LONG-TERM NET SOIL-EROSION AS DETERMINED BY CS-137 REDISTRIBUTION IN AN UNDISTURBED AND PERTURBED TROPICAL DECIDUOUS FOREST ECOSYSTEM

Erosion is the main cause of soil degradation in tropical regions, whe re the lack of methods for long-term studies is the principal constrai nt to addressing soil erosion problems. Recently, the analysis of Cs-1 37 redistribution within the landscape has been used for assessing lon g-term soil erosion and net deposition. In the present study, measurem ents of(137)Cs distribution were used to calculate longterm soil erosi on in a Mexican tropical deciduous ecosystem under undisturbed forest and pasture conditions. Sheet erosion processes caused Cs-137 redistri bution within the landscape. The crests had significantly higher Cs-13 7 activity than midslopes and lower concentration than the footslopes. There was no clear relationship between Cs-137 redistribution and loc al topographic variables in our study (that is, slope). Soils in a gen tle midslope had lower Cs-137 activity than those in a steeper midslop e. However, hill morphology explained Cs-137 redistribution within lan dscape, that is, high Cs-137 activity was associated with sites at the base of hillslopes. Thus, net erosion was found to be strongly influe nced by hill morphology. Calculated erosion and deposition rates for t he undisturbed watershed were 13.2 Mg ha(-1) yr(-1) and 4.9 Mg ha(-1) yr(-1), respectively. Net erosion within the pasture conversion plots was strongly influenced by rainfall erosivity during the year followin g perturbation. High net erosion was associated with plots with high a nnual erosivity immediately after forest burn. This suggests that the first year after slash and burn is critical for susceptibility to soil erosion. Based on erosion rates calculated in the present study, the top 5 cm of soil could be removed in only 25 years. This represents so il productivity loss, as this top layer represents the principal soil nutrient pool for the Chamela region.