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

Stability of humus in the plow layer soil is considered to affect the quantity and quality of dissolved organic matter leached from the plow layer soil. Therefore, a model experiment was conducted to analyze the effect of soil reduction under submerged conditions on the stability of humus in the plow layer soil. The changes in the stability of humus in the plow layer soil during submerged incubations with and without rice straw application were evaluated based on the changes in the binding type of humus. Binding type of humus in the plow layer soil was analyzed by successive extractions of organic matter 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 NaBH₄. Amounts of Fe, Mn, and Mg in each fraction were also determined to estimate the relationships between humus and metals.

The successive extraction of humus indicated that the amount of organic carbon which was extractable with the (NaBH₄ +0.1 M Na₄P₂O₇) solution decreased while that of the 0.1 M Na₄P₂O₇ (pH 7.0}-extractable organic carbon increased during submerged incubation with rice straw application. The origin of the increase in the amount of organic carbon in the Na₄P₂O₇ (pH 7.0)-extractable fraction during submerged incubation was investigated further by another incubation experiment using ¹³C-glucose as a reducing agent. Atom- 13C% analysis showed that the contribution of organic carbon derived from compounds other than glucose to the increase in the contents of humic acids and fulvic acids in the Na₄P₂O₇ (pH 7.0)-extractable fractions was ca. 80%. Therefore, it was concluded that the binding type of humus changed from (NaBH₄ + Na₄P₂O₇)-extractable to Na₄P₂O₇ (pH 7.0)-extractable humus under reducing conditions. Since the amounts of organic carbon and Fe increased in the Na₄P₂O₇ (pH 7.0)-extractable fraction and decreased in the (NaBH₄ +0.1 M Na₄P₂O₇)-extractable fraction simultaneously, iron reduction was presumably associated with the change in the binding type of humus in submerged paddy soil.