Origin and properties of humus in the subsoil of irrigated rice paddies IV. Effect of water percolation on the binding type of humus in a submerged plow layer soil

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 Na4P2O7 (pH 7.0), 0.1 M Na4P2O7 (pH 10.5), and 0.1 M Na4P2O7 (pH 10.5) with the addition of NaBH4. The amount of humus in the Na2SO4- and Na4P2O7 (pH 7.0)-extractable fractions increased during the incubation. Since only the amount of Na4P2O7 (pH 7.0)-extractable humus increased during hooded 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 Na4P2O7 (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.