The effect of rare earth elements on phosphorus leaching in intact soil columns

Abstract

Phosphorus (P) has been linked to eutrophication in surface waters because it is a limiting nutrient for algal growth, and recent studies have shown that P transport through subsurface flow is of growing concern. In this study we amended dairy and poultry manures with lanthanum (La) and ytterbium (Yb) chlorides to assess the efficacy of using rare earth element (REE) chloride amendments in reducing P leaching through intact soil columns and to determine the major pathways of applied P leaching. Significant reductions in dissolved P (DP; 56% to 64%), particulate P (PP; 22% to 36%), and total P (TP; 41% to 51%) in leachate were seen when dairy manure was amended with REE-chlorides, but no significant reductions in these P fractions were seen in amended poultry litter. Differences in P leaching losses between the two REE-amended manures were likely due to better mixing and dissolution of the REE-chlorides and better precipitation of an insoluble particulate REE-phosphate salt in the liquid manures prior to being applied. Very little vertical transport of REEs was observed in soil leachate over repeated events. Elevated concentrations of REEs along soil macropores at depths greater than 15 cm (5.9 in) suggest the importance of this pathway. However, due to the extremely low concentrations of REEs found at depth and the much higher values of soil P, the ability to “label” the manures with REEs to track P through macropores in the soil was only suggestive and by no means conclusive. Results point to the efficacy of REE-chlorides in lowering P solubility in liquid manures but limited potential in tracking subsurface transport pathways of applied manure P in soils.