Three upland soils from Thailand, a natural forest, a 16-year-old reforested site, and an agricultural field, were studied with regard to methane uptake and the community composition of methanotrophic bacteria (MB). The methane uptake rates were similar to rates described previously for forest and farmland soils of the temperate zone. The rates were lower at the agricultural site than at the native forest and reforested sites. The sites also differed in the MB community composition, which was characterized by denaturing gradient gel electrophoresis (DGGE) of pmoA gene fragments (coding for a subunit of particulate methane monooxygenase) that were PCR amplified from total soil DNA extracts. Cluster analysis based on the DGGE banding patterns indicated that the MB communities at the forested and reforested sites were similar to each other but different from that at the farmland site. Sequence analysis of excised DGGE bands indicated that Methylobacter spp. and Methylocystis spp. were present. Sequences of the "forest soil cluster" or "upland soil cluster ␣," which is postulated to represent organisms involved in atmospheric methane consumption in diverse soils, were detected only in samples from the native forest and reforested sites. Additional sequences that may represent uncultivated groups of MB in the Gammaproteobacteria were also detected.The current atmospheric mixing ratio of the greenhouse gas methane (CH 4 ) is 1.75 ppm by volume (8). An estimated 30 Tg of CH 4 year Ϫ1 is consumed via microbiological oxidation in upland soils, accounting for about 6% of the global methane sink (17). Atmospheric methane oxidation has been detected in many different upland soils, including arctic and subarctic tundra soils, grasslands and arable soils of the temperate zone, tropical forest soils, savannah soils, and even arid desert soils (25,31). Most studies have been performed to estimate the methane uptake capacity of upland soils of the temperate zone, and few data are available for tropical and subtropical soils (10,21,22,27,28). Most of these data indicate that the methane uptake rates of tropical and subtropical forest soils are comparable to those of forest soils of the temperate zone, but higher oxidation rates have been reported for some tropical soils (29).Among upland soils, forest soils are much more efficient methane sinks than cultivated soils (1, 3). Changes in land use, especially cultivation of formerly undisturbed soils, reduce the sink strength for atmospheric methane by 60 to 90% (30, 31). Such reductions have been reported for tropical soils as well (10, 21). However, the methane oxidation rate seems to recover much faster after abandonment of agricultural activities (22) compared to systems in the temperate zone.Methane oxidation in upland soils is mediated by methanotrophic bacteria (MB). Seven recognized genera of MB belong to the group containing the type I methanotrophs (Gammaproteobacteria), while the group containing the type II methanotrophs consists of four genera of MB (Alphaproteobacteria)...

Nitrous oxide (N 2 O) emissions from the soil surface of five different forest types in Thailand were measured using the closed chamber method. Soil samples were also taken to study the N 2 O production pathways. The monthly average emissions (AESD, n ¼ 12) of N 2 O from dry evergreen forest (DEF), hill evergreen forest (HEF), moist evergreen forest (MEF), mixed deciduous forest (MDF) and acacia reforestation (ARF) were 13.0 AE 8.2, 5.7 AE 7.1, 1.2 AE 12.1, 7.3 AE 8.5 and 16.7 AE 9.2 mg N m À2 h À1, respectively. Large seasonal variations in fluxes were observed. Emission was relatively higher during the wet season than during the dry season, indicating that soil moisture and denitrification were probably the main controlling factors. Net N 2 O uptake was also observed occasionally. Laboratory studies were conducted to further investigate the influence of moisture and the N 2 O production pathways. Production rates at 30% water holding capacity (WHC) were 3.9 AE 0.2, 0.5 AE 0.06 and 0.87 AE 0.01 ng N 2 O-nitrogen (N) g-dw À1 day À1 in DEF, HEF and MEF respectively. At 60% WHC, N 2 O production rates in DEF, HEF and MEF soils increased by factors of 68, 9 and 502, respectively. Denitrification was found to be the main N 2 O production pathway in these soils except in MEF.