Green Manure and Overlapped Rhizosphere Effects on Pb Chemical Forms in Soil and Plant Uptake in Maize/Canola Intercrop Systems: A Rhizobox Study

Abstract

To investigate the effects of single and mixed cultures of maize and canola, green manure addition, and distance from the rhizosphere on chemical forms of soil lead (Pb) and its uptake by plants, a greenhouse experiment was conducted using rhizobox systems. After plant harvest, the soils from different parts of the rhizoboxes were sampled and analyzed for dissolved organic carbon (DOC) concentrations and pH values. A sequential extraction procedure was also carried out to differentiate the chemical forms of Pb in the soil samples. Results showed that root activity significantly affected soil DOC concentration, pH and Pb chemical forms, while the effects generally ceased from the rhizosphere through the bulk soil. The pH value increased and DOC concentration decreased with distance from the roots. Green manure addition enhanced DOC concentration and exchangeable, carbonate-associated, and organic matter-bound Pb fractions. Green manure addition reduced the Pb concentration and uptake of maize but increased those of canola. The concentration of Pb in the roots of maize and canola was increased in intercropping compared with monocropping systems. The conclusion is that chemistry and bioavailability of soil Pb are interactively influenced by overlapped rhizospheres as compared to individual rhizospheres, use of green manure, and distance from the root surfaces.