Low-molecular-weight organic acids and metal speciation in rhizosphere and bulk soils of a temperate rain forest in Chitou, Taiwan

In this study, we investigated the basic soil physiochemical properties, low molecular weight organic acids (LMWOAs), and the associated speciation and sequential extractability of selected metals in rhizosphere and bulk soils of 3 trees, namely Chamaecyparis formosensis, Cryptomeria japonica, and Phyllostachys pubescens, of montane forests in Taiwan. LMWOAs were identified by gas chromatography, and the basic physiochemical properties of the soils were determined by conventional methods. Various fractions of metals in rhizosphere and bulk soils were extracted by a multiple-step extraction method. The pH of rhizosphere soils was found to be lower than that of bulk soils. The mean cation-exchange-capacity (CEC), organic carbon, nitrogen, and clay contents of rhizosphere soils as well as oxalic, malonic, fumaric, and succinic acids were greater than those in bulk soils (p < 0.05). Metal speciation fractionated by multiple-step extractions showed that Al, Fe, Mn, Cu, and Zn were distributed in different exchangeable, carbonate-bound, organic complex-bound, amorphous mineral colloid-bound, and residual fractions. No significant differences were observed among the tree species in physicochemical properties or metal fractionation except for LMWOAs. Concentrations of extractable Al and Fe showed high correlation with oxalic acid and total LMWOAs (p < 0.01). However, the extractable Mn, Cu, and Zn in the rhizosphere soils showed poor correlations with LMWOAs.