Acidification Mechanism of Tropical Forest Soils
and its Application to Fertility Management
Kazumichi Fujii (Forestry and Forest Products Research Institute, Division of Forest Soils)
fkazumichi@affrc.go.jp
Soil acidification is one of land degradation issues that limit crop productivity in humid
tropics, whilst acidification is an essential process that promotes weathering (pedogenesis)
and nutrient supply to plants. To clarify complex linkage between soil acidity andproductivity in tropical ecosystems, (1) proton budgets in soils were quantified to identify themajor proton-generating and -consuming processes. Then, (2) the effects of acidification on
nutrient cycling were studied in relation to tree root and soil microbial activities. Finally, (3)the effects of land-use change on acidification and neutralization were evaluated in tropicalcropland soils to develop low-cost fertility management that can mitigate soil acidification .
(1) Contribution of plants and microorganisms to soil acidification : Proton budgets have been
quantified for ion fluxes associated with plant uptake and solute leaching in plant-soil systems.
Proton budgets showed that plants and microorganisms promote acidification to acquire bases,
even in highly acidic tropical soils in Indonesia. The nitric and organic acids can contribute tomobilization of basic cations and their uptake by plants. The differences in soil sensitivity toacid load between glacial till-derived soils, tropical soils, and volcanic soils can bequantitatively explained based on acid neutralizing capacities and proton budgets in the soils.
(2) Soil acidification and adaption of plants and microorganisms : Acidification can generally
limit tree root and microbial activities, but the specific tree roots and rhizospheremicroorganisms can release low-molecular-weight organic acids (e.g. citrate and malate) tosolubilize non-labile phosphorus. Solubilization of lignin-rich organic matter, which
generally retards nutrient cycling, can also be enhanced in acidic organic layers by fungalperoxidases. The large fluxes of dissolved organic matters from the acidic organic layers canfunction in mobilization of bases without increasing leaching loss. This contrasts with thewell-known acidification process via bicarbonate in the moderately acidic tropical soils.

(3) Forest organic matters can neutralize acidity in tropical cropland soils: Conversion offorest to continuous cropping accelerated soil acidification through nitrification and productremoval. Most of the acidity can be neutralized by mineralization of organic anions
associated with loss of soil organic matter. Acid neutralization capacity depends on stocks ofsoil organic matter accumulated in fallow periods. Proton budgets in the soils under different
land use in Indonesia suggested that the long-term forest fallow (aroma oil production) or
short-term Imperata grass fallow (healthy herb production) in land-use schemes can increaseorganic matter stocks and ameliorate soil acidification without decreasing farmers’ benefits.
References[1] Fujii K et al. (2008) Contribution of different proton sources to pedogenetic soil
acidification in forested ecosystems in Japan. Geoderma, 144, 478 -490.
[2] Fujii K (2014) Soil acidification and adaptations of plants and microorga nisms in Bornean
tropical forests. Ecological Research, 29, 371-381. 【Suzuki Award Paper】[3] Fujii K et al. (2009) Quantification of proton budgets in soils of cropland and adjacentforest in Thailand and Indonesia. Plant and Soil, 316, 241-255.