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

Abstract

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.