The clearing and burning of forested land for agriculture results in a loss of carbon and the emission of greenhouse gases (GHG) from both the above-ground biomass and soils. Greenhouse gas emissions subsequent to slash-and-burn will depend upon the intensity of land use, including vegetation type and density, use of tillage and fertilisers. The rates and magnitudes of C sequestration and GHG fluxes are critical for evaluating the impact of land-use interventions. Three results from ASB stand out:

• Soil carbon stocks (top 20 cm) do not vary much with land use, whereas there are significant differences in above-ground carbon among these systems (Figure 1; Palm et al., 1998).
• The various tree-based systems have comparable average carbon stocks during the course of their rotation. In order to compare the potential for carbon sequestration in a system, it is necessary to compute the time-averaged carbon stocks, or the average carbon stored in a system over the rotation time of the system. ASB calculations indicate that tree-based land uses sequester about six times more carbon than annual crops or pastures.
• As a result of the first two observations, there is potential for C sequestration in the soils through the rehabilitation of degraded pastures and grassland, but the largest sequestration potential is above-ground through the adoption of tree-based land uses.
• The general trend from the data collected in Cameroon, Indonesia and Peru on greenhouse gas emissions indicates that all the upland systems are sinks of methane -- greatest for the forest and least for continuous cropping (Figure 2; Palm et al. 1998). The sink strength is reduced with increasing land-use intensity. The data suggest the possibility of designing mosaics of land-use practices to balance the sources and sinks of GHG at the landscape level.

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Page preparation by Dr. Erick C.M. Fernandes, Cornell University.
--ASB Global Coordinator (1998-1999)-