Safe utilization of farmland contaminated with heavy metals in China: Progress and outlook

Farmland heavy metal pollution is now a serious problem in China. Food heavy metal contents in some agricultural regions exceed the national limits and threaten human health and the development of economy and society. Meanwhile, farmland resources are very limited in China. Therefore, farmland heavy metal pollution needs to be resolved urgently. Among available remediation tools, the physical or chemical ones are costly and are not suitable for the slightly/moderately contaminated farmland at a large scale. The phytoextraction method is cost-effective and environmentally friendly, but requires a long time. Overall, techniques aiming at heavy metal removal have limitations in solving heavy metal pollution in farmland. Safe utilization is to produce safe agricultural products, without removing the heavy metal content in soil purposefully. Safe utilization of heavy metal contaminated farmland is preferable for China at this stage. Cultivation of low-accumulation crops is an important option for the safe utilization scheme. A number of low-accumulation cultivars of grain crops and vegetables have been screened out in China. Genetic engineering has potential in breeding of low-metal-accumulation crops. Soil additives such as clay minerals, organic wastes and biochar can inhibit heavy metal uptake of crops by immobilization of heavy metals in soil, via ion-exchange, precipitation, etc. However, use of soil additives may impact soil qualities by changing soil structure, loss of nutrition and secondary pollution. Some microorganisms showing strongly resistant to heavy metals have already been used to inhibit the heavy metal uptake of crops. Genetically modified microbes possessing stronger immobilization ability should be paid more attention. Furthermore, agronomic strategies including fertilization, water management, and intercropping can transform the heavy metal forms in soil and influence the heavy metal uptake by crops. However, the crop yield and quality like nutrient content can also be influenced by the safe utilization measures. Efforts should be made to get a balance between low accumulation of heavy metals and crop yield and qualities. In the future, researches may focus on the integration of various remediation techniques for the large-scale field implementation.