ORGANIC-MATTER STABILIZATION IN 2 SEMIARID TROPICAL SOILS - SIZE, DENSITY, AND MAGNETIC SEPARATIONS

Soil organic matter (SOM) in many tropical soils is less stable than t hat of temperate soils, in part due to faster decomposition processes in the tropics, but possibly also due to different mechanisms of organ omineral stabilization, We studied the mineral associations of SOR-I i n the surface horizons of two semiarid tropical soils tan Oxisol and a n Alfisol) using particle-size, density, and magnetic separations. The inorganic phases of fractionated materials were examined using chemic al extraction and x-ray diffraction. Organic matter associated with th e sand fraction accounted for 21 to 30% of soil C, mostly as coarse, f loating material. This coarse material was more abundant in the Alfiso l, One-half the organic C of both soils was associated with the silt-s ized fraction, which contained a substantial portion of microaggregate s resistant to sonication, Carbon in silt-sized aggregates was mainly stabilized by association with poorly crystalline oxides and by aggreg ation with crystalline oxides and clays, The C as light matter (<1.8 g cm(-3)) liberated from silt-size microaggregates by successive sonica tion treatments accounted for approximate to 23% of total soil C for b oth soils. The SORI in the <1.8 g cm(-3) fraction seemed younger and l ess decomposed than the organic materials in the temperate soil fracti ons with similar density and equivalent sizes. In clays, C was concent rated in fractions of intermediate magnetic susceptibility, whereas bo th the highly magnetic fraction of predominantly well-crystallized Fe and the nonmagnetic fraction were C poor. The study points to the impo rtant role of particle sire and crystallinity of Fe oxides and of micr oaggregation between oxides, organic matter, and other minerals in sta bilizing tropical soil organic matter.