Soil O2 profile affected by gas diffusivity and water retention in a drained peat layer

ABSTRACTPeatlands consist of hundreds of gigatons of soil carbon storage. Once a peatland is drained, however, it is transformed into a carbon source, since better aerated conditions promote the aerobic microbial decomposition of the peat soil layers. Since the aerobic depth should depend on the balance between the capability of the peat layers to transport the O2 and the intensity of the O2 consumption by aerobic microbes, our aim in the present study was to characterize the soil gas diffusivity (Ds/D0) and the soil water retention curves (SWRCs) of two types of peat soils found in sub-boreal and tropical peatlands. We also numerically simulated the effects of Ds/D0 and SWRCs on the aerobic depth into which the atmospheric O2 can penetrate in unsaturated peat layers with various groundwater table levels. For the numerical simulations, we modified the Millington-Quirk-type Ds/D0 model, while the O2 consumptive behavior in the peat layers was described by the Monod-type equation. The Ds/D0, as a function of the air-filled porosity, was consistently larger for the tropical-peat samples than for the sub-boreal ones, implying that the tropical-peat samples had less tortuous pore geometry than the sub-boreal ones. However, the features of the SWRCs indicated that the tropical-peat samples showed higher water retentivity than the sub-boreal ones; and therefore, lowering the groundwater table would not raise the air-filled porosity of the tropical-peat samples more than that of the sub-boreal ones. Through numerical simulations contrasting the features of the Ds/D0 and the SWRC curves for the tropical-peat samples with those of the sub-boreal ones, unsaturated anaerobic layers were seen to form more easily in the simulations representing the tropical-peat samples than in those representing the sub-boreal ones, suggesting high water retentivity. In turn, the difficulty in aeration inhibited the gaseous diffusion in the tropical-peat soils even though the Ds/D0 values for the tropical-peat samples were higher than those for the sub-boreal ones for a series of air-filled porosity. Furthermore, the development of unsaturated anaerobic layers caused a “ceiling” for the increase in the surface CO2 emission rate associated with the fall in the groundwater table.