Rare earth elements in acid sulfate soils under long-term paddy rice cultivation in Thailand

Abstract

There is little published information on amounts and associations of rare earth elements (REEs) in tropical acid sulfate soils (AS soils) with sulfuric horizons (pH < 4). This research focused on the distribution and partitioning of several REEs (Sc, Y, La, Ce, Nd, and Gd) in four tropical AS soils with sulfuric horizons under long-term paddy rice cultivation on the Lower Central Plain of Thailand. The bulk soil samples were analyzed for total concentrations of REEs using x-ray fluorescence and an aqua regia digest with inductively coupled plasma-optical emission spectrometry. The partitioning of REEs between soil constituents was assessed using a sequential extraction procedure. Some of REEs in the topsoil had been added in phosphatic fertilizers. The REEs had not been leached from the AS soils with sulfuric horizons as they progressively developed more oxic and acidic conditions under agricultural development. Rare earth elements are mostly associated with the residual fraction, followed by acid soluble and pyrite fractions. The acid soluble fraction of REEs is highest in the topsoil. Amounts of OC, poorly-crystalline Fe-(hydr)oxides and iron monosulfides affect the distribution of REEs in the topsoil of the tropical AS soils with sulfuric horizons after a long period of paddy rice cultivation. When AS soils with sulfuric horizons are managed by liming to increase soil pH for rice production, more of the REEs become adsorbed onto organic matter.