SOIL-FORMING PROCESSES IN ALIC MELANUDANDS UNDER JAPANESE PAMPAS GRASS AND OAK

Current soil-forming processes and the effect of Japanese oak (Quercus serrata Thunb.) tree invasion on soils developed under Japanese pampas grass (Miscanthus sinensis Anders.) were investigated in Alic Melanudands forming on recent dacitic tephra deposits in Japan. The Andisols examined are members of the nonallophanic group of Andisols whose clay-sized mineralogy is dominated by Al/Fe humus complexes and hydroxy-Al interlayered 2:1 minerals, rather than short-range-order aluminosilicates common to most Andisols. Two study sites were chosen that had similar soil-forming factors except for the present vegetation. One site has remained under Japanese pampas grass, while the other site was invaded by Japanese oak 50 yr ago. To examine soil processes, soil solutions were collected for a 2-yr period, using tension lysimeters. Results indicate that translocation of Fe, Al, and dissolved organic C is not a major process in these Andisols. Aqueous concentrations of Si are sufficient for the formation of allophane/imogolite; however, the Al appears to be preferentially incorporated into the interlayer of 2:1 layer silicates and Al-humus complexes and is, therefore, unavailable for synthesis of short-range-order minerals. Invasion of the oak vegetation resulted in a lowering of soil solution pH due to production of organic acids from the decomposing oak litter, and to removal and storage of base cations in the oak biomass. The lowering of the solution pH resulted in increased Al concentrations and decreased HCO3 concentrations in the A horizons of the oak site. When compared with the pampas grass site, the oak vegetation appears to promote greater cycling of Ca, Mg, K, and Cl in the upper 20 cm of the soil profile.