Vegetation conditions and soil fertility of fallow lands under intensified shifting cultivation systems in Sarawak, Malaysia
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概要
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This study was conducted in order to evaluate the vegetation conditions and soil fertility status of fallow lands under intensive shifting cultivation systems in the Engkari river area in Sarawak, Malaysia. The shifting cultivation by the several Iban communities selected at Engkari area for upland rice farming were currently being conducted with 2 to 7 years of continuous cropping followed by 1 to 6 years of fallow with application of chemical fertilizers and agrochemicals. Since agricultural management methods and land productivity under the current shifting cultivation systems conducted by the Iban farmer were our concerns, we carried out vegetation and soil surveys on fallow lands of which suitability for rice cropping using shifting cultivation had been classified by Iban landowners from their knowledge of vegetation conditions; 20 suitable and 20 unsuitable sites were analyzed. The results of our vegetation survey showed no substantial differences between suitable and unsuitable sites in the species composition and the sizes of tree species, as measured by the stem diameter at breast height (DBH). The plant density of grass and fern groups tended to be higher in most of the sites even after extensive fallow duration. As compared to our previous studies, the DBH of the trees in most of the Engkari area study sites was small even after extensive fallow duration. The soil properties in the study sites could be characterized by a strongly acidic nature with low levels of exchangeable bases. No substantial differences were found in the soil properties in terms of site suitability and fallow duration. Our results suggest that the rationality of farmers' traditional knowledge related to vegetation-based site selection is no longer applicable under intensive shifting cultivation practices in the Engkari area. Considering the high acidity and poor nutrient contents of the soils, as well as poor aboveground biomass added to soil as ash during burning, certain measures will be necessary for the farmers in the Engkari area to continue rice cropping through the intensive shifting cultivation system. These include effective types of fertilizer and appropriate application methods as well as countermeasures to soil erosion in order to prevent the loss of fertilizer nutrients from soil ecosystems.
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