Soil Erosion and Sustainability of Different Land Uses of Smallholder Imperata Grasslands in Sea

摘要

Soil erosion is a prominent environmental problem in cultivated upland areas and Imperata grasslands in tropical Asia. Imperata grasslands cover about 35 million ha in Asia. In Indonesia, Imperata grasslands cover about 8.5 million ha. About 17% of the surface cover of the Philippines is Imperata grasslands. Imperata cylindrica is the dominant species in these grasslands which generally represent degraded, acidic, low organic matter content and dry areas susceptible to soil erosion. Grasslands in tropical areas have expanded rapidly and encroached indiscriminately due to deforestation and consequent proliferation of shifting cultivation. In typical upland Imperata areas of Southeast Asia, shifting cultivators face falling economic returns as fallow lengths shorten. The environmental consequences of shifting cultivation in upland areas can be severe. Conversion of Imperata grasslands into upland crop farms planted to rice and maize is triggered by the interacting factors of rapidly increasing population, landholding policies and declining area of arable land per farmer in the lowlands. Attractive market-driven demand for fast-growing timber species like Gmelina arborea is also another driving force for the development of smallholder timber plantations. Long-term sustainability of these different land uses of smallholder Imperata grasslands was examined and simulated using Soil Changes Under Agroforestry (SCUAF) model. SCUAF is a simple, deterministic model that can be used to predict crop yield as a function of changes in soil carbon, nitrogen and phosphorus content. Changes in soil carbon, nitrogen and phosphorus contents are results of erosion, recycling of plant materials and mineral uptake by plants in a specified system within a given environment. Soil erosion is predicted in SCUAF using the FAO Modified Universal Soil Loss Equation (MUSLE) and was calculated based on climatic, soil erodibility, slope and crop cover factors. Simulation results showed that a change in land use from Imperata grassland or maize cropping system to Gmelina plantation system can provide significant improvements to a range of on-site and off-site biophysical quality measures. The Gmelina system appears to be superior with the other systems studied since it has the least cumulative soil loss, highest organic carbon retained in the plant-soil system, greater amounts of nitrogen and phosphorus conserved or recycled in the soil. Simulation results of maize monocropping system showed the highest reduction in total soil C, total soil organic N and total soil organic P as well as highest cumulative soil loss. Imperata grassland system had a lower cumulative soil loss compared with maize monocropping system.