Neutral rhizoplane pH of local rice and some predominant tree species in South and Central Kalimantans : A possible strategy of plant adaptation to acidic - soil

To investigate the acid sulfate soil-adapting strategies of South Kalimantan local rice varieties, their rhizoplane pH was preliminarily examined using a glass-made micro-electrode. Although raw acid-sulfate soils in paddocks in South Kalimantan were generally pH 3.5-4.5 and sometimes pH 2.5, pH values of the rhizoplane of living roots directly measured by the microelectrode always converged to approximately 7.0 (variance 0.02-0.22, n=10). On the other hand, the pH of dead roots, which was measured after soil was thoroughly washed from the surface, was relatively low and often close to the soil pH itself. Similarly constant neutral rhizoplane pH was also observed in the major Central Kalimantan tree species, Cratoxyllum arborescens (upland) and Combretocarpus rotundatus (swampy forests). Whilst, two other predominant grasses, Xyris complanata and Imperata cylindrica, inhabiting acidic soils in upland and lowland, had relatively low rhizoplane pH (6.05 and 5.86, respectively), suggesting that these grasses have different strategies than the paddy rice varieties for adapting to acidic conditions. Rhizoplane bacteria are likely to be one of the factors for the maintenance of neutral rhizoplane pH. In fact, some Sphingomonas sp. raised medium pH, originally set at pH 4.0, to 5.5. These findings suggested the importance of cationic charge in the rhizoplane of local rice varieties for adapting to acid-sulfate soil, which is similar to strategy of some arboreal trees inhabiting acidic peat soil.