Fluxes of methane and carbon dioxide from soil under forest, grazing land, irrigated rice and rainfed field crops in a watershed of Nepal

Field evolution of CH^sub 4^ and CO^sub 2^ from soils under four dominant land uses in the Mardi watershed, western Nepal, were monitored at 15-day intervals for 1 year using closed chamber techniques. The CH^sub 4^ oxidation rate (mean±SE, μg CH^sub 4^ m^sup -2^ h^sup -1^) in the forest (22.8±6) was significantly higher than under grazing land (14±2) and an upland rainfed maize and millet system (Bari) (2.6±0.9). Irrigated rice fields (Khet) showed an oxidation rate of 6±0.8 μg CH^sub 4^ m^sup -2^ h^sup -1^ in the dry season (December-May) but emitted a mean rate of 131 μg CH^sub 4^ m^sup -2^ h^sup -1^ in the rainy season and autumn (June-October). The evolution of CO^sub 2^ ranged from 10 mg CO^sub 2^ m^sup -2^ h^sup -1^ in the Bari in January to 1,610 mg CO^sub 2^ m^sup -2^ h^sup -1^ in the forest in July. Higher evolution of CO^sub 2^ (mean±SE, mg CO^sub 2^ m^sup -2^ h^sup -1^) was observed in the Bari (399±39) and forest (357±36) compared to Khet (246±25) and grazing (206±20) lands. The annual emission of CO^sub 2^ evolution varied from 86.6 to 1,836 g CO^sub 2^ m^sup -2^ year^sup -1^. The activation energy for CH^sub 4^ and CO^sub 2^ varied between 16-283 and 80-117 kJ mol^sup -1^, respectively. The estimated temperature coefficient for CO^sub 2^ emission varied from 2.5 to 5.0. Temperature explained 46-51% of the variation in CO^sub 2^ evolution, whereas it explained only 4-36% of the variation in CH^sub 4^ evolution.[PUBLICATION ABSTRACT]