Response Of The Rhizosphere Microenvironment Of Robinia Pseudoacacia L. Seedlings To Elevated CO₂ Combined With Cd-and Pb-contaminated Soils

Heavy metal-contaminated soils including lead(Pb) and cadmium(Cd) is widespread in the world due to industry waste, mine, and fertilizrtion. In addition, increasing atmospheric CO2 due to combustion of fossil fules affects the function and the stability of terrestrial ecosystems over the past century. Therefore, study on effects of heavy metal and elevated atmospheric CO2 on plants is increasing recently. Most studies have focused on separate factors on plants; however, few studies on rhizosphere microenvironment of woody plants under the combination of these environmental factors. We inverstaged is the effect of elevated atmospheric CO2 combined with Cd- and Pb-contaminated soils on rhizosphere microenvironment of Robinia pseudoacacia L. seedling using open-top chambers(OTCs) and pot experiment. Results indicated:(1) Elevated atmospheric CO2 significantly affected total soluble sugars, amino acids, organic acids and total phenolic contents exudating from roots of Robinia pseudoacacia L. seedlings. Total soluble sugars, amino acids and organic acid concentrations increased under Cd or Pb with the increasing Cd; however, total phenolic content in root exudates reduced. Under Cd + Pb, total sugars, amino acids and organic acids content in root exudates increased but total phenolic acid content decreased compared to Cd alone. Under elevated CO2 + Pb + Cd, total soluble sugars, amino acids, organic acids, and total phenolic contents were higher than under either elevated CO2 or individual heavy metal.(2) Elevated atmospheric CO2 was associated with the increase in urease, dehydrogenase, β-glucosidase, and invertase activity and the decrease in asparaginase activity. Under Cd + Pb, urease, dehydrogenase, β-glucosidase, invertase, and asparaginase activity decreased with increasing Cd levels. β-glucosidase and asparaginase activity were more sensitive to Cd, Pb, and Cd/Pb than other enzyme activity but invertase activity showed little changes, which suggested different enzymes showed different sensitivity to the same heavy metal. Enzyme activity decreased under Cd + Pb compared to individual heavy metals, which suggesed that the stress of multiple heavy metals in soils might be stronger than individual heavy metals. Under elevated CO2 + Pb, Cd or Cd + Pb, urease, dehydrogenase, β-glucosidase, and invertase activity was higher than that under Pb, Cd or Cd + Pb, which suggested that elevated atmospheric CO2 might alleviate the inhibition of heavy metals on soil enzyme activity.(3) Bacteria, fungi, and actinomycetes abundance, microbial biomass carbon, microbial quotient, and FDA activity in rhizosphere soil of Robinia pseudoacacia L. seedlings increased significantly under elevated atmospheric CO2 compared to ambient atmospheric CO2. Individual heavy metals and the combination of metals resulted in the decrease in microbial abundance, microbial biomass carbon, microbial quotient, and FDA activity; moreover, the decrease was more obvious with increasing Cd levels in soil. Overall, bacterial was more sensitive to Cd stress, following by actinomycetes. The stress of Cd + Pb soil microbial activity was greater than individual heavy metals. Under the stress of the same heavy metal, soil microbial activity in the rhizosphere of Robinia pseudoacacia L. seedlings increased under elevated atmospheric CO2 compared to under ambient atmospheric CO2.(4) In conclusion, elevated atmospheric CO2 can alleviate the adverse effect of Cd and Pb on rhizosphere soil micro-environment of Robinia pseudoacacia L. seedlings.