PROCESSES OF SOIL ACIDIFICATION IN TROPICAL DURANDEPTS, NICARAGUA

Masaya volcano in west-central Nicaragua has been emitting large amounts of SO2 and HCl since 1979; discharges of HCl exceed 1400 metric tons (t) per day. A wedge-shaped region of west-central Nicaragua, downwind of the volcano, is consistently exposed to acidified rainfall containing washed-out HCl and H2SO4. The effects of this acid rain on the ecosystems receiving it are strikingly different from those observed in northeastern North America, due to the influence of tropical vegetation and amorphous silicate soil minerals derived from volcanic glass. Contact of the rain with ubiquitous Malan-thora and Lantana vegetation produces at least a 100-fold drop in acidity between bulk deposition and throughfall. In the soil, plant root systems release acid cations allowing plant uptake of base cations, especially potassium. Abundant allophane, produced in the weathered volcanic ash, apparently is titrated by the level of plantadded acidity. The interaction between the allophane and changing soil solution compositions controls soil solution sulfate concentrations. The preferred hypothesis proposes that decreases in soil pH from added acidity increase the anion exchange capacity (AEC) in impacted soils. Because of the increased AEC, sulfate leaching from the soil decreases, while base cations released from the allophane enter the soil solution. Thus, soils exposed to chronic acid input exhibit depleted exchangeable cation populations. Subsequent deposition of nonacidified rain appears to reverse the process: sulfate release to the soil solution increases, and base cation release to the solution decreases, implying that the AEC drops and the CEC increases.