Soil carbon dynamics in regrowing forest of eastern Amazonia

The future flora of Amazonia will include significant areas of secondary forest as degraded pastures are abandoned and secondary succession proceeds. The rate at which secondary forests regain carbon (C) stocks and re-establish biogeochemical cycles that resemble those of primary forests will influence the biogeochemistry of the region. Most studies have focused on the effects of deforestation on biogeochemical cycles. In this study, we present data on the recuperation of carbon stocks and carbon fluxes within a secondary forest of the eastern Amazon, and we compare these measurements to those for primary forest, degraded pasture, and productive pasture. Along a transect from a 23-y-old degraded pasture, through a 7-y-old secondary forest, through a 16-year-old secondary forest, and to a primary forest, the delta(13)C values of soil organic matter (SOM) in the top 10 cm of soil were -21.0, -26.5, -27.4, and -27.9 parts per thousand, respectively, indicating that the isotopic signature of SOM from C3 forest plants was rapidly re-established. The degraded pasture also had significant inputs of C from C3 plants. Radiocarbon data indicated that most of the C in the top 10 cm of soil had been fixed by plants during the last 30 years. Differences in soil C inventory among land use types were small compared to uncertainties in their measurement. Root inputs were nearly identical in primary and secondary forests, and litterfall in the secondary forest was 88% of the litterfall rate of the primary forest. In contrast, the secondary forest had only 17% of the above ground biomass. Because of rapid cycling rates of soil C and rapid recovery of C fluxes to and from the soil, the below ground C cycle in this secondary forest was nearly identical with those of the unaltered primary forest.