Origin and properties of humus in the subsoil of irrigated rice paddies

Abstract

To examine the effect of downward percolation of flooding water under reduced soil conditions on the binding type of humus in the plow layer soil of paddy fields, a model experiment was conducted. Changes in the binding type of humus were analyzed by successive extraction of humus with water, 0.25 m Na₂SO₄, 0.1 m Na₄P₂0₇ (pH 7.0), 0.1 m Na₄P₂0₇ (pH 10.5), and 0.1 m Na₄P₂0₇ (pH 10.5) with the addition of NaBH₄. The amount of humus in the Na₂SO_4^- - and Na₄P₂0₇ (pH 7.0)-extractable fractions increased during the incubation. Since only the amount of Na₄P₂0₇ (pH 7.0)-extractable humus increased during flooded incubation without percolation (Maie et al. 1998: Soil Sci. Plant Nutr., 44, 331-345), it was concluded that water percolation as well as the development of reducing soil conditions was essential for the increase in the solubility of humus in soil water. Humic acids (HAs) that leached from the plow layer soil accounted for less than 0.7% of dissolved organic matter in the leachate and almost all of them belonged to Type Rp. Thus, HAs with a high humification degree, which were extractable with Na₄P₂0₇ (Maie et al. 2000: Soil Sci. Plant Nutr., 46, 163-175) were not leached from the plow layer soil with percolation even after the development of reduced soil conditions.

Key Words:

• binding type
• DOC
• humus
• reducing conditions
• water percolation