Soil carbon dynamics in regrowing forest of eastern Amazonia

The future flora of Amazonia will include significant areas of secondary fo rest as degraded pastures are abandoned and secondary succession proceeds. The rate at which secondary forests regain carbon (C) stocks and re-establi sh biogeochemical cycles that resemble those of primary forests will influe nce the biogeochemistry of the region. Most studies have focused on the eff ects of deforestation on biogeochemical cycles. In this study, we present d ata on the recuperation of carbon stocks and carbon fluxes within a seconda ry forest of the eastern Amazon, and we compare these measurements to those for primary forest, degraded pasture, and productive pasture. Along a tran sect 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 tha t the isotopic signature of SOM from C3 forest plants was rapidly re-establ ished. 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 ha d been fixed by plants during the last 30 years. Differences in soil C inve ntory among land use types were small compared to uncertainties in their me asurement. Root inputs were nearly identical in primary and secondary fores ts, and litterfall in the secondary forest was 88% of the litterfall rate o f 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 r ecovery 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 f orest.