Organic matter lability in a tropical oxisol: Evidence from shifting cultivation, chemical oxidation, particle size, density, and magnetic fractionations

When temperate soils are cultivated, rates of organic matter mineralization rarely exceed 50% of total carbon (C) for about 50 to 100 years of cultivation, and the remaining soil organic matter contributes to soil quality by storing nutrients and providing aggregation, In weathered tropical soils, rates of C loss caused by cultivation are many times faster than those for temperate soils, with a substantial deterioration in soil quality often in less than 10 years. Here we examine the role of minerals and microaggregation in organic matter stability in the search for a stable organic C pool in a semiarid tropical soil (Oxisol) from the Chapada de Araripe in Pernambuco, NE Brazil. In this soil, 14% of the C and N was lost in 6 years of manual shifting cultivation, after which it was abandoned. Carbon in the sand fraction accounted for 22% of total soil C and declined by 40% upon cultivation, Although silt and silt-sized microaggregates accounted for only 10% of total soil mass, they contained nearly half the soil C, which declined by 13% during cultivation. Clay-associated C (27% of soil C) showed no decline because of a mass transfer from sand and silt-sized microaggregates to clay, probably as a result of their destabilization under cultivation. Chemical oxidation with 0.03 M KMnO4 was able to predict the proportion of labile C and indicated a decreased organic matter stability after cultivation. Density and magnetic fractionation indicated that organo-mineral complexes were broken down during cultivation, with a subsequent mineralization of C and redistribution to finer, lighter fractions. The most stable fraction appeared to be in silt-sized microaggregates and in clays of intermediate magnetic susceptibility, indicating its association with Fe of low crystallinity. Even this most resistant fraction showed a C-14 age of only about 100 years, approximately double that of the whole soil but only one-tenth of values typical of resistant fractions in temperate soils, This result indicates that organic matter of weathered tropical soils may be less stable than assumed, that rapid degradation of soil quality is possible, and that organic matter management should be a priority for sustainable agriculture.