Lead accumulation and low-molecular-weight organic acids secreted by roots in Sonchus asper L.-Zea mays L. intercropping system

There is a close relationship between low-molecular-weight organic acids (LMWOAs) secreted by plant roots and plant accumulation of heavy metals when intercropping accumulators and crops in heavy metal-contaminated soils. However, the specific mechanism behind this remains unclear. Sonchus asper L. Hill is a native hyper accumulator that can be found in Yunnan. Plant growth, amounts of LMWOAs secreted by plant roots, rhizosphere soil extractable lead (Pb) contents, and plant Pb accumulation characteristics were studied under S. asper-Zea mays L. intercropping in soil that was collected from a farmland surrounding a mining area, in order to investigate the effects of the LMWOAs produced by intercropping roots on heavy metal bioavailability in soils. The results showed that the aboveground and root biomass, root length, root inner diameter, and root volume of S. asper were significantly increased (P < 0.05), and that the root biomass, root length, root diameter, and root volume of Z. mays were significantly increased (P < 0.05) under the intercropping systems compared to under the monoculture systems. The major LMWOAs secreted by the roots of both S. asper and Z. mays were citric acid and oxalic acid. Intercropping resulted in an increase and decrease in the LMWOA contents secreted by S. asper and Z. mays roots, respectively. The bio-available Pb content in the rhizosphere soil of S. asper increased by 85.2% (P < 0.05), and that of Z. mays decreased by 26.1% (P < 0.05) under intercropping. The Pb content, enrichment factor, and transfer coefficient increased by 18.0%-43.2%, increased by 26.0%, and reduced by 42.0% in S. asper under intercropping, respectively. Compared to the monoculture system, the shoot Pb content and Pb transfer coefficient decreased by 24.3% and 43.1% (P < 0.05) in Z. mays under the intercropping system, respectively. The amounts of citric acid and oxalic acid secreted by S. asper roots were significantly positively correlated with the soil bio-available Pb content, and the bio-available Pb content in soil was significantly positively correlated with S. asper shoot and root Pb contents. The results indicate that intercropping increases the uptake and accumulation of Pb in S. asper. These are both closely correlated with increases in soil bio-available Pb, which is mediated by plant roots that secrete LMWOAs.