Fluxes and production pathways of nitrous oxide in different types of tropical forest soils in Thailand

Nitrous oxide (N2O) 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 N2O production pathways. The monthly average emissions (+/- SD, n = 12) of N2O from dry evergreen forest (DEF), hill evergreen forest (HEF), moist evergreen forest (MEF), mixed deciduous forest (MDF) and acacia reforestation (ARF) were 13.0 +/- 8.2, 5.7 +/- 7.1, 1.2 +/- 12.1, 7.3 +/- 8.5 and 16.7 +/- 9.2 mu g N m(-2) h(-1), 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 N2O uptake was also observed occasionally. Laboratory studies were conducted to further investigate the influence of moisture and the N2O production pathways. Production rates at 30% water holding capacity (WHC) were 3.9 +/- 0.2, 0.5 +/- 0.06 and 0.87 +/- 0.01 ng N2O-nitrogen (N) g-dw(-1)day(-1) in DEF, HEF and MEF respectively. At 60% WHC, N2O production rates in DEF, HEE and MEF soils increased by factors of 68, 9 and 502, respectively. Denitrification was found to be the main N2O production pathway in these soils except in MEF.