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Soil moisture condition is essential to regulate the release of soil carbon from a drained peatland since aerobic microbial activities can be encouraged through oxygen supply associated with dewatering the soil layer while they may be discouraged under too dry conditions. Aiming to characterize the soil moisture condition in a reclaimed tropical peatland, we monitored the volumetric water content at 5 cm depth ( 5 cm ), groundwater level (GWL) and rainfall for 20 months from March 2010 to November 2011 in an oil palm field in NakhonSi-Thammarat, Thailand. We also measured the soil water retention curve and the unsaturated hydraulic conductivity (k) for a series of matric potential (h) to simulate the moisture condition monitored in the field by using the Buckingham-Darcy's flux law. During the dry season in 2010, the 5 cm consistently stayed lower than 0.35 m 3 m -3 with the GWL lower than a depth of 30 cm. In the transition from the dry season to the rainy season in 2010, the GWL rose to the land surface with peaks and dips across the time for about one month with the 5 cm increasing toward saturation. During the rainy season where the GWL stayed near or above the land surface, the 5 cm remained the field-saturated value of 0.58 m 3 m -3 on average, less than the laboratory-saturated value of 0.63 m 3 m -3 , suggesting the development of a significant amount of entrapped air-phase. Hysteretic behavior in the measured 5 cm -GWL relation also supported that the top soil layer refuses to absorb water in wetting processes. The simulated 5 cm based on the measured k(h) and soil water retention curves demonstrated that the ease with which the top soil dries during a dry season was due mainly to the low k(h) value in the dried condition, while the slope of the (h) curve was so moderate that the soil layer could retain moisture for maintaining liquid water supply to the surface from the dropped GWL. Sensitivity analyses while varying the magnitude of both k(h) and evaporation rate (E) suggested that the k(h) function was more deterministic than the value of E in making the land surface easily dried. As the GWL stayed lower than 30 cm in depth for a total of 187 days out of the year monitored, while surface-ponding conditions took place for 120 days of the year, it was concluded that either the extremely dried condition or the saturated-moisture condition had dominantly occurred in the study site through a year and, thus, there may only be a limited time when soil organic matter near the land surface is in favorable moisture conditions for aerobic decomposition.

A soil water retention curve (SWRC) is usually measured in a laboratory (lab SWRC), and is used to analyze in-situ soil moisture conditions. However, it is rarely verified whether and how a lab SWRC is in agreement with its equivalent relation between matric potential (h) and volumetric water content (θ) in a natural field (in-situ SWRC). In addition, most SWRCs show moisture hysteresis through which the drying process gives a larger θ at a given h than the wetting process, while an in-situ SWRC must be produced through the cycles of drying and wetting in the field. Thus, it can be hypothesized that an insitu SWRC shows a lower value of θ than a lab SWRC for any h that the soil layer ordinarily experiences. To give experimental proofs for this hypothesis, this study aimed at quantifying seasonal behaviors of in-situ SWRCs and at comparing them with their corresponding lab SWRCs. To obtain a series of in-situ SWRCs, the h and θ were coincidently monitored at four points with three depths each in a meadow for 2.5 years using tensiometers and a capacitance-type soil moisture sensing system. As the equivalent to the in-situ SWRCs, the lab SWRCs were also measured. The in-situ SWRCs tended to have roughly 10% smaller θ than the lab SWRCs for the series of h observed in the study site, suggesting that an in-situ SWRC can hardly be reproduced by a lab SWRC only. In addition, when the driest condition in the recent 3 years was exerted on the study site, some in-situ SWRCs shifted along the θ axis on the θ(h) charts, suggesting that the most dried condition had changed the soil moisture regime of these soil layers, resulting in the reduction of monthly or annual means of soil water content in the field. Since the shifts of the in-situ SWRCs were accompanied by the increases in both the gradients 'dθ/dh' and the variation of measured h, it was implied that an extraordinary drying of a soil layer promotes the development of soil pore structure or an increase in the fraction of plant available water.

Tropical peatlands are potentially the highest-ranked carbon sources among various types of soil in the world. The O 2 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 O 2 influx at the soil surface in relation to below-ground O 2 consumption in a palm oil plantation field on a tropical peatland in Thailand. The surface O 2 influx rate was measured using a closed-chamber method. Below-ground O 2 concentrations were also measured. The O 2 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 O 2 concentrations in the chambers dropped to different steady-state concentrations even in the two chambers that showed similar surface O 2 influx rates to each other, suggesting a difference in the effective depth range of O 2 consumption. The O 2 concentrations at depths of 5, 10 and 20 cm reached 0.181, 0.131 and 0.070 m 3 m )3 , respectively, at one monitoring point, whereas the concentrations at the other point were 0.194, 0.149 and 0.144 m 3 m )3 , respectively. The drop in O 2 concentrations after the installation of the O 2 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 O 2 consumption rate surpassing the diffusive O 2 transport at the depth, suggesting that because of low soil gas diffusivity the depth range of O 2 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.