Mechanisms of soil acidification under leguminous fast growing tree plantations in the humid tropics and its prediction

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This study was conducted in the Acacia mangium plantation in the Province of South Sumatra, Indonesia for the purpose of elucidating the possibility of soil acidification caused by establishment of leguminous fast growing tree plantation in the humid tropics and its mechanism, and to develop the methodology to predict the acidification. The progresses obtained are the followings.1)Based on the multipoint soil survey covering relatively large area, we illustrated in an empirical way that conversion of degraded Imperata grassland into Acacia tree plantation causes obvious soil acidification, and the decline in exchangeable base cations in soils resulted from nutrients trans location from soil to growing tree biomass is the major cause of the acidification. Furthermore, it was indicated that effective cation exchange capacity of the humid tropical soils containing minerals of variable charge characteristics might decline with soils acidification, and it may possibly accelerate further soi … More l acidification.2)For the two 7-year-old Acacia stands respectively on a heavy clay soil and a light clay soil, allometry formulas were developed by destructive measurements, and nutrients concentrations were determined for every organ of trees. Based on these information, nutrients export through tree harvest was estimated, and possible number of replanting-harvest rotation was simulated on a supposition that nutrients is input only through precipitation. The results of simulation suggested that Ca and K will become deficient in near future and its magnitude is different by the soil characteristics especially clay content.3)Nutrients transported by litterfall, throughfall, stemflow and water percolation through litter layers (Ao horizon, 0 horizon), and nutrients and dissolved organic matter released from litter decomposition were monitored for 3 years at the permanent plot, and the results showed every related parameters display obvious seasonality between drier and wetter seasons mostly in conjunction with each other.4)A model to predict the soil acidification was formulated based on ion balance and mass balance, and soil change during a period of three rotations was predicted by using data collected by field observation. The simulation output by the model indicated possible rapid and significant soil acidification, however also it became clear that the model still have weakness to be improved. Less