Processes of Nutrient Transfer in a Tropical Rain Forest

The process of cation leaching in a tropical soil was investigated at Finca La Selva, Costa Rica. It was concluded that carbonic acid is the agent responsible for most cation leaching in this soil, just as it is in a temperate Douglas-fir forest soil in western Washington. Calculated carbon dioxide pressures in the tropical soil are approximately an order of magnitude greater than those in the temperate soil; thus it is presumed that the greater soil carbon dioxide production in the tropical soil has played a major role in depleting the reserve of exchangeable cations therein. Carbon dioxide evolution studies conducted at the site indicate that probably less than half the total soil carbon dioxide evolved comes from initial litter decomposition. Consequently, high decomposition rates cannot necessarily be directly related to high soil-leaching potential. Other aspects of mineral transfer in solution are discussed with special reference to storm duration, storm intensity, and uptake by vegetation. IT HAS LONG BEEN KNOWN that litter decomposition rates are greater in the tropics than in temperate regions (Jenny eXt al. 1949), a fact that must have a profound effect on the mineral cycling schemes of the two regions. In some tropical rain forests, for instance, base saturation is below 5 percent (Bourgeois et al. 1972), and presumably most of the nutrient capital lies in the forest biomass (Jordon 1970; Edmisten 1970). Therefore it appears that rapid and efficient uptake of minerals released from decomposing litter occurs, thus preventing further depletion of the total nutrient capital. In addition, rapid decomposition may aggravate nutrient loss by producing greater amounts of carbon dioxide (CO2), which in turn will result in higher carbonic acid concentrations in the soil solution. McColl (1969) has shown that carbonic acid is responsible for most soil-leaching losses from a 45-year-old Douglas-fir forest in western Washington. As a wetting front enters the soil, it encounters the high levels of gaseous C02 normally present in the soil atmosphere. According to Henry's law, C02 enters this incoming solution until the concentration of dissolved CO2 is proportional to the partial pressure of gaseous C02 in the soil, according to the equation [CO2 (aq) I K,Pco2, where Khl is Henry's law constant. Aqueous C02 hydrolizes with water to