Kazutoshi Osawa

Surface O2 influx related to soil O2 profiles in a drained tropical peatland.

Abstract 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.

Sediment and nutrients transport in watershed and their impact on coastal environment.

Sediment and nutrients yields especially from farmlands were studied in a watershed in Ishigaki island, Okinawa, Japan. The transport processes of these materials in rivers, mangrove, lagoon and coastal zones were studied by using various observation methods including stable isotope analysis. They were simulated by using a WEPP model which was modified to be applicable to such small islands by identifying several factors from the observations. The model predicts that a proper combination of civil engineering countermeasure and change of farming method can reduce the sediment yield from the watershed by 74%. Observations of water quality and coral recruitment test in Nagura bay indicate that the water is eutrophicated and the corals cannot grow for a long time. Based on these observations, a quantitative target of the reduction of sediment and nutrients yield in watershed can be decided rationally.

A seasonal behavior of surface soil moisture condition in a reclaimed tropical peatland

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 (θ 5 cm), groundwater level (GWL) and rainfall for 20 months from March 2010 to November 2011 in an oil palm field in Nakhon-Si-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-Darcys flux law. During the dry season in 2010, the θ 5 cm consistently stayed lower than 0.35 m3 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 θ 5 cm increasing toward saturation. During the rainy season where the GWL stayed near or above the land surface, the θ 5 cm remained the field-saturated value of 0.58 m3 m–3 on average, less than the laboratory-saturated value of 0.63 m3 m–3, suggesting the development of a significant amount of entrapped air-phase. Hysteretic behavior in the measured θ 5 cm–GWL relation also supported that the top soil layer refuses to absorb water in wetting processes. The simulated θ 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 θ(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.

FUNDAMENTAL STUDY ON ENDOCRINE DISRUPTERS PURIFICATION BY SUBMERGED AQUATIC VEGETATION

Phytoremediation that attracts the attention is an effective technology of removing endocrine disrupters in soils, ground water and surface water. In this study, the absorption experiment of 17β-estraziol (E2) and bisphenol A (BPA) using Egeria densa and the field survey in the Nikaryo-yosui were performed in order to investigate whether submerged vegetation in a river can apply to the phytoremediation of endocrine disrupters. The absorption experiment revealed that Egeria densa absorbed E2 and BPA. From the result of field survey, the reduction rate of E2 and BPA concentrations in the area with vegetation was larger than that in the area without vegetation. Moreover, Egeria densa in a river absorbed E2 and BPA, and possibly removed.

Soil Runoff Analysis Using the WEPP Model in Okinawa, Japan

In Okinawa, red soil runoff from developed lands and consequent ocean pollution soil has been a major issue since the 1970’s. The Okinawa Prefecture Department of Agriculture must move to introduce some soil runoff prevention measures. For such countermeasures to be properly formulated, we must be able to estimate their efficiency in the early planning stages. To this end, the USLE has been in use but this model has shortcomings in its inability to calculate short-term runoff or runoff on the scale of entire watersheds. The USDA also developed the WEPP model in 1985. In the WEPP model, watersheds are composed of slopes and channels. Changes of vegetation and soil condition on the slopes are calculated to yield seasonal changes in erosion or soil loss. Calculations for the channel include considerations of detachment and deposition in consonance with flow rates. Therefore, the WEPP model can be used to calculate spatial and temporal soil erosion processes. However, there are few examples of applications of the WEPP model in Japan. Then, WEPP model have been applied to an agricultural watershed in Okinawa and affectivity of the model is checked. In the application, parameters on soil and farming schedules were drawn from observations made in the watershed and the other parameters were defaults shown in the WEPP model. As the results demonstrate, the WEPP model can be used to calculate changes of crop and soil conditions clearly. Calculated soil erosion was almost the same as actual observed parameters in the event scale.