Long-term net soil erosion as determined by 137Cs redistribution in an undisturbed and perturbed tropical deciduous forest ecosystem

Abstract

Erosion is the main cause of soil degradation in tropical regions, where the lack of methods for long-term studies is the principal constraint to addressing soil erosion problems. Recently, the analysis of 137Cs redistribution within the landscape has been used for assessing long-term soil erosion and net deposition. In the present study, measurements of 137Cs distribution were used to calculate long-term soil erosion in a Mexican tropical deciduous ecosystem under undisturbed forest and pasture conditions. Sheet erosion processes caused 137Cs redistribution within the landscape. The crests had significantly higher 137Cs activity than midslopes and lower concentration than the footslopes. There was no clear relationship between 137Cs redistribution and local topographic variables in our study (that is, slope). Soils in a gentle midslope had lower 137Cs activity than those in a steeper midslope. However, hill morphology explained 137Cs redistribution within landscape, that is, high 137Cs activity was associated with sites at the base of hillslopes. Thus, net erosion was found to be strongly influenced by hill morphology. Calculated erosion and deposition rates for the 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 following perturbation. High net erosion was associated with plots with high annual 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 soil productivity loss, as this top layer represents the principal soil nutrient pool for the Chamela region.