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

Abstract

Forests are the largest C sink (vegetation and soil) in the terrestrial biosphere and may additionally provide an important soil methane (CH 4) sink, whilst producing little nitrous oxide (N 2O) when nutrients are tightly cycled. In this study, we determine the magnitude and spatial variation of soil–atmosphere N 2O, CH 4 and CO 2 exchange in a Eucalyptus delegatensis forest in New South Wales, Australia, and investigate how the magnitude of the fluxes depends on the presence of N 2-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 CO 2 exchange. CH 4 was taken up by the forest soil (−51.8 μg CH 4-C m −2 h −1) and was significantly correlated with relative saturation ( S r) of the soil. The soil within creek lines was a net CH 4 source (up to 33.5 μg CH 4-C m −2 h −1), whereas the wider forest soil was a CH 4 sink regardless of distance from the creek line. Soil N 2O emissions were small (<3.3 μg N 2O-N m −2 h −1) throughout the 2-week period, despite major rain and snowfall. Soil N 2O emissions only correlated with soil and air temperature. The presence of A. dealbata in the understorey had no influence on the magnitude of CH 4 uptake, N 2O emission or soil N parameters. N 2O production increased with increasing soil moisture (up to 50% S r) in laboratory incubations and gross nitrification was negative or negligible as measured through 15N isotope pool dilution. The small N 2O emissions are probably due to the limited capacity for nitrification in this late successional forest soil with C:N ratios >20. Soil–atmosphere exchange of CO 2 was several orders of magnitude greater (88.8 mg CO 2-C m −2 h −1) than CH 4 and N 2O, and represented 43% of total ecosystem respiration. The forest was a net greenhouse gas sink (126.22 kg CO 2-equivalents ha −1 d −1) during the 2-week measurement period, of which soil CH 4 uptake contributed only 0.3% and N 2O emissions offset only 0.3%.