Abstract
The SO4-S inputs exceed SO4-S outputs in rain forests at San Carlose de Rio Negro, Amazonas, Venezuela and at La Selva, Costa Rica. Hypotheses to explain excess of inputs over outputs include accumulation of S in biomass, accumulation of S in soil, conversion of SO4-S to organic S compounds which leave the system in drainage water, conversion of SO4-S to volatile S compounds which leave as gases, and estimation errors. Acid rain occurs at both sites. If the S were volatilized out of the forest, oxidized in the atmosphere to SO4, then washed out of the atmosphere by rain, the resulting quantity of H2SO4 would be sufficient to account for the rainfall acidity observed in the field in Costa Rica.