Sub-millimeter distribution of labile trace element fluxes in the rhizosphere explains differential effects of soil liming on cadmium and zinc uptake in maize

• Published in: The Science of the total environment Vol. 738; p. 140311
• Main Authors: Smolders, Erik, Wagner, Stefan, Prohaska, Thomas, Irrgeher, Johanna, Santner, Jakob
• Format: Journal Article
• Language: English
• Published: Netherlands Elsevier B.V 10.10.2020
• Subjects: Cadmium; Cadmium - analysis; LA-ICP-MS; Liming; Rhizosphere; Soil; Soil Pollutants - analysis; Trace Elements; Zea mays; Zinc - analysis; Cadmium; Rhizosphere; LA-ICP-MS; Liming
• ISSN: 0048-9697; 1879-1026
• DOI: 10.1016/j.scitotenv.2020.140311

Trace element concentrations in the rhizosphere were quantified to better understand why soil liming often fails to reduce cadmium (Cd) uptake by plants. Maize seedlings were grown on a soil with natural background levels of Cd and zinc (Zn). Soil liming increased soil pH from 4.9 to 6.5 and lowered the soil solution free ion activities by factor 7 (Cd) and 9 (Zn). In contrast, shoot Cd concentrations were unaffected by liming while shoot Zn concentrations were lowered by factor 1.9. Mapping of labile soil trace elements using diffusive gradients in thin films (DGT) in combination with laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) revealed an almost complete depletion of Cd in the rhizosphere in all soil treatments, showing that Cd uptake is controlled by diffusion. The flux of Cd from soil to the DGT, with direct contact between the soil and the binding gel, was unaffected by liming whereas it decreased by factor 3 for Zn, closely mimicking the contrasting effects of liming on Cd and Zn bioavailability. This evidence, combined with additional flux data of freshly spiked Cd and Zn isotopes in soil and with modelling, suggests that the diffusive transport of Cd in unsaturated soil is more strongly controlled by the labile adsorbed metal concentration than by its concentration in solution. This is less the case for Zn because of its inherently slower desorption compared to Cd. [Display omitted] •First experimental evidence that cadmium is depleted in the rhizosphere, more so than other metals•The combination of DGT and Sector Field Laser Ablation ICP-MS allowed to trace that depletion.•Liming soil does not reduce cadmium uptake in contrast to uptake of almost all other trace metals.•Higher lability of soil cadmium than that of zinc explains the contrasting responses to liming.