Abstract

ABSTRACT Soil acidification is a natural pedogenetic process in forest ecosystems, but is accelerated by acidic deposition. Soils on Yakushima Island, in southwestern Japan, are derived from granitic bedrock and volcanic parent materials influenced by eolian dust inputs. Acid deposition and low acid-neutralizing capacity (ANC) of granite-derived soils are hypothesized to be the cause of acidic stream water that has been widely observed on Yakushima. It remains unclear, however, whether ANCs are also limited in volcanic soils, which contain weatherable minerals and have high buffering capacities. To compare acidification rates between granite-derived soils and volcanic soils, we estimated changes in ANCs between granite and volcanic parent materials and the resulting soils at a pedogenetic timescale, using the mass balance of titanium. After 7300 years of soil development following the eruption of the Kikai caldera, decreased ANCs and total base reserves were observed in both granite-derived and volcanic soils, compared to their respective parent materials. No difference in soil ANC was observed between granite and volcanic parent materials. Different cations contribute to acid neutralization in soils: decreasing ANC results primarily from the loss of Al in granite-derived soils, while acidity is neutralized by the loss of base cations from volcanic soils. In granite-derived soils, resistance to weathering and K retention in 2:1 type silicate clays limit the loss of bases while increasing the accumulation of exchangeable Al. In volcanic soil profiles, base leaching and the sorption of organic acids onto short-range order minerals can neutralize acidity, although erosion and admixing of granite or eolian dust-derived materials lead to a reduction of ANC in surface soil horizons.

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