The effects of deforestation on carbon balance in a natural tropical peat swamp forest were evaluated by micrometeorological monitoring.The monitoring sites were the primary forest at To-Daeng and the secondary forest at Bacho in Narathiwat Province, Thailand.The concentration gradient method (CG) and the relaxed eddy accumulation method (REA) were used to estimate the CO2 flux above the canopies. We estimated the annual amount of carbon absorption for both sites, using the relationship between CO2 flux and solar radiation for each site. An annual net carbon absorption of 5.32t C ha-1 year-1, was estimated for the primary forest. The accumulation of the released organic matter from the forest into logged-water caused the net carbon absorption in the peat swamp forest. In the secondary forest, annual net carbon absorption was estimated at 5.22 t C ha-1 year-1, which is almost the same range of absorption as the primary forest, because of plant growth in the secondary forest. A greater quantity of net carbon absorption, 9.69t C ha-1 year-1, was expected, assuming that the ground surface remained water-logged throughout the year. On the other hand, assuming it remained dry throughout the year, a net carbon emission of -0.40 t C ha-1 year-1, was expected.
The major advantages of the true eddy accumulation technique are that it is a direct method for flux measurements and applicable to any trace gases which need either a slow response sensor or the installation of an adsorbent for condensation. However, it is di$cult to accurately take the sample air with high time resolution according to vertical wind velocity and direction. Therefore, we have developed an air sampling system for the true eddy accumulation technique. The air sampling system is designed to take updraft and downdraft air samples in proportion to vertical wind velocity with high time resolution using syringes connected to a .* Hz stepping-motor. The system is also designed to accumulate air samples in separate reservoirs for updraft and downdraft, and to convey air samples to the gas analyzer alternately to detect their gas concentration automatically. To test how precisely air samples are taken with high time resolution, the sample volumes by the system under conditions of di#erent suction durations and input voltages were measured. The results of this experiment showed the system with the back flow route was able successfully to take air samples in proportion to vertical wind velocity on a base of 1 Hz considering the problem of suction errors. The preliminary test in a grass field showed that the accumulated sample volume during one measurement sequence was proportional to the amount of accumulated vertical wind velocity. The linear relations were recognized between CO, flux measured using the system and that measured using the eddy correlation technique. These results lead to the conclusion the developed air sampling system for the true eddy accumulation technique is available to measure CO, flux.
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