Mechanisms of acid buffering and formation of secondary minerals in vitric Andosols A. HERRE a , F. L ANG a ,C H.S IEBE b , R. DOHRMANN c & M. KAUPENJOHANN a a Institut fu ¨r O ¨ kologie, FG Bodenkunde, TU Berlin, Salzufer 11–12, 10587 Berlin, Germany, b Instituto de Geologı ´a, Universidad Nacional Auto ´noma de Me ´xico, Cd. Universitaria, C.P. 04510, Coyoacan, Me ´xico D.F., Mexico, and c Landesamt fu ¨r Bergbau, Energie und Geologie (LBEG), Bundesanstalt fu ¨ r Geowissenschaften und Rohstoffe (BGR), Stilleweg 2, 30655 Hannover, Germany Summary Andosols in the vicinity of active volcanoes receive large inputs of SO 2 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 Poa´s and Masaya. Two types of short-term (24 hour) acidifica- tion experiments at pH 3 were conducted to simulate an open system in which leaching prevails (extrac- tion 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 Poa´s sam- ples 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 crys- tals but did not result in protonated carboxyl groups. In Masaya samples mineral and/or glass dissolu- tion 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 basalu- minite at Masaya while it seems to be susceptible to leaching at Poa´s. Acid buffering resulted in the formation of amorphous silica at both sites. Introduction Input of acids and subsequent effects on soils have been inten- sively studied in temperate regions of the northern hemisphere (Van Breemen, 1990). Acid deposition in this area has been mainly of anthropogenic origin and reached up to 6 kmol H þ ha –1 year 1 in the 1980s in strongly polluted forested areas of western and central Europe (Van Breemen, 1990). In contrast, few studies exist on the impact of acid emissions from active volcanoes on surrounding soils, where estimated H þ deposi- tion rates can reach up to 109 kmol H þ ha –1 year 1 (Delmelle et al., 2001). While anthropogenic acid emissions are domi- nated by HNO 3 and H 2 SO 4 , active volcanoes release pre- dominantly H 2 SO 4 or HCl with minor amounts of HF. According to Bruggenwert et al. (1990) and Ulrich (1990), different acid buffering mechanisms with specific buffer capacities and rates are distinguishable in soils. In the temper- ate regions, protonation of variable charge sites with subsequent liberation of basic cations and dissolution of sesquioxides are the major short-term acid buffering mechanisms (Van Breemen et al., 1983). In Andosols various buffering mechanisms can be relevant, depending on acid deposition rates, the pH of the soil solution and the Andosol subtype. Besides, leaching con- ditions are expected to influence acid buffering as the accumu- lation of cations in soil solution can lead to product limitation and thus hamper buffering processes. Changes in buffer mech- anisms and rates may occur over short periods of time, as vol- canic emissions show a strong temporal variability, causing large pH fluctuations in surrounding Andosols (Parnell, 1986). Field and laboratory experiments on the effects of acid input in different Andosol subtypes suggest the protonation of allo- phane and humic substances as well as the dissolution of allo- phane, iron oxides and primary silicate minerals or glass as buffering mechanisms (Table 1). As a consequence, cations and Si are released into the soil solution and can be leached or precipitated as secondary mineral phases. Correspondence: Andrea Herre. E-mail: andrea.herre@tu-berlin.de Received 30 May 2006; revised version accepted 13 November 2006 European Journal of Soil Science, April 2007, 58, 431–444 doi: 10.1111/j.1365-2389.2007.00890.x # 2007 The Authors Journal compilation # 2007 British Society of Soil Science 431