Impacts of soybean growth on Cu speciation and distribution in two rhizosphere soils

Abstract

The effects of soybean growth on Cu speciation and distribution in two artificially Cu-contaminated soils (phaiozem, a Mollisol, and burozem, an Alfisol) were investigated using a rhizobox culture system followed by the sequential chemical extraction technique. The rhizosphere soils in the rhizobox were separated with nylon cloth into 11 sections: a central zone (4 mm in width) and ten side rhizosphere subzones (1 mm in width). The Cu fractions in Cu-contaminated phaiozem could be ranked as EXCH (exchangeable Cu)< ACID (HAc extractable Cu)< FeMnOX (bound to Fe-Mn oxides)< OM (bound to organic matter)< RES (residual Cu), and in Cu-contaminated burozem as EXCH < FeMnOX < OM < ACID < RES. Soybean growth resulted in an obvious increase of EXCH-Cu in the rhizosphere soil in the first 4 weeks of incubation, with the biggest increase in the central zone. Compared with the unplanted control soil, the content of ACID-Cu in soybean rhizosphere increased in the first 2-3 weeks of incubation and then decreased towards the end of the experiment (6 weeks). The increasing tendency was also observed in the FeMnOX-Cu fraction during the first 3 weeks and the OM-Cu fraction during the first 4 weeks, and their increments were obviously larger than those of the EXCH-Cu and ACID-Cu fractions. This suggested that more RES-Cu activated by the action of soybean roots was partitioned into the less phytoavailable forms, FeMnOX-Cu and OM-Cu. This may be of a benefit to the normal growth of soybean in Cu-contaminated soils. By the end of cultivation, the content of FeMnOX-Cu decreased, but only a small part of Cu was absorbed by soybean, being most of the Cu loss partitioned into the RES-Cu fraction.