The effect of soil structure on the bioavailability of trace metals

The majority of traditional soil analyses disrupt the soil structure to diagnose soil quality, toxicity or nutrient requirements. They are based on batch extractions on dried, sieved and homogenized samples. However, opening up the soil structure overestimates the reactive surface that controls nutrient and contaminant availability. It is unclear if soil structure needs to be accounted for in bioavailability assessment. This study was set up to quantify the effects of soil structure on the mobility and the plant availability of five different trace metals, thereby using soils that are either or not disturbed by sieving and using both added metals and metals naturally present in soil. Maize was grown in 7.4 L soil columns spiked with ⁶²Ni, ⁶⁵Cu, ⁷⁰Zn, ¹⁰⁸Cd and ²⁰⁴Pb isotopes added to the soils as spike solutions with 2.2 mm initial irrigation. Five different types of soil and three degrees of disturbance were used: intact soil columns, soil sieved at 8 mm, and soil sieved at 2 mm. Soil analyses showed deeper and more variable penetration of metal isotopes in undisturbed than in sieved soils, logically related to macropores in the former. Maize plants grown on intact soils contained higher concentrations of spiked metal than those grown on sieved soil, with mean differences (among soils) ranging between 1.5 (Ni) and 3.8 (Cu). This indicates an increased availability of freshly added metal in the intact soils compared to sieved soils due to the higher reactive surfaces in the latter and due to the colocalization of the spike with the roots in macropores. Conversely, for the native metal, the trend was reversed; more native metal was taken up in the sieved treatments, and differences were highest (almost factor 2) for Cd. The second effect can be explained by an increased contact area between roots and soil due to sieving, thereby reducing the diffusion limitation of the native metals from soil to roots. This study shows that soil structure affects trace metal bioavailability and suggests that soil testing for bioavailability may need to be revisited to account for this effect.