Factors affecting the geochemistry of rare earth elements in soils in tropical rain and montane forests in Sri Lanka across an elevation gradient

Abstract

Studying the distribution of rare earth elements (REE) in the soil is crucial for understanding how natural factors influence the geochemical behaviour of such components in tropical lowland rainforests (TLRF) and tropical montane forests (TMF) that are differentiated based on their elevational range and floristics. Since little is known about REE in forest soils in Sri Lanka, eight (8) forest areas along an elevation gradient were investigated to determine the abundance of REE and its relationship to soil physicochemical properties, climatic factors, and vegetation parameters. Sampling was carried out over an area of a one-hectare plot; 17 representative soil samples were taken to a depth of 25 cm, and the REE content was quantified using ICP-MS. The mean REE content varied in all forest plots in the order Ce > La > Nd > Pr > Gd > Sm > Dy > Er > Tb > Yb > Eu > Ho > Lu > Tm. Higher light REE content (La-Nb) with depleted Eu content was a key feature in the forest soils. Altitude showed significant (p < 0.05) relationships with all studied parameters except clay content and heavy rare earth metal contents. The REE contents showed significant positive correlations (p < 0.05) with climatic factors such as precipitation and average temperature, as well as the vegetation parameters such as the Shannon-Wiener Vegetation Diversity Index and above-ground biomass. The results of this study highlight the significant influence of climate and vegetation on REE geochemistry. Further studies are required to elucidate the clay mineral adsorption of REE in forest soils of Sri Lanka.