Influence of Leaf-Cutting Ant Nests on Secondary Forest Growth and Soil Properties in Amazonia

Leaf-cutting ants (Atta spp.) often increase in abundance following deforestation and may have an important effect on forest succession on abandoned land. In this study, we evaluated the effects of leaf-cutting ant (Atta sexdens) activity on physical and chemical properties of soil, root distribution, and tree growth in a 17-yr-old secondary forest in eastern Amazonia, Brazil. We compared the soil properties and root distribution in shafts excavated into five mature nests and at 15 m distanced from each nest mound. We evaluated the nest effects on vegetation measuring the stem diameter growth of nine tree species and predawn leaf water potential of the tree species, Banara guianensis, along a gradient of increasing distance from the nest mounds. The growth of seedlings (Cecropia sp.) in pots containing different proportions of mineral soil and organic matter removed from nest refuse chambers was also compared. The deep soil beneath A. sexdens nests at different depths (100, 200, and 300 cm) presented a low (fivefold, P < 0.01) resistance to penetration and was rich in Ca (three- to fourfold, P = 0.06-0.02), K (7-14-fold, P < 0.05), and Mg (two- to threefold, P = 0.09 for 200 cm depth) when compared to non-nest soil. These changes in nest soil properties were accompanied by increases in coarse root biomass (>2 mm diameter, three- to fourfold) and fine root biomass (