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Abstract
Plant residue-sand mixtures were incubated for 28 d at 25°C under upland conditions, and water extracts were obtained periodically. Three kinds of plant residues (rapeseed meal, orchard grass shoots, and rice straw) were used and mixed with sand at a rate of 20 g kg-1. The water extracts were fractionated into three fractions, i.e., humic acids (HAs), polyvinylpyrrolidone (PVP)-adsorbed fulvic acids (FAs), and PVP-non-adsorbed FAs. High performance size exclusion chromatography (HPSEC) on an Asahipack GS-320H column was performed on the water extracts and the fractions. The amount of organic C in the extract tended to decrease with time, and at the same time the formation of water-soluble humic substances (HS) was observed by HPSEC. The molecular weight (MW) of HS at peak maximum varied with the plant residues used. A large proportion of the organic substances contained in the plant residues belonged to the PVP-non-adsorbed FA fraction, and this fraction was considerably reduced by the incubation. On the other hand, increases in the amount of water-soluble HS, especially in the HA fraction, were detected by HPSEC. Using polyethylene glycols to calibrate the column, the MWs of water-soluble HS at peak maximum ranged from 2,300 to 4,900 and 1,200 to 1,900 for the HA and PVP-adsorbed FA fractions, respectively.
