ORGANIC-MATTER LABILITY IN A TROPICAL OXISOL - EVIDENCE FROM SHIFTINGCULTIVATION, CHEMICAL OXIDATION, PARTICLE-SIZE, DENSITY, AND MAGNETICFRACTIONATIONS

When temperate soils are cultivated, rates of organic matter mineraliz ation rarely exceed 50% of total carbon (C) for about 50 to 100 years of cultivation, and the remaining soil organic matter contributes to s oil quality by storing nutrients and providing aggregation, In weather ed tropical soils, rates of C loss caused by cultivation are many time s faster than those for temperate soils, with a substantial deteriorat ion in soil quality often in less than 10 years. Here we examine the r ole of minerals and microaggregation in organic matter stability in th e search for a stable organic C pool in a semiarid tropical soil (Oxis ol) 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 cultivatio n, after which it was abandoned. Carbon in the sand fraction accounted for 22% of total soil C and declined by 40% upon cultivation, Althoug h 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 decl ine because of a mass transfer from sand and silt-sized microaggregate s to clay, probably as a result of their destabilization under cultiva tion. Chemical oxidation with 0.03 M KMnO4 was able to predict the pro portion 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 s ubsequent mineralization of C and redistribution to finer, lighter fra ctions. The most stable fraction appeared to be in silt-sized microagg regates and in clays of intermediate magnetic susceptibility, indicati ng its association with Fe of low crystallinity. Even this most resist ant 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 org anic matter of weathered tropical soils may be less stable than assume d, that rapid degradation of soil quality is possible, and that organi c matter management should be a priority for sustainable agriculture.