How could phytoextraction reduce Cd content in soils under annual crops? Simulations in the French context.

Human populations are threatened by chronic exposure to the Cd accumulated in foods after being taken up from soils by crops. To evaluate how phytoextraction with the hyperaccumulator Noccaea caerulescens as an annual crop or as a cover crop could modify the Cd mass balance in French agricultural soils, we simulated this process according to two scenarios. If current practices are maintained (first scenario), the average soil Cd content will increase by 2.9% after a century. If Cd content in P fertilizers is limited according to the European regulation project (second scenario), the decrease will be of about 4%. A phytoextraction crop with a 10 t dry matter (DM) ha yield every 25 years would bring down the soil Cd content from 0.31 mg kg to around 0.11 mg kg. However, this scenario is relatively unrealistic, because high dry matter yield is unlikely and the cost of the process is elevated. Phytoextraction as a cover crop every four to five years would decrease the soil Cd content more quickly. This requires a 2.5 t DM ha yield, which appears realistic. This cover crop phytoextraction would be cheaper. It would need annual sowing of 4 million ha and the production of around 10 million t of dry biomass. To meet such a requirement, any breeding of the hyperaccumulator should favour traits allowing a 3-4 month cultivation period in the autumn. Processes also have to be developed to recover energy, metal or beneficial compounds from the biomass produced by phytoextraction.