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[C-3.2.3 Studies on Estimation of Acid-Buffering Capacity of Soils]

[Contact Person]

Tamon Fumoto
Laboratory of Soil Conservation
National Institute of Agro-Environmental Sciences
Ministry of Agriculture, Forestry and Fishery
3-1-1 Kannondai, Tsukuba, Ibaraki 305 Japan
Phone +81-298-38-8270, Fax +81-298-38-8199
E-mail tamon@niaes.affrc.go.jp

[Total Budget for FY1993-1995]

7,114,000 Yen (FY1995 2,102,000 Yen)

[Abstract]

(1) Soil columns of a volcanic ash soil and a granitic soil were leached with simulated precipitations for 85 weeks to compare acid-buffering capacities. Significantly higher concentration of Al was dissolved from acidified volcanic ash soil. Volcanic ash soil had significantly higher SO₄^2- adsorption capacity, but adsorption of input SO₄^2- was limited due to large amount of native SO₄^2-. Magnesium and Ca release rates from mineral dissolution in volcanic ash soil, estimated according to mass balances, were one order of magnitude higher than those rates reported for soil environments in Europe and North America. (2) Element release rates were measured for a volcanic ash soil and a granitic soil, employing flow-through column technique. Steady-state Mg and Ca release rates of volcanic ash soil at pH 4 were almost ten times higher than granitic soil, indicating high cation release rates from mineral dissolution. (3) Extractable SO₄^2- contents and SO₄^2- adsorption isotherms were determined on forest volcanic ash soils to investigate the effect of accumulated SO₄^2- on acid buffering capacity. Sulfate adsorption capacity was found to depend largely on Al content in allophane and clay content. Large amount of SO₄^2- had already been adsorbed to soils with high adsorption capacities, thus increase in adsorbed SO₄^2- was estimated to be relatively small even if soil solution SO₄^2- increased. Therefore, acid buffering capacity by SO₄^2- adsorption was considered not to correspond to SO₄^2- adsorption capacity of soil. (4) Throughfall and soil solution were observed in a forest to asess the effect of precipitated proton on soil. Nitrate and base cation concentrations were significantly higher in O horizon soil solution than in throughfall, suggesting that large amount of protons were being produced by nitrification and neutralized by base cation release in the O horizon. Proton production by nitrification in O horizon was estimated as much as 5 kmol_c ha^-1 yr^-1. Therefore, the adverse effect of precipitated protons was considered to be small on the studied soil.

[Key Words]

Acid-buffering Capacity, Sulfate Adsorption, Volcanic Ash Soil, Soil Column, Mineral Dissolution