|Abstract||The ability to understand soil organic matter dynamics in forest-derived soils is essential in the search for more intensively managed and environmentally sound and sustainable practices. Evaluation of sustainability of newly developed systems requires sensitive methods that allow an early detection of changes in. Soil fertility before degradation becomes apparent.
Soil samples from under different humid tropical land use systems were obtained from intact as well as recently converted forests, under i) rubberlike systems and lmperata grassland in Sitiung, Indonesia, and under ii) Peltophorum dasyrachis and G/iricidia sepium woodlots, lmperata grassland and sugarcane plantations at 5.1 Lampung,-Indonesia. Soil was sieved and the 0.15 - 2 mm size fraction was then separated by density (<1.13, 1.13-1.3, > 1.3 g cm'3). The fraction dry weights suggested that land use systems without burning, soil cultivation and intensive weeding can maintain soil organic matter close to forest levels, but in frequently burnt lmperata grasslands stocks are depleted.
The origin of soil organic matter from rainforest and crops was determined by δ13C methodology for a forest - sugarcane conversion series in Lampung, as well. As a pure Brachiaria humidicola pasture and a B. humidicola-Desmodium ovalifolium mixture after rainforest clearing in Bahia, Brazil. Results showed that, rainforest plant species at both sites were mainly C3 plants differing in “C content with the C4 sugarcane and grass pasture, and thus allowed identification of the soil-C origin from the two groups. Loss of rainforest-C after clearing amounted to, around 50% under both sugarcane and pastures nine and eight years after rainforest clearing respectively. The contribution of crops to the buildup of new soil organic matter was higher in the pasture systems than under sugarcane presumably due to burning activities and associated loss of organic matter inputs in the sugarcane system.