Conversion of secondary forest into agroforestry and monoculture plantations in Amazonia: consequences for biomass, litter and soil carbon stocks after 7 years

Large areas of primary forest in Amazonia have been cleared for cropping or pasture, thereby releasing carbon into the atmosphere. Part of this carbon is re-assimilated by secondary forest after the land has been abandoned. Agroforestry and tree crop plantations are options for the economic valorization of previously cleared land in the humid tropics however, for evaluating the consequences of these land uses for regional carbon flows when established on secondary forest land, information is needed on carbon accumulation in the biomass and soil of these land use systems and of the successional vegetation that they replace. Above- and belowground biomass and litter accumulation were measured for three multistrata. agroforestry systems and five tree crop monocultures seven years after their establishment on secondary forest land on a xanthic Ferralsol in central Amazonia. The biomass of the tree crop systems was compared with that of the 14-year-old secondary forest that would have covered the area in their absense. The agroforestry systems were studied at two fertilization levels. Allometric relationships were developed for estimating the aboveground biomass of eight tree crop species, and root systems were excavated to determine root-shoot-ratios. Depending on species composition and fertilizer input, the multistrata systems had an aboveground biomass of 13.2-42.3 t per ha, a belowground biomass of 4.3-12.9 t per ha, and a litter mass of 2.3-7.2 t per ha. The monocultures had an aboveground biomass of 7.7-56.7 t per ha, a root biomass of 3.2-17.1 t per ha and a litter mass of 1.9-5.6 t per ha. The combined biomass and litter was highest in the peach palm for fruit (Bactris gasipaes) monoculture, followed by two of the multistrata systems. The 14-year-old secondary forest had a combined biomass and litter stock of 127 t per ha. The soil carbon stocks tended to be lower in the agricultural systems than under adjacent primary forest in the topsoil, but not when summed over the soil profile to 2 in depth. Multistrata agroforestry had several advantages over monocultures as it allowed to combine high and long-term biomass accumulation with early generation of income from annual and semiperennial intercrops, increased growth and earlier yields of certain tree crops, long-term accumulation of capital in larger trees, and more complete occupation of the soil than in common tree crop monocultures of the region. Trees with low litter quality seemed to have a favourable effect on soil carbon even when associated with species with high litter quality and could be used as an insurance against soil organic matter loss under tree crop agriculture. (C) 2002 Elsevier Science B.V. All rights reserved.