Dynamics of Soil Carbon in Primary and Secondary Tropical Forests in Colombia

Atmospheric CO 2 concentration has been increasing since the beginning of industrial revolution (Keeling, 1997; Wuebbles et al., 1999) and is expected to continue growing at progressively higher rates in coming years, with significant consequences on global climate and ecosystem dynamics (IPCC, 1996; Schimel et al., 1994). The potential sinks of this atmospheric CO 2 are terrestrial ecosystems and oceans (Fan et al., 1998). Tropical forests could play an important role in the absorption of excess CO 2 because they are the largest pool of organic C among terrestrial ecosystems. Tropical forests cover only 17% of terrestrial surface of the planet and store about 42% of C contained in biomass, 59% of C in forests (Dixon et al., 1994), and 27% of C in soils; they also account for 36% of all terrestrial net primary productivity NPP (Melillo et al., 1993; Phillips et al., 1998). Tropical deforestation is a critical global environmental problem because the 15.4 million hectares cleared each year mainly for agriculture and grazing (FAO, 1993; Nepstad et al., 1991) are a disproportionately large source of atmospheric carbon (1.1–1.6 Pg C; 1 Pg = 10 g) (Batjes & Sombroek, 1997). Simultaneously, secondary forests are extensive in the tropics because of the increasing abandonment of many lands in this region. In 1990 they accounted for about 40% of the tropical forest area (Brown, 1990), with the potential of being significant carbon sinks (Silver et al., 2000) Forest carbon cycles involve inputs, storage, transfers and outputs of carbon between two large pools: vegetation and soils. Main processes in vegetation are photosynthesis, growth of above and belowground plant organs, and production of dead material. Carbon in plant biomass goes back to the atmosphere through