Biosphere-atmosphere exchange of trace gases in the tropics: evaluating the effects of land use changes.
| Authors: | Michael Keller, M Matson |
| Year: | 1994 |
| Type: | Miscellaneous Publication |
| Station: | International Institute of Tropical Forestry |
| Source: | Plenum Press |
Abstract
Tropical soils and vegetation represent globally significant sources of a range of atmospheric gases, including CO2, CO, N20, NO, CH4, and volatile organic compounds. Conversion of tropical forests to agriculture and other uses, and intensification of agricultural practices in both forest and savanna areas are occurring very rapidly; yet the impacts of these land use practices on biogeochemical cycles, trace gas emissions, and atmospheric chemistry are not well understood. We present early results of IGAC's Biosphere-Atmosphere Trace Gas Exchange in the Tropics (BATGE) Activity that evaluate the importance of tropical land use change on fluxes of a number of gases.
Soils are important sources and sinks for trace gases, especially for oxides of nitrogen. On-going studies in Brazil, Costa Rica, Puerto Rico, and Mexico suggest that conversion of forest to pasture results in elevated soil emissions of nitrogen oxides in the first decade or less after clearing, but that older pastures, unless intensively managed, have lower fluxes than forests. Forest conversion to pasture also diminishes the soil sink strength for methane; in some cases the direction of the flux is reversed and pasture soils become methane sources. Likewise, lesults from a number of tropical regions suggest that agricultural sites recently converted from native vegetation have elevated fluxes of nitrogen and carbon gases, but that high emissions of nitrogen oxides and reduced uptake of methane in older agricultural systems are maintained only with intensive management and fertilizer use. Moreover, the magnitude of fluxes occurring in response to fertilization depends on soil type and management considerations such as crop type, fertilizer type,application rate, and application methods. The great spatial and temporal variability in trace gas responses to tropical agricultural practices complicates estimates of their importance at regional and global scales.
Few attempts have been made to study vegetation sources and sinks for trace gases in the tropics. It is clear that tropical vegetation can produce large quantities of volatile organic compounds. Forests may be strong sinks for oxidants and odd-nitrogen compounds. Changes in the species composition and structure of vegetation will almost certainly change these system properties; but our ability to predict the magnitude of these changes is ~everely hampered by a dearth of relevant data.