This paper discusses the effects of trees on soil fertility, with a focus on agricultural systems in Amazonia. Relevant literature concerning the effects of trees on soil physical and chemical properties in tropical, subtropical, and temperate regions is reviewed, covering both natural ecosystems and agroecosystems. Soil carbon, in the form of organic matter, is considered as an indicator of biological activity as well as in relation to policy issues such as carbon sequestration and climate change. In the case of tropical soils and Amazonia, information on the effects of trees on soils is discussed in the context of traditional agriculture systems, as well as in regard to the development of more sustainable agricultural alternatives for the region. Lastly, attention is given to a case study in the savanna region of Roraima, northern Brazil, where a chronosequence of indigenous homegarden agroforestry systems showed clear effects of management practices involving trees on soil fertility. The use of diverse tree species and other practices employed in agroforestry systems can represent alternative forms of increasing soil fertility and maintaining agricultural production, with important practical applications for the sustainability of tropical agriculture. 1. Introduction According to a study by the World Agroforestry Centre, ICRAF, 43% of the planet’s agricultural lands (more than a billion hectares) has more than 10% tree cover [1]. A lesser but still significant area of agricultural land, 160 million hectares, has more than 50% tree cover. The potential of trees to bring improvements in nutrition, income, housing, health, energy needs, and environmental sustainability in the agricultural landscape has guided ICRAF’s mission, with the presence of trees being the principal component of an “evergreen agriculture” [2]. Within the array of benefits brought by trees, an important element is the positive effect of trees on soil properties and consequently benefits for crops. This paper explores current knowledge as to this relation between trees and soil, based on agroforestry systems research, as well as studies innon-agricultural or natural environments that demonstrate effects of trees on soil. Although we consider information from various ecosystems and biomes, the focus will be on Amazonia, where the authors have most of their experience. This focus on Amazonia is also due to the strong policy demands for the development of more sustainable agricultural systems in the region, as alternatives to forms of land use that have shown significant and negative
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