Soil methane oxidation in both dry and wet temperate eucalypt forests shows a near-identical relationship with soil air-filled porosity

Well-drained, aerated soils are important sinks for atmospheric methane (CH₄) via the process of CH₄ oxidation by methane-oxidising bacteria (MOB). This terrestrial CH₄ sink may contribute towards climate change mitigation, but the impact of changing soil moisture and temperature regimes on CH₄ uptake is not well understood in all ecosystems. Soils in temperate forest ecosystems are the greatest terrestrial CH₄ sink globally. Under predicted climate change scenarios, temperate eucalypt forests in southeastern Australia are predicted to experience rapid and extreme changes in rainfall patterns, temperatures and wild fires. To investigate the influence of environmental drivers on seasonal and inter-annual variation of soil–atmosphere CH₄ exchange, we measured soil–atmosphere CH₄ exchange at high-temporal resolution (< 2 h) in a dry temperate eucalypt forest in Victoria (Wombat State Forest, precipitation 870 mm yr⁻¹ ) and in a wet temperature eucalypt forest in Tasmania (Warra Long-Term Ecological Research site, 1700 mm yr⁻¹ ). Both forest soil systems were continuous CH₄ sinks of −1.79 kg CH₄ ha⁻¹ yr⁻¹ in Victoria and −3.83 kg CH₄ ha⁻¹ yr⁻¹ in Tasmania. Soil CH₄ uptake showed substantial temporal variation and was strongly controlled by soil moisture at both forest sites. Soil CH₄ uptake increased when soil moisture decreased and this relationship explained up to 90 % of the temporal variability. Furthermore, the relationship between soil moisture and soil CH₄ flux was near-identical at both forest sites when soil moisture was expressed as soil air-filled porosity (AFP). Soil temperature only had a minor influence on soil CH₄ uptake. Soil nitrogen concentrations were generally low and fluctuations in nitrogen availability did not influence soil CH₄ uptake at either forest site. Our data suggest that soil MOB activity in the two forests was similar and that differences in soil CH₄ exchange between the two forests were related to differences in soil moisture and thereby soil gas diffusivity. The differences between forest sites and the variation in soil CH₄ exchange over time could be explained by soil AFP as an indicator of soil moisture status.