Rapeseed increases cadmium concentration of wheat in a rotation system through suppressing root mycorrhizal activity of wheat

Aims Rapeseed (Brassica napus L.) is an extensively used rotation crop that provides various benefits to the subsequent crops. One of these benefits is the suppression of soilborne pathogens through its release of biocidal metabolites, which, however, can also have detrimental effects on arbuscular mycorrhizal fungi (AMF). Reduction in AMF activity is associated with increased cadmium (Cd) accumulation in aboveground plant parts. We investigated the effect of rapeseed rotation on Cd concentration of the following wheat crop. Methods Wheat plants were grown in pots treated with low, medium and high rates of phosphorus (P) as i) rapeseed-wheat and ii) wheat-wheat rotations and analyzed for Cd concentrations. An additional experiment was conducted to study the effects of the rapeseed and wheat rotations on mycorrhizal root colonization and shoot Cd of subsequent crops. Results There was a substantial increase in shoot, spike and grain Cd concentration and content of wheat plants grown after rapeseed, compared with those grown after wheat. The differences in shoot Cd concentration and accumulation between the rapeseed-wheat and wheat-wheat rotations were minimized at the highest P-application rate, likely because the wheat plants were poorly colonized by mycorrhizal fungi. Increased shoot Cd concentrations and reduced AMF colonization in wheat plants grown after rapeseed were confirmed in an additional experiment. Conclusion The results clearly demonstrate elevated Cd concentrations in wheat plants when cultivated in rotation with non-mycorrhizal rapeseed plants. These results also point to a potential Cd concern in crop rotation systems including non-mycorrhizal crops such as rapeseed.