Soil-atmosphere greenhouse gas exchange in a cool, temperate Eucalyptus delegatensis forest in south-eastern Australia

Forests are the largest C sink (vegetation and soil) in the terrestrial biosphere and may additionally provide an important soil methane (CH₄) sink, whilst producing little nitrous oxide (N₂O) when nutrients are tightly cycled. In this study, we determine the magnitude and spatial variation of soil-atmosphere N₂O, CH₄ and CO₂ exchange in a Eucalyptus delegatensis forest in New South Wales, Australia, and investigate how the magnitude of the fluxes depends on the presence of N₂-fixing tree species (Acacia dealbata), the proximity of creeks, and changing environmental conditions. Soil trace gas exchange was measured along replicated transects and in forest plots with and without presence of A. dealbata using static manual chambers and an automated trace gas measurement system for 2 weeks next to an eddy covariance tower measuring net ecosystem CO₂ exchange. CH₄ was taken up by the forest soil (-51.8μg CH₄-Cm⁻² h⁻¹) and was significantly correlated with relative saturation (S r) of the soil. The soil within creek lines was a net CH₄ source (up to 33.5μg CH₄-Cm⁻² h⁻¹), whereas the wider forest soil was a CH₄ sink regardless of distance from the creek line. Soil N₂O emissions were small (20. Soil-atmosphere exchange of CO₂ was several orders of magnitude greater (88.8mg CO₂-Cm⁻² h⁻¹) than CH₄ and N₂O, and represented 43% of total ecosystem respiration. The forest was a net greenhouse gas sink (126.22kg CO₂-equivalents ha⁻¹ d⁻¹) during the 2-week measurement period, of which soil CH₄ uptake contributed only 0.3% and N₂O emissions offset only 0.3%