Rare Earth Element Fluxes in Diverse Soils and Their Absorption by Coconut Palm

Abstract

The pattern of uptake of six rare earth elements (REEs) [lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm), and gadolinium (Gd)] by coconut palm (Cocos nucifera L.) from three types of soils varying widely in their chemical characteristics and monazite content were studied through soil and plant tissue analysis employing ICP‐MS method. Absorption of the REEs by the palm varied with soil type with highest from laterite soil (Ultisol) followed by alluvial soil (Entisol), and least from sandy soil (Entisol). The predominant source of REEs in these soils was monazite as it was possible to explain the differential absorption of the REEs by the relative abundance of the mineral in the three soils. Concentrations of the six REEs in a given plant tissue (leaf or root) or in soil were highly interrelated. Roots accumulated more REEs than did the leaves and consequently REE concentration ratios (CRs) were higher for roots than for leaves. Assessment of bioavailability of REEs as a function of their total concentrations in the substrate does not seem applicable to all types of soils. Although native soil REE content is a fairly good indicator of plant availability in sandy soil, it is not so for alluvial and laterite soils. The results point to the need for fractional approach for evaluating the plant‐available REE pool in relatively more reactive and finer textured soils. The CR approach in assessing the plant uptake of REEs also failed to validate its assumptions and theoretical expectations. Based on the significant relationships between concentrations of REEs in sandy soil and plant tissue, a concept of “relative absorption coefficient” was proposed. Based on the coefficients obtained for the REEs studied, it was shown that although Sm was the least accumulated lanthanide in coconut palm tissues, it was the most favored element by the palm on a comparable scale of substrate concentration.

Keywords:

• Rare earth elements
• Coconut palm
• Concentration ratio
• Relative absorption coefficient
• Monazite