Introduction

Soil acidification results from excessive application of chemical fertilizer or acid deposition. Acid neutralization mechanism includes cation exchange, acid adsorption and dissolution of solid-phase aluminum (Al) in acid soils. Acid neutralization capacity and Al dissolution rate are very important for determining soil productivity and estimating environmental load capacity of soils. In several studies, the rate and mechanism of acid neutralization, or Al dissolution rates of Andosols, was investigated using a few soil samples (Wada and Momonura,1991 , Wada et al., 1994 , Dahlgren and Saigusa, 1994). Allophanic soils showed high neutralization capacity due to acid adsorption and soils dominated by Al-humus complexes released greater amounts of Al to neutralize acid, compared with allophanic soils. However, acid neutralization capacity of silandic and aluandic Andosols has not been comprehensively estimated because these Andosols have diverse contents of allophanic clay, Al-humus complexes and exchangeable cation. The objective of this study was to evaluate comprehensively acid neutralization capacity of silandic and aluandic Andosols using diverse soil sample population.

Materials and methods

Experimental methods: Acid neutralization capacity of soils was estimated by amounts of Al dissolved after soils reacted with acid. Experimental methods was constructed mostly according to Wada and Momomura (1991). After adding 20 mL of 5 mM nitric acid to 2 g of soils and shaking for 24 hours at 25 C, solution pH and the concentration of Al, silica (Si) and bases (Ca, Mg, K and Na) in the solution were measured. Proton was added to soils at a rate of 5 cmol kg^-1. Residual soil was extracted 4 times using 25 mL of 1 M ammonium chloride solution and amounts of Al and basic cations which were adsorbed to solid were determined. Acid oxalate extractable Al (Alo) and Si (Sio), pyrophosphate extractable Al (Alp) and extracted Al and bases with 1 M ammonium chloride solution (exchangeable Al and bases) were measured.

Soils: Sixteen silandic soils and 15 aluandic soils were selected to keep diversity of sample population in the contents of Alo, Alp and exchangeable Al and bases.

Results and discussion

Soil pH (H₂O) ranged from 5.2 to 6.0 and from 4.8 to 5.4 for silandic and aluandic soils, respectively. Silandic and aluandic soils showed diverse properties for their soild-phase Al contents. Total active Al derived from allophanic clay and Al-humus complexes ranged from 20 to 71 and from 8.6 to 31 (g kg^-1) for silandic and aluandic soils used in this study, respectively. Exchangeable Al ranged from 0 to 3.2 and from 3.1 to 11 (cmol(+) kg^-1) for silandic and aluandic soils, respectively. Active Al species of silandic and aluanidic soils were dominated by allophane-imogolite (Alo-Alp) and Al- humus complexes, respectively. The major clay minerals were allophane-imogolite and 2:1-2:1:1 intergrade for silandic and aluanidic soils, respectively.

Solution pH decreased to 3.8-4.6 and 3.8-4.4 in silandic and aluanidic soils after each soil reacted with acid, respectively. Added acid was almost completely neutralized. Aluminum concentrations in the solution, which were released from soils, increased with the decreases in exchangeable base contents of soils in the range of less than 5 cmol(+) kg^-1, which was equal to the amount of added protons. When exchangeable base contents were equal, the amounts of Al released into solution were greater in aluandic soils than in silandic soils. It was made clear that allophanic clay would dissolve by acid because solution Si concentrations were greater in silandic soils than in aluandic soils. Total amounts of Al dissolved by acid treatment were calculated by subtracting exchangeable Al of original soils from the sum of solution and exchangeable Al of acid treated soils. Total dissolved Al showed a significant negative correlation with exchangeable bases and Al derived from allophanic clay (Alo-Alp), and a significant positive correlation with Al-humus complexes (Alp-exchangeable Al). The multiple regression equation was expressed as follows; dissolved Al contents = -0.34*exchangeable bases -0.0049*Al derived from allophanic clay +0.0097*Al-humus complex +5.1 (units of all fractions were cmol(+) kg^-1, n=31, R²=0.89) All fractions were significant factors (P < 0.001).

It was concluded that acid neutralization capacity of Andosols were determined by exchangeable bases and composition of reactive solid-phase Al pools, and when exchangeable bases were equal, silandic Andosols were estimated to show higher acid neutralization capacity than aluandic Andosols due to acid adsorption by allophanic clay and high solubility of Al-humus complexes.