uses that replace forests and the underlying causes of deforestation: socioeconomic factors, environmental factors, and government policy. Fearnside (1987) divided the causes of deforestation in Brazil into proximate and ultimate causes. Repetto (1989) stressed the economic incentives set by government policies. One approach is no more correct than another, although Repetto's approach may be the most useful for determining how to change current incentives. From the perspective of sustainable agriculture, however, there is yet another approach to assigning cause to deforestation—most deforestation in the tropics has been, and still is, due to the development of new agricultural land. The expansion of agricultural land, and thus deforestation, could be reduced by adopting methods of sustainable agriculture.

When forests and woodlands are cleared for cultivated land, an average of 90 to 100 percent of the aboveground biomass is burned and immediately released to the atmosphere as CO₂. Up to an additional 25 percent of carbon in the 1 m of surface soils is also lost to the atmosphere (Table A-3). Most of the loss occurs rapidly within the first 5 years of clearing; the rest is released over the next 20 years.

The wood harvested for products subsequently oxidizes, but it does so much more slowly than does the wood felled for cultivated land. The material remaining above and below the ground decays, as does the organic matter of newly cultivated soil. The rates of decay vary with climate, but in the humid tropics, most material decomposes within 10 years (John, 1973; Lang and Knight, 1979; Swift et al., 1979). However, recent work has indicated that many tropical woods take up to several decades to decompose (S. Brown and A. E. Lugo, personal observations). A small fraction of burned organic matter is converted to charcoal, which resists decay (Comery, 1981; Fearnside, 1986; Seiler and Crutzen, 1980). When croplands are abandoned, the lands may return to forests at rates determined by the intensity of disturbance and climatic factors (Brown and Lugo, 1982, 1990b; Uhl et al., 1988).

Cultivation of staple food crops in fields is common in the humid tropics—as it is elsewhere in the world—and is sustainable on good soils. Rice, maize, and cassava are the principal crops. Rice is usually cultivated in flooded fields or paddies, and the productivity and sustainability of wet rice cultivation is enhanced by reducing soil acidity under anaerobic conditions. This improves nutrient availability and the fertilization capabilities of the algae, decayed stubble, and