 enrichment (increasing with height) in relation to stem height from ground. The average lower tree stem delta C-13-CH4 (0-40 cm) of Melaleuca and Casuarina (-53.96 parts per thousand and -65.89 parts per thousand) were similar to adjacent flooded soil CH4 ebullition (-52.87 parts per thousand and -62.98 parts per thousand), suggesting that stem CH4 is derived mainly by soil sources. Upper stems (81-200 cm) displayed distinct delta C-13-CH4 enrichment (Melaleuca -44.6 parts per thousand and Casuarina -46.5 parts per thousand, 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 delta C-13-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.

Automated summary

Mechanisms moderating methane behavior along the soil-tree stem-atmosphere continuum remain unclear. Stable isotope analysis in subtropical lowland species revealed that methane in tree stems is mainly derived from soil sources. Significant methane enrichment was observed in upper tree stems, suggesting distinct gas sources and transport processes. Diel experiments showed higher oxidation rates in the afternoon compared to the morning.