Bromide and lithium transport in soils under long-term cultivation of alfalfa and wheat

Abstract

The combined effects of two soil textures and two types of crop management were investigated using lithium and bromide tracers transport under saturated flow conditions. Leaching tests were carried out through intact columns of two soils, a clay loam (CL) and sandy loam (SL), each cropped with either wheat (Triticum aestivam) (W) or alfalfa (Medicago sativa) (A) for 11 years. A saturated steady state flow condition was established using tap water prior to injecting a pulse of 0.005M LiBr solution onto the surface of the soil columns. Breakthrough curves (BTCs) for leached Br− and Li+ in the soil columns (especially under alfalfa cultivation) exhibited an early arrival time and greater concentrations, indicating preferential flow effects. Relative 5% arrival times were 0.07, 0.11, 0.24 and 0.31 for CL-A, SL-A, CL-W, and SL-W, respectively; its smaller values confirmed the higher possibility of preferential flow under alfalfa than under wheat. The Br− concentration was higher than the Li+ concentration. With the exception of soils under alfalfa (CL-A and SL-A), the peak of the BTCs for Br− occurred earlier than that for Li+, by about 0.4 and 1.2 pore volumes for the CL-W and SL-W cases, respectively. Clay loam soil under alfalfa showed higher Br− and Li+ concentration levels when compared to sandy loam soil under alfalfa crop production. In the soils under alfalfa cultivation, structural cracks, root channels, and earthworm burrows were the cause of higher leached concentrations for both tracers when compared to the soil under wheat. Therefore, alfalfa-induced changes in soil structure lead to continuous macropores, a result of the decomposition of penetrating roots. Our results show that agricultural-management practices (i.e. the type of cropping) can play an important role in making groundwater vulnerable to leached solutes.