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Abstract
Soils amended with plant residues (20 g kg-1 soil) were incubated for 56 d at 25°C under upland conditions, and water extracts were obtained periodically. The decomposition process was monitored using high performance size exclusion chromatography (Asahipack GS-220M column) for the extracts detected by absorbance at 280 nm. Two types of soils (Udifluvent and Melanudand) and three kinds of plant residues (rapeseed meal, orchard grass shoots, and rice straw) were used for the experiment. The rates of decomposition were in the order of rapeseed meal > orchard grass shoots > rice straw, and the decomposition of rice straw caused the immobilization of inorganic N throughout the incubation period. In all the treatments, the amount of water-soluble organic C decreased sharply during the early period of incubation. The amount of organic substances contained in the plant residues rapidly disappeared from the water extract, and simultaneous formation of water-soluble organic substances was observed by high performance size exclusion chromatography. The amount of newly formed organic substances decreased with time. Using polyethylene glycols to calibrate the column, the molecular weight of the UV-absorbing water-soluble organic substances at peak maximum was 1,200 and 500 for the Udifluvent and Melanudand samples without plant residue amendment, respectively. For the newly formed UV-absorbing water-soluble organic substances, the molecular weight at peak maximum was 1,200 regardless of the soil types. However, the molecular weight of the UV-absorbing water-soluble organic substances formed from plant residues in the Melanudand sample tended to decrease during the incubation period.
