Interactions of humates and chlorides with cadmium drive soil cadmium chemistry and uptake by radish cultivars

In contrast to some salts such as chlorides (Cl) that enhance cadmium (Cd) phyto-uptake, complex soil organics like humates (HA) potentially minimise Cd uptake, but are depleted in soils low in organic matter. Organically-depleted and salt-affected areas frequently coincide in (semi)arid agroecosystems where inappropriate management practices may load topsoils with Cd. We evaluated the impact of HA (0–100 mg/kg) and NaCl (0–60 mM) in Cd-contaminated (0–5 mg/kg) soil on the chemical changes in the rhizosphere and Cd uptake by two radish (Raphanus sativus L.) cultivars. In the rhizosphere solution the significant HAxCd interaction resulted in a decrease in Cd concentration with increasing HA rates, whereas the NaClxCd interaction was brought about by an increase in Cd concentration with NaCl rising. Also, the NaClxCd interaction increased Cd concentration in radish hypocotyl with increasing NaCl addition; in contrast, the HAxCd interaction reduced Cd concentration in hypocotyl, notably at the highest Cd rate, with increasing soil humification. The addition of HA acted as a biostimulant in both radish cultivars and decreased Cd accumulation (up to 44%), whereas NaCl stress reduced the root growth and enhanced total Cd accumulation (by almost 50%). Dose-dependent severity of Cd toxicity was confirmed in both cultivars by reduced growth and progressive (up to 2 orders of magnitude) Cd accumulation (vs. uncontaminated soil). Ion speciation modelling suggested that chemistry of deprotonated humates and chlorides is crucial for complexation of the most bioavailable Cd²⁺ species, thus driving Cd mobility within the soil matrix, including uptake by plants. Detected differences between the tested cultivars (e.g. lower Cd concentration in Sparkler vs. Cherry Belle) and their impacts on rhizosphere chemistry and Cd soil-plant acquisition/root-hypocotyl-shoot (re)distribution, suggest that genetic improvements (by developing and introducing salt- and/or metal-resistant varieties) should be exploited in phytoremediation of contaminated soils or for minimising metal accumulation in sustainable food production.