ROLE OF SOIL ORGANIC-MATTER IN STABILIZATION OF WATER-STABLE AGGREGATES IN SOILS UNDER DIFFERENT TYPES OF LAND-USE

Three Andosols (used as grassland, upland field, and paddy field) and two Gray Lowland soils (used as upland field and paddy field) were exa mined to study the role of organic matter in the stabilization of soil aggregates. It was found that prolonged use of Andosols as grassland or as upland field enabled to maintain a higher level of soil organic carbon than use as paddy field. However, paddy soil exhibited a greate r aggregate stability (determined by wet-sieving procedure) than soil under upland field or grassland management. Comparison between two Gra y Lowland soils also showed that aggregates in the paddy soil were mor e stable than in the upland field soil. Among organic constituents exa mined (such as sodium pyrophosphate (SPP)-extractable organic matter a nd carbohydrates), only the contents of hot water (HW)-extractable car bohydrates of the soils were significantly correlated with aggregate s tability. Paddy soils which showed a greater aggregate stability conta ined a larger amount of HW-extractable carbohydrates in the soils, and a larger amount of HW-extractable carbohydrates regardless of the siz e of their aggregates compared with other soils. These findings sugges t that HW-extractable carbohydrates which represents easily decomposab le organic materials may be of special importance for the stabilizatio n of aggregates in the soils.