Geochemical fingerprinting of volcanic soils used for wetland rice in West Sumatra, Indonesia

Abstract

Cultivation of paddy (rice) in volcanic soils is commonly practiced in West Sumatra, Indonesia. This study aims to provide a detailed geochemical fingerprinting of topsoils of paddy fields derived from volcanic materials in the vicinity of mountain (Mt) Marapi, Mt. Sago, Mt. Singgalang, Mt. Tandikek and Caldera Maninjau in West Sumatra, Indonesia as a function of different geochronology of volcanic parent materials. Seventy-nine topsoil samples were collected along an altitudinal gradient ranging from 44m in the Maninjau area to 1220m above sea level (a.s.l.) at Mt. Singgalang. In addition to conventional physical and chemical analysis, geochemical analysis was carried out using a portable X-ray fluorescent spectrometer (XRF) and organic matter composition was analyzed using mid-infrared Fourier transform infrared spectroscopy (FTIR). The chemical composition of the volcanic paddy soils in this area is controlled by the origin of parent materials and weathering processes. Soils of Mt. Sago have lower soil pH (5.46) and smaller cation exchange capacity (CEC=16.5cmolckg−1) compared to soils from the other three mountains. On the other hand, soils of Mt. Marapi have higher pH (6.05) and larger CEC (19.8cmolckg−1). Linear discriminant analysis revealed that the major geochemical elements in volcanic paddy soils can be ascribed to the different volcanic origin. The results of Mahalanobis distance statistics clearly separated soils of Mt. Sago with the other four soils. Soils from Mt. Marapi were also dissimilar with the other three soils, while soils from Mt. Singgalang-Tandikek and Maninjau were more related. Clear differentiation among weathering indices was also observed. Soils of Mt. Sago again showed higher weathering stages when evaluated using indices with immobile elements (Al2O3, Fe2O3, TiO2 and Zr). The following sequence of the degree of weathering can be concluded: Sago>Maninjau>Marapi>Singgalang-Tandikek. Soil analysis using FTIR revealed that labile aliphatic (CH) compounds were the dominant organic matter fractions in these soils with abundances between 64 and 77%. Soils with total C less than 2% tend to be dominated by aromatic fractions, while soils greater than 2% C are dominated by the more labile aliphatic fractions. In conclusion, although the soils have been cultivated with paddy for hundreds of years, they still retain distinct geochemical signatures that can be revealed using a portable XRF.