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Traditional slash-and-burn agriculture has been practiced for centuries in Southeast Asia. Our past study has identified distinctly different soil profile characteristics over a very short distance within a sloping field. In order to accurately estimate the potential of the traditional slash and burn cropping system for soil C sequestration as well as the maintenance of soil fertility, the spatial variation of soil C stock, soil nutrient status, and soil reactions were studied in Sabah, Malaysia. A triangular grid was used to conduct intensive soil sampling with an interval of about 5.8 m and a total of 71 locations. Total C (TC) stock was high with a mean of 34.9 Mg ha−1 and the spatial variation was extreme with a CV of 57.3% within a field. The spatial range was also short at 14.3 m. All soil nutrients had high CVs especially for available phosphate at 90.2% and the lowest for exchangeable potassium at 44.0%. Available phosphate was the most limited major nutrient when referencing the Japanese fertilizer guidelines, and its spatial distribution was affected by soil acidity status, especially exchangeable Al. Potential improvements of the production systems can be made by the application of lime materials in locations where TC content was low. However, the introduction of any external inputs requires careful assessment of not just crop productivity but also its consequences for long-term sustainability of the system. Future studies are anticipated in order to understand the response of crop productivity by pH improvement as well as the use of fertilizers. [ABSTRACT FROM AUTHOR]

In Southeast Asia, peatland is widely distributed and has accumulated a massive amount of soil carbon, coexisting with peat swamp forest ( PSF). The peatland, however, has been rapidly degraded by deforestation, fires, and drainage for the last two decades. Such disturbances change hydrological conditions, typically groundwater level ( GWL), and accelerate oxidative peat decomposition. Evapotranspiration ( ET) is a major determinant of GWL, whereas information on the ET of PSF is limited. Therefore, we measured ET using the eddy covariance technique for 4-6 years between 2002 and 2009, including El Niño and La Niña events, at three sites in Central Kalimantan, Indonesia. The sites were different in disturbance degree: a PSF with little drainage ( UF), a heavily drained PSF ( DF), and a drained burnt ex- PSF ( DB); GWL was significantly lowered at DF, especially in the dry season. The ET showed a clear seasonal variation with a peak in the mid-dry season and a large decrease in the late dry season, mainly following seasonal variation in net radiation ( Rn). The Rn drastically decreased with dense smoke from peat fires in the late dry season. Annual ET forced to close energy balance for 4 years was 1636 ± 53, 1553 ± 117, and 1374 ± 75 mm yr−1 (mean ± 1 standard deviation), respectively, at UF, DF, and DB. The undrained PSF ( UF) had high and rather stable annual ET, independently of El Niño and La Niña events, in comparison with other tropical rainforests. The minimum monthly-mean GWL explained 80% of interannual variation in ET for the forest sites ( UF and DF); the positive relationship between ET and GWL indicates that drainage by a canal decreased ET at DF through lowering GWL. In addition, ET was decreased by 16% at DB in comparison with UF chiefly because of vegetation loss through fires. [ABSTRACT FROM AUTHOR]

Melastoma malabathricum L. (melastoma) is an Al-accumulating woody plant that grows in tropical Southeast Asia in acid soils with high aluminum (Al) concentrations and low nutrient concentrations. Because oxalate serves as a ligand for Al accumulation in melastoma leaves and citrate is the ligand associated with Al translocation from roots to shoots, we investigated the role of organic acids in the adaptation of melastoma to growth on these soils. Phosphorus starvation increased oxalate concentration in the rhizosphere, enabling melastoma to solubilize insoluble aluminum phosphate in the rhizosphere. Increased availability of P and Al in the rhizosphere enhanced growth. In the xylem sap, the concentration of citrate increased with increasing Al concentration. In contrast, the concentrations of malate, succinate and α-ketoglutarate in the xylem sap decreased with increasing Al concentration, suggesting that tricarboxylic acid cycle enzymes were affected by Al treatment. [ABSTRACT FROM PUBLISHER]