Carbon and nutrient storage in primary and secondary forests in eastern Amazônia

Abstract

We compared carbon and nutrient concentrations and stocks in aboveground vegetation and soils between secondary forests (10-, 20-, and 40-year-old stands subject to repeated cycles of slash-and-burn agriculture) and a primary forest fragment in the Bragantina region of Pará, Brazil. We hypothesized that repeated agricultural use would result in lower nutrient concentrations and/or stocks in the plant tissue and soils of the secondary forest stands relative to the primary forest fragment. Yet there were no significant differences in median foliar tissue concentrations of C, N, P, K, Ca, or Mg between the secondary forests and the primary forest. In woody tissue, the primary forest had a lower median Mg concentration (205 μg g −1) than all secondary forest plots (356–620 μg g −1) and a higher median N concentration (0.3%) than the 40-year-old secondary forest (0.2%). Foliar nutrient stocks were higher in the secondary forests than in the primary forest due to higher foliar biomass estimates for those plots. Aboveground woody nutrient stocks were greatest in the primary forest with the exception of Mg. Soil concentrations of exchangeable Ca decreased with increasing stand age; soil concentrations of exchangeable Mg were higher in all secondary plot soils than in the primary plot soil. Labile P stocks were greater in the primary forest soil than in all secondary forest soils. Soil labile P stocks were larger than aboveground P stocks in the 10- and 20-year-old secondary plots and the primary plot and approximately equal to aboveground stocks in the 40-year-old plot. Relative to other tropical and temperate locations, nutrient capital at these sites is low in both the vegetation and the soil, but a century of shifting cultivation does not yet appear to have introduced soil nutrient limitations to forest regrowth.