Land-use and erosion of a Costa Rican Ultisol affect soil chemistry, mycorrhizal fungi and early regeneration

Deforestation causes soil erosion, especially in the humid tropics where rainfall is heavy and terrain is often steep. Land-uses, such as overgrazing and planting annual crops on slopes exacerbate the resultant land degradation. Consequent loss of productivity in this area of the world is on a collision course with increasing human population density and the demand for food. Because of the serious nature of erosion, its effects on tropical soil, especially biological characteristics that help reestablish soil fertility, need more study. Here, we used apparent erosion intensity, land-use history, and soil color to find eight sites representing an a priori spatial gradient of soil degradation on an overgrazed Costa Rican farm. We tested the gradient by measuring several chemical factors that indicate fertility of these tropical Ultisols. These factors decreased with increasing degree of soil degradation. Next, we assessed spore density and diversity of arbuscular mycorrhizal fungi (AMF) along the gradient. We found that the diversity and composition of AMF changed across the gradient although not in the same pattern as the chemical factors. Finally, three years of vegetative regeneration after cattle exclusion had not improved the soils chemically but some improvement in AMF status was suggested for the less damaged sites. These results show that local farmers can use common sense cues to determine the chemical and biological status of their soils, that they can use these cues in future land-use decisions, such as planting hardy trees in the most degraded sites, and that they must expect severely degraded sites to require many years for recuperation. It is possible that intervention to improve the AMF status of soils could hasten recovery, since this process seems to be the first to occur. (C) 2001 Elsevier Science B.V. All rights reserved.