The few data that exist on exchange of atmospheric methane with tropical soils are equivocal. Upland temperate soils are thought to be sinks of CH4, and site disturbance usually decreases rates of CH4 uptake by soils, but these generalizations have not been tested widely in the tropics. Indeed, preliminary evidence suggests that high rates of soil respiration and low gas diffusivities in fine textured soils of the eastern Amazon can result in CH4 production in tropical forest soils. Moreover, knowledge of the effects of changing land uses on CH4 fluxes in the tropics is rudimentary at best. The first objective of this research is to assess variation in CH4 fluxes caused by land uses, diurnal cycles, seasonal cycles, within-site heterogeneity, and events in primary forests, secondary forests, active pastures of varying age, and abandoned pastures in eastern Amazon. The second objective is to test two mechanistic hypotheses: that gas diffusivity in soils controls CH4 fluxes; and that enhanced NH4+ availability can inhibit CH4 consumption. It is expected that diffusivity, estimated from profiles of 222Rn and from CClF3 tracer experiments, will be the dominant factor affecting production and consumption of CH4. Characterizing the effects of land use in this area of active logging and ranching, and describing these effects mechanistically will improve our understanding of how human activities impact atmospheric methane.
