Characterization of organic matter in topsoils under rain forest and pasture in the eastern Brazilian Amazon basin

In topsoils under forest and 7-, 12- and 17-year-old pastures, organic matt er was characterized by analysing C and N distribution in particle-size fra ctions, the C decomposition rates of soil and particle-size fractions and b y employing density-fractionation of macro-organic matter (>150 mu m). The C and N associated with clay fractions increased with increasing age of pas ture. The weight (%) of macro-organic matter and its heavy fractions (>1.37 g cm(-3)) also increased with increasing age of pasture. However, in a lon g-term incubation (100 days), these changes seemed to involve an increase i n the C decomposition rate in the topsoil of the oldest pasture. Using the C decomposition rates of particle-size fractions, it appeared that silt and clay contributed differently to C decomposition in the whole soil. C assoc iated with silt contributed to the C decomposition rate during the first 40 days of incubation, while C associated with clay contributed to C decompos ition in the long-term incubation (after 40 days), especially when the clay fraction appeared to reach saturation point with respect to its ability to bind organic compounds and thus protect the soil from C loss.