Copper availability and bioavailability are controlled by rhizosphere pH in rape grown in an acidic Cu-contaminated soil

We evaluated how root-induced changes in rhizosphere pH varied and interacted with Cu availability and bioavailability in an acidic soil. Rape was grown on a Cu-contaminated acidic soil, which had been limed at 10 rates. Soil Cu bioavailability was not influenced by liming. However, liming significantly decreased CaCl2–extracted Cu for pH between 3.7 and 5.1. Little effect was found for pH above 5.1. For soil pH < 4.4, CaCl2–Cu contents were smaller in rhizosphere than uncropped soil. Rhizosphere alkalisation occurred at pH < 4.8, while acidification occurred at greater pH. This explained the changes of CaCl2–Cu in the rhizosphere at low pH and the absence of pH dependency of Cu bioavailability to rape. In addition, apoplastic Cu in roots increased with increasing soil pH, most probably as a result of increased dissociation and affinity of cell wall compounds for Cu.