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

Abstract

To study the accumulation mechanism of dissolved organic carbon (DOC) leached from the plow layer soil in the subsoil of irrigated rice paddies, leaching experiments using a series of plow layer soil columns and subsoil columns were conducted. The binding types of organic matter in the subsoil columns were estimated by successive extraction of organic matter with water, Na2S04, Na4P2O7 (pH 7.0), Na4P2O7 + NaOH, and NaOH solutions. The ability of the subsoil constituents to adsorb DOC was investigated by removing them. Successive removal of indigenous organic matter and iron oxides and hydroxides from the subsoil samples decreased the adsorption of DOC to the subsoil column. The decrease appeared in the Na2S04- and Na4P207 (pH 7.0)-extractable fraction for the organic matter removed subsoil column (Org.(—) subsoil column) and in the Na4P2O7 (pH 7.0)-extractable fraction for the organic matter and iron oxides and hydroxides removed subsoil, column (Org. (—) \\d. Fe(—) subsoil column), respectively, suggesting the importance of polyvalent cations in the subsoil and in the leachate for binding DOC to the subsoil. The effect of drainage on the accumulation of DOC was investigated in the case three repetitions of leaching incubation followed by stand incubation with or without drainage. The sum of the amounts of DOC retained in the subsoil columns during the 3 periods of leaching incubation was larger in the treatment with drainage than in the treatment without drainage. Since the net increase in the total organic carbon content in the subsoil columns was less pronounced in the treatment with drainage than in the treatment without drainage (untreated subsoil columns), or similar between the two treatments (Org.(—) subsoil columns), the drainage was assumed to promote the adsorption of DOC to the subsoil, and also to promote organic matter decomposition. In the treatment with drainage, Na4P207 (pH 7.0)-extractable organic carbon mainly accumulated. Drainage also resulted in the impossibility of extracting water-extractable organic carbon with water, and promoted the formation of humin.