Climate impact on phytoremediation efficacy by the Cd/Zn hyperaccumulator Arabidopsis halleri in heavy metal contaminated agricultural soils

Soil and plant health are major drivers of food production. Heavy metals are present in agricultural soils and their concentrations have been increasing due to anthropogenic activities. When toxic elements reach harmful concentrations in soils and become available, they adversely affect the environment, plant performance and consequently, crop production. Our group has shown that solubility of the toxic and non-biodegradable element Cd is expected to increase in agricultural soils under IPCC-projected climatic conditions likely for the year 2100. This increased mobility of the toxic Cd potentially enhances its transfer into crops, causing a threat for agricultural production. By contrast to agricultural crops, the heavy metal hyperaccumulating plants possess physiological traits that allow them to tolerate and accumulate high concentrations of heavy metals without exhibiting toxicity symptoms. Due to these traits the hyperaccumulators show great potential for phytoremediation, but the impact of climate change on their efficacy to remove heavy metals from the soil and translocate them to aboveground tissues is still unknown. We conducted greenhouse experiments growing A. halleri on five agricultural soils that vary in Cd contents ranging from 0.07 to 14 mg kg^-1 dry soil. Temperature and atmospheric CO₂ concentration were controlled during the experiments, so that today’s ambient climate was compared to future climate with +4°C and doubled atmospheric CO₂ concentration. Cd accumulation in aerial parts was higher under future climatic conditions compared to plants grown under today’s climate. However, the metal transfer from soil to roots and from roots to shoots depended on the combination of soil biogeochemical processes as well as plant growth and physiology. If the soil had a Cd content between 0.1 and 0.5 mg kg^-1 dry soil, hyperaccumulation might not be triggered, even if the mobility of Cd was still higher under future climatic conditions. On the other hand, despite that high concentrations of soil Cd stimulated metal accumulation, simultaneous presence of high content of other elements, such as Pb that is ultimately toxic to this species, negatively impacted the efficacy of Cd phytoextraction due to the increased plant stress. For soils with Pb contents that were not toxic and Cd concentrations above 0.5 mg kg^-1 dry soil, in combination with future climatic conditions, Cd availability as well as Cd translocation and transfer from soil to root were affected, increasing phytoextraction. Our results indicated that translocation towards the shoots and a potential high efficacy of phytoextraction by A. halleri may be triggered by climate change mainly in soils with moderate Cd content. Under such growth conditions, phytoremediation could be considered as a feasible option to reduce mobile Cd fractions by using metal hyperaccumulators as cover crops in agricultural ecosystems.