Abstract

Effects of tropical forest conversion to nonforest uses greatly affects low-activity soils with substantial variable charge, but quantitative changes are poorly studied. We evaluated forest conversion effects on Ultisols and Oxisols by establishing a spatial comparison study on alluvial fans and terraces in the Rio General Valley of Costa Rica. Soils from pairs of forest and pastures on similar local landforms were sampled to evaluate differences in physical and chemical properties to a depth of 120 cm. In the upper 30 cm, soils under forests had lower bulk densities than under pastures (0.81 vs. 1.04 g cm ⁻³), and soil C under pasture was less than that under forest by about 15.9 ± 10.4 Mg ha⁻¹ Compared with forest soils, those under pastures had a greater soil pH_H2O throughout the upper 120 cm (0.44 ± 0.10 pH units). Soil effective cation-exchange capacity (ECEC) was 39 ± 19.9 kmol_c ha⁻¹ greater under pastures than under forests, a difference in ECEC that was mainly associated with an additional 19.3 ± 17.7 kmol_c ha⁻¹ of exchangeable Ca. Forest clearing and burning followed by conversion reduced soil C moderately but apparently increased the ability of mineral soils to retain exchangeable-cationic nutrients because of increased soil pH. Water-stability of aggregates was less in pastures than under forests (36.5% less for aggregates >2 mm). Reduced soil C was statistically associated with changes in the percentage of water-stable aggregates >0.25 mm (R² = 0.34, P < 0.05). Our understanding of the effects of land uses on advanced weathering-stage soils would benefit from monitoring studies that directly observe temporal soil changes caused by management.