Reduced Root Mycorrhizal Colonization As Affected by Phosphorus Fertilization is Responsible for High Cadmium Accumulation in Wheat

Phosphorus (P) fertilizers are often considered an important source of cadmium (Cd) in crop plants. However, increased plant Cd concentrations are not strictly related to the Cd content of P fertilizers. Considering this, we hypothesized that, alternatively, reduction of arbuscular mycorrhizal colonization by P fertilization enhances Cd accumulation in plants. Wheat and canola as mycorrhizal and non-mycorrhizal species, respectively, were grown under greenhouse conditions with and without soil sterilization. Phosphorus fertilizers with 0.09, 5, and 28 mg Cd kg−1 were applied at different rates with varied zinc (Zn) fertilization. In wheat, all three P fertilizers markedly increased shoot and grain Cd concentrations as P supply was increased, irrespective of the Cd concentration in the fertilizers. These increases were pronounced with soil sterilization or at low zinc supply. Adding mycorrhizal fungi to sterilized soil substantially decreased shoot Cd concentrations. We found a strong negative relationship in wheat between mycorrhizal colonization and shoot Cd concentration, for both high- and low-Cd fertilizers. In contrast to wheat, shoot Cd concentrations in canola showed virtually no response to P supply or soil sterilization. High Zn application also reduced plant Cd concentrations, especially at high P rates. Our findings demonstrate the critical importance of mycorrhizal colonization in reducing Cd accumulation in wheat, and suggest that factors suppressing root mycorrhizal activity including P fertilization, will increase Cd uptake in mycorrhizal plants.