Organic Matter Stabilization in Two Semiarid Tropical Soils: Size, Density, and Magnetic Separations

Soil organic matter (SOM) in many tropical soils is less stable than that of temperate soils, in part due to faster decomposition processes in the tropics, but possibly also due to different mechanisms of organomineral stabilization. We studied the mineral associations of SOM in the surface horizons of two semiarid tropical soils (an Oxisol and an Alfisol) using particle-size, density, and magnetic separations. The inorganic phases of fractionated materials were examined using chemical extraction and x-ray diffraction. Organic matter associated with the sand fraction accounted for 21 to 30% of soil C, mostly as coarse, floating material. This coarse material was more abundant in the Alfisol. One-half the organic C of both soils was associated with the silt-sized fraction, which contained a substantial portion of microaggregates resistant to sonication. Carbon in silt-sized aggregates was mainly stabilized by association with poorly crystalline oxides and by aggregation with crystalline oxides and clays. The C as light matter (<1.8 g cm -3 ) liberated from silt-size microaggregates by successive sonication treatments accounted for 23% of total soil C for both soils. The SOM in the <1.8 g cm -3 fraction seemed younger and less decomposed than the organic materials in the temperate soil fractions with similar density and equivalent sizes. In clays, C was concentrated in fractions of intermediate magnetic susceptibility, whereas both the highly magnetic fraction of predominantly well-crystallized Fe and the nonmagnetic fraction were C poor. The study points to the important role of particle size and crystallinity of Fe oxides and of microaggregation between oxides, organic matter, and other minerals in stabilizing tropical soil organic matter.