Effects of forest conversion to pasture on soil carbon content and dynamics in Brazilian Amazonia

Soils play an important role in the carbon cycle, and deforestation in the tropics affects both soil carbon storage and CO2 release into the atmosphere. The consequences of deforestation and conversion to pasture for soil carbon content and dynamics were examined in two soil types differing mainly by their texture. Two chronosequences were selected, each consisting of an intact forest and three pastures of different ages (4, 8, 15 years and 3, 9, 15 years, respectively). One chronosequence is located in the central part of the Brazilian Amazon basin, where the soils are clayey ferralsols, and the second in the Eastern Brazilian Amazon Basin, where the soils are sandy clayey acrisols. In the upper layer the C content of clayey soils was three times higher than in the sandy soils, but despite the differences in soil texture, the C distribution in the particle-size fractions was quite similar. In the two chronosequences, the conversion to pasture induced a slight increase in C content. Bulk density increases were greater on soils with lower clay contents. The 13C measurements, which allowed to calculate the distribution of C derived from forest and from pasture, showed that all the particle-size fractions incorporated C derived from pasture and that a significant proportion of the young organic matter is rapidly trapped in the finest fractions. Although the proportions of pasture-derived C were higher in the sandy soils than in the clayey soils, the amounts of pasture-derived C in the particle-size fractions were 2–3 times larger in the clayey soils than in the sandy soils.