Field-scale remediation of cadmium-contaminated farmland soil by Cichorium intybus L.: Planting density, repeated harvests, and safe use of its Cd-enriched biomass for protein feed

Chicory (Cichorium intybus L.) is a perennial herb cultivated worldwide that has a wide range of industrial uses, produces large yields, and also has strong cadmium (Cd) accumulation ability. In field experiments in two farmlands with acidic soils, chicory potential to remove Cd from contaminated soils was evaluated at different planting densities and with repeated harvesting. To determine whether Cd-contaminated biomass could be used as a feed ingredient, efficacy of detoxification was measured with successive extractions using different concentrations of HCl. Biomass per plant decreased with an increase in planting density, and maximum biomass per plant in the two experimental fields was 17.11 and 13.89 g·plant−1. However, the increase in plant numbers at high planting density increased total biomass per hectare, and maximum accumulated biomass at the highest planting density in the two experimental fields was 14,226 and 22,020 kg·ha−1. The highest Cd concentrations were at the first harvest and were 27.68 and 19.27 mg·kg−1 Cd in the two fields, resulting in bioconcentration factors (BCFs) of 24.94 and 24.39, respectively. With the increase in number of harvests, aboveground Cd concentrations and BCFs decreased. Planting density had no consistent effect on aboveground Cd concentrations at the first two harvests, whereas Cd concentration decreased gradually with the increase in density at the last three harvests. Collectively, after five repeated harvests, total Cd uptake was highest at the highest planting density, reaching 237.55 g·ha−1 (field 2), and the corresponding phytoextraction efficiency was 11.57%. Two consecutive extractions with 0.5% HCl removed 96.30% of Cd in leaf meal, which reduced Cd concentration below the threshold for feed safety while retaining most of the crude protein, there remained a substantial crude protein content with 173.1 g·kg−1. Thus, biomass of forage crops used for phytoremediation may also provide economic gain.