Seasonal changes in tree stem and soil methane fluxes along a soil moisture gradient in a subtropical Australian rainforest

Abstract Natural soil microbial methane (CH 4 ) uptake is well documented, but uncertainty remains about uptake rates in subtropical forest and the extent to which tree stem emissions may offset the soil CH 4 sink. We compared seasonal CH 4 fluxes from trees and soils in a subtropical Australian rainforest. We conducted four campaigns to measure in situ CH 4 fluxes from tree stems and soils across three plots along a moisture gradient (valley floor, lower slope, and upper slope plots). Tree stem and soil CH 4 fluxes showed a significant positive correlation with soil moisture. Valley floor soil CH 4 fluxes transitioned between a CH 4 sink (end dry season: − 151 µmol soil m −2 d −1 ) to a source (end wet season: 830 µmol soil m −2 d −1 ). Drier soils of the slope plots acted as a net CH 4 sink year-round, with average soil CH 4 fluxes of − 135 µmol soil m −2 d −1 (lower slope) and − 156 µmol soil m −2 d −1 (upper slope). In the sloped plots, tree stems emitted negligible CH 4 , except for two “high-emitting” trees, featuring CH 4 fluxes 200 and 300-fold higher than the adjacent trees. In the valley floor at the end of the wet season, tree stem emission contributed an extra 27% to the soil CH 4 source, while in the slope plots, tree CH 4 emissions offset < 1% of soil CH 4 uptake. This study adds to the limited knowledge of high temporal and spatial heterogeneity of upland tree and soil CH 4 fluxes, and it highlights the importance of rainforest soils as important global CH 4 sink.