ORGANIC MATTER CYCLING BY TROPICAL AGROFORESTRY SYSTEMS: A REVIEW

review. The trend in shifting tropical agriculture to shorter fallow periods and ultimately to attempts at continuous cultivation usually leads to land degradation and reduced productivity. This often results in the clearing of more forest land. Although the effects of these practices are most apparent at local scales, large releases of carbon dioxide and other greenhouse gases from forest clearing and land use change also have implications for the global environment. Agroforestry appears to be a promising technique to achieve sustainable land use by conserving soil organic matter. This paper compares the organic matter dynamics of agroforestry systems to successful long fallow agricultural systems. For the studies surveyed, agroforestry systems on average returned 7.4 t ha-1 y ' (±0.8) of organic matter to the soil surface in the form of prunings. This is within the range of litter production observed for long fallow systems. There is also evidence that the sustainability of agroforestry systems may be constrained by soil properties. On infertile soils, a limited potential for increasing nutrient inputs results in reduced plant growth, litterfall, and nutrient cycling. Implementation of agroforestry systems as an alternative to continuous cropping, however, should slow the loss of. soil organic carbon and extend the cropping period