Seasonal changes in organic compounds in soil solutions obtained from volcanic ash soils under different land uses

We investigated the organic matter composition in soil solutions as influenced by land use. Undisturbed mineral soils were collected periodically from mineral layers of Andosols under coniferous and deciduous forest, grasslands and upland fields for 1 year using soil core samplers. Soil solutions were obtained by centrifugation under - 1.55 x 10(6) Pa condition, which equals to 4.2 pF. The dissolved organic carbon (DOC), hexoses, pentoses, phenolic acids and amino acids in soil solutions were determined. The concentrations of DOC under forest and grassland stands were not influenced by precipitation, but were negatively correlated to the yield of soil solution (P<0.01). Temporal variations in concentrations of hexoses were similar to those of DOC at all Fujisawa sites but not at the Tsukui sites. Except for one forest stand, the concentrations of hexoses increased from autumn to winter. Concentrations of pentoses and phenolic acids showed similar seasonal variation in the forests and grasslands, and peaked in autumn and spring. Variations in the concentrations of pentoses corresponded to litter fall and ground grass cutting in forests, drying of grass at grasslands and harvest at upland fields. Although the concentrations of phenolic acids varied with seasons, differences between land uses were not clear. The release of phenolic acids might depend on the variation of the temperature. Correlations between pentoses and phenolic acids and DOC were weak. The concentrations of amino acids were lower than those of other organic compounds at all sites. At the upland fields, the concentrations of amino acids were significantly (P<0.01) correlated with those of hexoses. The composition of dissolved organic matter (DOM) varied throughout the year. Seasonal changes in organic matters in soil solutions depended on yields of soil solution, temperature variations and the timing of litter supply under different land use. (C) 2002 Elsevier Science B.V All rights reserved.