Salinity decreases dissolved organic carbon in the rhizosphere and increases trace element phyto-accumulation

Soil degradation processes, such as organic matter (OM) depletion, accompanied by metal contamination and salinization are becoming a serious threat to crop production and human food security. A glasshouse study was conducted to assess a factorial combination of salinity (0, 20, 40 and 60 mm NaCl) and cadmium (Cd) (0.3, 2.5 and 5.4 mg Cd kg-1) in organic soil (>90% OM) and their influence on dissolved organic carbon (DOC) in the rhizosphere and phyto-accumulation in radish (Raphanus sativus L. var. sativus). A 34-day exposure to increasing NaCl salinity significantly decreased DOC concentration in the radish rhizosphere solution and increased trace element (copper, Cu; zinc, Zn; and Cd) concentrations in the rhizosphere as well as in leaf/fruit tissues of radish. Soil contamination by Cd progressively raised concentrations in soil solution, but markedly reduced total concentration of Cu and Zn in the rhizosphere and leaves of radish. The NICA-Donnan chemical speciation/distribution modelling confirmed the predominance of dissolved organic reactive surfaces (from fulvic acid for example) in Cu and Cd chemisorption/complexation processes over the whole range of applied NaCl and Cd treatments. In contrast, Zn speciation was dominated by an organically-complexed pool at low salinity (020 mm NaCl), and free Zn2+ was the most important species at increased salinity (=40 mm NaCl). In conclusion, because of the diminished pool of DOC under excessive salinity, the biogeochemistry of Cu, Zn and Cd in the rhizosphere can be affected in a way that would enhance solubility and phyto-accumulation of these trace metals in food crops.