Intercropping of citrus with Hylotelephium spectabile reduces Cd uptake by removing Cd in rhizosphere soil of citrus

Objectives Intercropping of hyper accumulator with cash crops is an effective measure to preventing cash crop from injury of heavy metals in contaminated soil. So, we studied the effect of H. spectabile and citrus intercropping on the absorption and accumulation of Cd by citrus in order to confirm a cost-effective measurement for safe citrus production.

Methods Both early and late maturing citrus and HHylotelephium spectabile were used as test materials in a pot experiment, in which the total Cd content in contaminated and uncontaminated soil were 0.92 mg/kg and 0.06 mg/kg, respectively. The seven treatments included early and later maturing citrus monoculture, intercropped with H. spectabile with and without limitation of root growth space, and H. spectabile monoculture. When plants were grown for 74 days (July) and 218 days (December), taking plants and soil samples to investigate plant biomass and Cd content, soils were tested for total Cd and available Cd content, respectively.

Results Under intercropping, the Cd content of citrus leaves decreased by 26.7% compared with that of citrus under monoculture, and had no significantly difference with that in citrus leaves grown in uncontaminated soil (0.011 mg/kg). The total and available Cd contents in intercropped citrus rhizosphere soil was significantly lower than those of monocrop citrus (P < 0.05), with the total Cd from 0.92 mg/kg to 0.75 mg/kg, and the available Cd decreased from 0.82 mg/kg to 0.78 mg/kg, respectively. The reduction of total Cd in intercropped citrus rhizosphere soil was 2.83 times of that in citrus-H. spectabile restrictive intercropping (6.52%). Due to intercropping with citrus, the annual extraction and annual removal rate of soil Cd by H. spectabile were 1.40 mg/pot and 3.3%, respectively, compared with H. spectabile monoculture (1.50 mg/pot, 3.7%), the removal efficiency was reduced by 10.6%.

Conclusions Although the citrus-H. spectabile intercropping has certain negative effect on the Cd remove efficiency of H. spectabile, it significantly reduces total and available Cd contents in the citrus rhizosphere soil, so decreases the Cd contents in citrus leaves. The intercropping of citrus with H. spectabile is thus a cost-effective measurement for achieving the target of safe production and remediation of Cd contaminant at the same time.