DETERIORATION OF SOIL FERTILITY BY LAND-USE CHANGES IN SOUTH SUMATRA,INDONESIA - FROM 1970 TO 1990

We monitored the land use changes in a hilly area of West Lampung, Sou th Sumatra, Indonesia, from 1970 to 1990. The main data sources were t he land use maps produced in 1970, 1978, 1984 and 1990 covering the ar ea of 27 km x 27 km. Transmigration and the resultant effect of increa sed population were the major driving forces in land use changes. Fift y-seven per cent of the study area was covered with primary forests in 1970, but only 13% in 1990. Areas under plantations, which were absen t in 1970, increased to 60% in 1990. In addition, the change from mono culture plantations (mostly coffee plantation) to mixed plantations wa s noticeable from 1984 to 1990. Total upland areas including upland ar eas under shifting cultivation and upland fields with crops and vegeta bles decreased from 21% in 1970 to 0.1% in 1990. Soil chemical propert ies (total organic C, total N, available P, total P, exchangeable cati ons, cation exchangeable capacity (CEC), etc.) were analysed for lands under different land use forms after deforestation in the study area. Soil samples (surface layers, 0-20 cm, and subsurface layers, 20-40 c m) were collected from three different locations, each comprised of fo ur different land use systems: i.e. primary forests, secondary forests , coffee plantations and cultivated lands. The contents of total organ ic C, total N, available P, total P, exchangeable cations and CEC decr eased significantly with land use change from primary forests to the o ther land use forms. Cultivated lands exhibited the lowest values. Alt hough less remarkable than in the surface layers, the amounts of total organic C, total N, total P, exchangeable cations and CEC were also d ecreased by forest clearing in the subsurface layers. Based on the lan d use changes from 1978 in the study area and the deterioration of soi l chemical properties by forest clearing, total decreases in the amoun ts of nutrients in the surface and subsurface layers were estimated. T he land use changes were estimated to have decreased the total amounts of total organic C, total N, available P, total P, exchangeable catio ns and CEC by 2-9% in 1984 and by 2-15% in 1990 in the surface layers, and by 1-6%% in 1984 and by 2-9% in 1990 in the subsurface layers fro m the levels in 1978, respectively. (C) 1998 John Wiley & Sons, Ltd.