Surface O2 influx related to soil O2 profiles in a drained tropical peatland
Tropical peatlands are potentially the highest-ranked carbon sources among various types of soil in the world. The O2 consumption rate is one of the deterministic factors for soil carbon release through aerobic decomposition of soil organic matter in reclaimed tropical peatlands. The present study examined in-situ O2 influx at the soil surface in relation to below-ground O2 consumption in a palm oil plantation field on a tropical peatland in Thailand. The surface O2 influx rate was measured using a closed-chamber method. Below-ground O2 concentrations were also measured. The O2 influx rates obtained from three sampling points were 3.06, 3.66 and 7.63 mmol m−2 h−1, and did not show marked responses to changes in soil temperature. When the surface chambers were kept closed beyond the influx measurement period, the O2 concentrations in the chambers dropped to different steady-state concentrations even in the two chambers that showed similar surface O2 influx rates to each other, suggesting a difference in the effective depth range of O2 consumption. The O2 concentrations at depths of 5, 10 and 20 cm reached 0.181, 0.131 and 0.070 m3 m−3, respectively, at one monitoring point, whereas the concentrations at the other point were 0.194, 0.149 and 0.144 m3 m−3, respectively. The drop in O2 concentrations after the installation of the O2 sensors into the monitoring depths were rapid and linear over time at the former monitoring point, in contrast to the slower convergent lowering behaviors observed at the other point. The fast linear lowering at a monitoring depth implied the O2 consumption rate surpassing the diffusive O2 transport at the depth, suggesting that because of low soil gas diffusivity the depth range of O2 consumption could be confined to just a shallow portion of the unsaturated zone in a peat soil layer and could make the other deeper portion anaerobic.