Carbon dynamics in slash and burn systems and land use alternatives: findings of the alternative to slash and burn programme

Abstract

Conversion of natural ecosystems to agriculture, particularly through tropical deforestation, is a major source of greenhouse gas emission into the atmosphere. Too often, fallow intervals become increasingly shortened or eliminated as land use intensifies in recently cleared forests resulting in a large net loss of total system carbon. Total system carbon (TSC) was calculated for 116 sites in the humid forest zones of S.E. Asia (Indonesia), Africa (Cameroon) and the Amazon (Brazil and Peru) based on estimates of tree, understorey and root biomass, surface necromass and soil organic carbon. The land use categories examined were original forests (10), managed and logged-over forests (9), recently burned croplands (IS), bush and young improved fallows (17), tree fallows (8), secondary' forests (8), pastures (9), /mpercra spp. grasslands (8). immature experimental agroforests (10) and mature agroforests and tree plantations (19). The land uses were arranged into chronosequences based upon land use transition and duration. TSC in a 20 year ’’traditional” slash-and-bum sequence were (t ha l): original forest (305) to burned cropland (52) to bush fallow (85) to tree fallow (136) to secondary forest (219). Logging reduced forest system carbon by 124 t ha '. Ten year-old pastures and 13 year-old Imperóla spp. grasslands contained less TSC than croplands (-4 and -5 t C ha , resDectively). Recently established agmforesrry systems contain more TSC than did croplands (+11 t C ha1). Mature agroforests (130 t C ha1) contained significantly greater TSC than croplands, pastures and grasslands (p<0.00l) but significantly less than secondary forests of similar age (p < 0.001). Soil organic carbon (SOC) represented 25% of the TSC stocks in original forests and 84%, 82% and 91% in croplands, pastures and Imperóla spp. grasslands, respectively. Aboveground C accounted for a majority of the loss from forests converted to croplands (82%) but the SOC content of croplands was also reduced by 17% to 35 t C ha1 (approximately 0-20 cm). Carbon sequestration rates were calculated for three land use patterns derivéd from croplands. Natural fellows re-accumulated 7.9 t C ha * yr‘ following land abandonment (r = 0.92). Agroforestry systems, established at the time of initial land clearing, sequestered 3.3 t C ha1 yr1 (r = 0.70) and pastures/grasslands tended to lose C at a slow rate (600 kg ha1 yr'1, n.s.). Land use systems where trees are planted and managed have greater potential to sequester C than field crops or pastures, but at sequestration rates less than those of natural succession.