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Soil erosion from shifting cultivation and other smallholder land use in Sarawak, Malaysia

Department of Agricultural Sciences, Faculty of Life Sciences, University of Copenhagen, Thorvaldsensvej 40, DK-1871 Frederiksberg C, Denmark
Agriculture Ecosystems & Environment (Impact Factor: 3.2). 05/2008; 125(1-4):182-190. DOI: 10.1016/j.agee.2007.12.013

ABSTRACT The sustainability of shifting cultivation systems and their impact on soil quality continues to be debated, and although a growing body of literature shows a limited impact on, e.g. soil carbon stocks, shifting cultivation still has a reputation as detrimental to the environment. We wished to compare soil erosion from three land use types in a shifting cultivation system, namely upland rice, pepper gardens and native forest. We used two sample sites within the humid tropical lowland zone in Sarawak, Malaysia. Both areas had steep slopes between 25° and 50°, and were characterised by a mosaic land use of native forest, secondary re-growth, upland rice fields and pepper gardens. Soil samples were collected to 90 cm depth from all three land use types, and analysed for various chemical parameters, including texture, total organic matter and 137Cs content. 137Cs is a radioactive isotope derived from nuclear fallout, and was used to estimate the retention of topsoil in the profiles. Soil chemical parameters in upland rice fields, such as extractable cations, pH and conductivity, indicated limited soil transportation downslope, and depletion of cations from upslope samples are most likely caused by leaching and losses via ashes after clearing and burning. The position on slope had no significant effect on soil texture, carbon or P content, indicating very limited physical movement of soil downslope. A soil carbon inventory to 90 cm depth on the three land uses only showed a higher carbon concentration in the top 5 cm of forest and upland rice plots. When corrected for soil density, there was no effect of land use on the carbon inventory. Moreover, the carbon content in the top 30 cm contributed <50% of the total carbon inventory, hence even significant effects of land use on carbon content in the upper soil layers, are unlikely to change the carbon inventory dramatically. 137Cs content in the soil profile indicated largest retention of original topsoil in the native forest plots, and a loss of 18 and 35% of topsoil from upland rice and pepper gardens, respectively, over the past 40 years. When comparing to 30 cm depth, soil loss was 30% from both upland rice and pepper fields. Low 137Cs activity in deeper soil layers rendered a total profile inventory impossible. It is concluded that shifting cultivation of upland rice in the current system is not leading to degradation of soil chemical and physical quality. The soil carbon inventory is not affected by land use in this analysis, due to the contribution from the deeper soil layers.

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    • "The sampling site in Sarawak (Belaga) was described previously. The samples for the measured data in Sarawak (Padawan District close to Kuching; De Neergaard et al., 2008) were collected from undisturbed plots including sacred forest , protected areas, and fields that had not been cultivated for more than 60 years. The data from the Sarawak (Belaga) are based Table 4 Synthesized list of measured total inventories of 137 Cs at reference sites in Southeast Asia. "
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    ABSTRACT: Low reference inventories of the fallout radionuclide 137Cs in low latitudes may limit its present and future application for studies of soil erosion and sediment redistribution in Southeast Asia. 137Cs reference inventories and concentrations in surface materials measured in nine and five areas, respectively, across Southeast Asia are here reported and reviewed. The compiled reference inventories decrease from north to south. Three global estimates of 137Cs total fallout are also reviewed and compared to the measured data while taking into account factors that affect the fallout estimates and the reference inventory. The results are presented as a schematic regional distribution map of 137Cs reference inventories for the year 2012. A relationship between a reference inventory and topsoil concentration is also provided. The measured 137Cs concentrations suggest that a minimum detectable activity (MDA) less than 0.5 Bq/kg is required for detection of 137Cs activity in topsoils in the lowest reference inventory areas. This sensitivity should allow, at present, 137Cs to be a useful tool for analysis of soil erosion in Southeast Asia, should also be a useful chronometer, and will be a useful tracer at least where the reference inventory is more than 500-600 Bq/m2. This level of MDA has been demonstrated in previous studies to be achievable by gamma-ray spectrometry using non-destructive sample treatment. As the nuclide decays, sufficient will remain to be useful until the middle of this century in most areas in Peninsular Malaysia and southern maritime Southeast Asia, and a few decades more in the rest of the region.
    Journal of Asian Earth Sciences 11/2013; 77:108-116. DOI:10.1016/j.jseaes.2013.08.012 · 2.83 Impact Factor
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    • "Previous studies in Peninsular Malaysia (Tanaka et al., 2009; Sultan and Shazili, 2009; Neergaard et al., 2008; Wüst et al., 2003; Phillips and Bustin, 1998; Midmore et al., 1996; Malmer, 1990) have mainly emphasized the importance of nutrient contents in soils and sediments as indicators of soil erosion and deposition and eutrophication. Neergaard et al. (2008), for instance, used the 137 Cs technique to investigate soil erosion resulting from the conversion of forest land into agricultural areas, although these authors did not examine the geochronology of deposition of the sediments in sink areas. A literature review also highlighted this absence of work in the application of radioisotopes to detect historical variations in nutrient contents and eutrophication in lake sediments. "
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    ABSTRACT: Lake Bera is the largest natural lake in Peninsular Malaysia and was designated as its first RAMSAR site in 1994. The lake has a total catchment area of 593.1 km2, although approximately 340 km2 of the original tropical rain forest cover has been converted to oil palm and rubber plantations since 1972. Research was conducted to determine the soil nutrient contents in the areas of developed land and to correlate historical variations in nutrient concentrations and eutrophication at the lake with anthropogenic activities. Thus, soil samples in areas of different land use in the catchment area were collected in addition to two cores in the bottom sediments of Lake Bera. In total, 132 samples were analyzed for total carbon (TC) and total nitrogen (TN) contents as well as fallout 210Pb and 137Cs radioisotope activities. Sediment profile dating was performed using the constant rate of supply (CRS) model; the resultant sediment ages were verified by 137Cs horizons. Soils in cleared forest areas exhibited the lowest average nutrient content and 137Cs inventory with an average loss of carbon, nitrogen and 137Cs, of 54.6%, 31.2%, and 74%, respectively, in comparison with soils in areas of undisturbed forest. Clear-felling and burning during forest conversion were identified as the two main mechanisms that disrupted the nutrient cycles in the lake catchment. The total concentrations of nutrients in the bottom sediment profiles in the main open water and in the north of Lake Bera decreased in the order of TOC > K > TN > S > Mg > C. The results highlight a clear correlation between variations of nutrient contents in the lake sediments with anthropogenic and natural events dates using the CRS model; the C/N ratio has remarkably increased four times since oil palm plantations were developed in 1981. This result indicates an upward increase in eutrophication during and following land-use changes. The results also suggest long-term increasing acidic conditions in Lake Bera, leading to a reduction in exchangeable cation contents (Ca, Mg, and K), organic matter preservation, and an incremental addition of SO4 (sulfate) and NO3 (nitrate) ions, particularly in the top layer of the sediment column. This situation will result in Lake Bera being on the verge of considerable ecological risk, as illustrated by very low dissolved oxygen contents, high levels of nitrate, and a reduction in the fish population.
    Agriculture Ecosystems & Environment 01/2013; 165:19-27. DOI:10.1016/j.agee.2012.12.004 · 3.20 Impact Factor
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    • "High erosion rates near Shillong are associated with cultivation on the steep slopes (up to 27 degrees), the downslope direction of tillage which favours translocation and shortening of the length of the fallow period from 5–10 years several decades ago to 1–3 years or less recently. Numerous studies in tropical and subtropical cultivated steeplands have shown an increase in soil erosion with increasing slope gradient (Kimaro, et al., 2005; Dupin et al., 2009), with a cultivation of vegetable crops especially if not planted on the contour (Zhang et al., 2004) and shortening of fallow length (Neergaard et al., 2008). Moreover, in the case of the bun cultivation system, the harvesting and sowing seasons coincide with the peak of the monsoon rainfall in June–August, when bare soil is exposed to the risks of raindrop-induced surface sealing, sheet erosion and rill formation within a dense infield furrow system. "
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    ABSTRACT: Rapid increase in population and growing demand for agricultural products has generated changes in land use and upland farming systems and resulted in erosion and land degradation over large tracts of northeast India. The spatial variation of soil erosion rates was studied, in a small catchment under a modified form of shifting cultivation on the hilly Meghalaya Plateau, using the 137Cs technique. Soil erosion and deposition patterns are much influenced by land use. The estimated annual soil loss from cultivated fields ranged from 32 to 79 Mg ha−1 y−1 and reached only 0·5 Mg ha−1 y−1 from degraded grassland. Calculated soil erosion rates from cultivated fields exceed that which can be produced by weathering of new soil from parent materials. Observed intensification of agricultural production will lead to progressive conversion of cultivated fields into degraded grasslands and other changes to land use in the upper part of the Meghalaya Plateau. Copyright © 2012 John Wiley & Sons, Ltd.
    Land Degradation and Development 07/2012; 23(4). DOI:10.1002/ldr.2147 · 2.06 Impact Factor
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