Mechanisms of acid buffering and formation of secondary minerals in vitric Andosols

Andosols in the vicinity of active volcanoes receive large inputs of SO2 and HCl. We studied (i) the mechanisms of acid buffering, (ii) the effect of cation removal on the short-term acid neutralization capacity and (iii) the consequences of acid buffering for secondary mineral formation in vitric Andosols around the Central American volcanoes Poas and Masaya. Two types of short-term (24 hour) acidification experiments at pH 3 were conducted to simulate an open system in which leaching prevails (extraction with protonated cation exchange resin) and a closed system with no leaching (pH(stat) titration with cation accumulation). Long-term buffering under field conditions (mean soil pH: 4.6) and its effect on secondary mineral formation were studied by analysis of samples from a transect of decreasing acid input by IR spectroscopy, microscopic methods and geochemical equilibrium modelling. In Poas samples the main short-term buffering mechanisms at pH 3 are plagioclase dissolution and protonation of organic matter. Long-term acid buffering under field conditions led to weathering of plagioclase crystals but did not result in protonated carboxyl groups. In Masaya samples mineral and/or glass dissolution are the dominant acid buffering mechanisms in laboratory experiments and under field conditions. For both sites, cation accumulation during pH 3 acidification experiments led to a decrease of the effective acid neutralization capacity. Due to different climatic conditions, Al is precipitated as basaluminite at Masaya while it seems to be susceptible to leaching at Poas. Acid buffering resulted in the formation of amorphous silica at both sites.