Inorganic sulphate extraction from SO 2 -impacted Andosols T. D ELFOSSE , P. D ELMELLE , C. G IVRON & B. D ELVAUX Unite ´ des Sciences du Sol, Universite´ catholique de Louvain, Place Croix du Sud, 2/10, 1348 Louvain-la-Neuve, Belgium Summary Sulphate sorption on to the surface of short-range ordered minerals and precipitation of Al-hydroxy sulphate contribute to the acid neutralizing capacity of soils. The correct measurement of total inorganic sulphate is thus essential in soils that are accumulating SO 4 2– anions. We extracted SO 4 2– by various solutions, namely 0.005 M Ca(NO 3 ) 2 , 0.016 M KH 2 PO 4 , 0.5 M NH 4 F and 0.2 M acidic NH 4 -oxalate (pH 3), from Vitric and Eutric Andosols exposed to prolonged deposition of acid and SO 2 from an active volcano (Masaya, Nicaragua). We attributed sulphate extractable by KH 2 PO 4 (20–3030 mg kg 1 ) to anion-exchangeable SO 4 2– , which was much smaller than NH 4 F- and oxalate-extractable SO 4 2– (400– 9680 and 410–10 480 mg kg 1 , respectively). Our results suggest the occurrence of a sparingly soluble Al-hydroxy-mineral phase extractable by both NH 4 F and oxalate. The formation of Al-hydroxy minerals would result from the combination of enhanced weathering caused by strong acid loading and simultan- eous occurrence of large SO 4 2– concentrations in soil solution. Oxalate extracted slightly more inorganic SO 4 2– than did NH 4 F, this additional amount of SO 4 2– correlating strongly with oxalate-extractable Si and Fe contents. Preferential occlusion of SO 4 2– by short-range ordered minerals, especially ferrihydrite, explains this behaviour. If we exclude the contribution of occluded sulphate then oxalate and NH 4 F mobilize similar amounts of SO 4 2– and are believed to mobilize all of the inorganic SO 4 2– pool. Introduction Sulphate adsorption in soils is governed mostly by poorly crystallized Al and Fe oxyhydroxides and short-range ordered alumino-silicates (Camps Arbestain etal., 1999). Owing to their mineralogical characteristics, Andosols are amongst soils with the largest SO 4 2– retention capacity. These soils can contain large SO 4 2– concentrations caused by the deposi- tion of sulphur either from marine aerosols (e.g.  7400 mg SO 4 2– kg 1 : Hasan etal., 1970) or from a volcanic source (e.g.  4500 mg SO 4 2– kg 1 : Takamatsu etal., 1992). Deposited SO 4 2– may be involved in the immobilization of organic S, sorption of inorganic SO 4 2– and precipitation as aluminium-hydroxy sulphate mineral [Al x (OH) y (SO 4 ) z ]. All these processes are expected to diminish or delay net acidifica- tion of soils. Immobilization of S in organic matter and SO 4 2– adsorption indeed reduce cation leaching (Strickland & Fitzgerald, 1984; Camps Arbestain etal., 1999). Both formation of [Al x (OH) y (SO 4 ) z ] and SO 4 2– adsorption on to poorly crystal- line minerals involve OH – release or H þ consumption (Adams & Hajek, 1978; Rajan, 1978). Therefore, inorganic SO 4 2– retention plays a key role in soils affected by acid deposition involving S input (Johnson etal., 1981; Fumoto etal., 1996) and is regarded as an important process in acid-buffering of Andosols (Fumoto etal., 1996). Yet this process stores acidity as SO 3 , which might eventually produce sulphuric acid, once released (van Breemen etal., 1983). The correct assessment of the content of inorganic S is thus essential in the estimation of the soil’s susceptibility to acid- ification. Furthermore, since one usually determines the con- tent of organic S in soil indirectly by subtracting inorganic S from total S, accurate evaluation of the inorganic S would quantify adequately the organic S content of the soil. Many methods have been used for the determination of diverse pools of inorganic SO 4 2– in soil, e.g. water-, NaCl-, CaCl 2 -extraction displacing soluble SO 4 2– (Curtin & Syers, 1990; Alewell & Matzner, 1996), and NaHCO 3 , K or NaH 2 PO 4 or Ca(H 2 PO 4 ) 2 extracting SO 4 2– adsorbed on to minerals (Curtin & Syers, 1990; Wada etal., 1994; Alewell & Matzner, 1996). However, these conventional methods under- estimate the pool of inorganic SO 4 2– (Prietzel & Hirsch, 2000), since they fail to extract SO 4 2– involved in [Al x (OH) y (SO 4 ) z ] mineral phases (Prietzel & Hirsch, 1998). Instead, Prietzel & Hirsch (2000) have recommended the use of 0.5 M NH 4 F to extract inorganic SO 4 2– in acid forest soils because of its ability Correspondence: B. Delvaux. E-mail: delvaux@sols.ucl.ac.be Received 11 August 2003; revised version accepted 16 March 2004 European Journal of Soil Science, February 2005, 56, 127–133 doi: 10.1111/j.1365-2389.2004.00647.x # 2004 British Society of Soil Science 127