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

The elevated CO₂ concentration has become the top issue of global environmental change.Soil contamination by heavy metals such as Cd and Pb is another important global environmental issue which has significant impact on modern society.Further increase in atmospheric CO₂ and heavy metals contamination of soil are expected to occur in the future,and combined effects of these factors could have important implications for the function and stability of terrestrial ecosystem.In recent years,some studies have shown that elevated CO₂ can impact the plants grown in heavy metals contamination of soil.The rhizosphere is a dynamic microenvironment characterized by interactions between roots,soil,and microorganisms.The nutrient and harmful substances in soils could through the rhizosphere microenvironment into plant body and affect the plants growth directly..Here,we used open-top chambers combined with pot experiment to examine the response of organic compounds,microbial flora and activity,microbial community structure,and enzyme activity to elevated CO₂?500 and 700 ?mol/mol?and Cd and Pb contamination,exploring the response of microenvironment of Robinia pseudoacacia L.seedlings to the combined effects.The main results are as follows:?1?Total souble sugars,amino acids,and organic acids in rhizosphere soils increased significantly with increasing CO₂ levels,while the content of phenolic acids was higher under 500 ?mol/mol CO₂ than under 700 ?mol/mol CO₂.Under Cd or Pb stress,total souble sugars,amino acids,and organic acids increased relative to the control,while phenolic acids decreased.Under Cd + Pb treatment,the content of total souble sugars,amino acids,and organic acids were higher than under individual metals.Under elevated CO₂ + Cd,Pb,or Cd + Pb treatments,the organic compounds in Cd-/Pb-contaminated soils increased with elevated CO₂ levels.?2?The soil pH decreased significantly under elevated CO₂,Cd,or Pb treatment.Cd + Pb treatment decreased the removal ratio of Cd and increased the removal ratio of Pb.The removal ratio of Cd and Pb increased with increasing CO₂ levels.?3?The population of bacteria and actinomycetes and total microbial activity?FDA?were higher under 500 ?mol/mol CO₂ than under 700 ?mol/mol CO₂ stress,while fungi population increased significantly under 700 ?mol/mol CO₂ stress.Cmic-to-Corg?the ratio of MBC to TOC?and MBC?microbial biomass carbon?increased with increasing CO₂ levels.Cd,Pb or Cd + Pb treatments led to an decrease in the population of bacteria and actinomycetes,MBC,Cmic-to-Corg,and FDA,and these factors decreased with increasing Cd levels,while fungi population did not vary significantly under different treatments,indicating that fungi are more tolerant to heavy metals than bacteria and actinomycetes.The microbial population and activities were higher under ECO₂ + Cd,Pb,or Cd + Pb than under heavy metals stress.?4?Under elevated CO₂,Cd,or Pb treatment,the bacterial and fungal community structure in the rhizosphere soils of Robinia pseudoacacia L.seedlings showed that the numbers of some species appeared/were enriched and disappeared/weakend,however the dominant species were constant.New species did not appear under elevated CO₂ condition,while the abundance of bacteria and fungi was increased.The abundance and diversity index of bacteria and fungi decreased significantly under Cd,Pb,or Cd + Pb stress,while new fungal species appeared/original species was enriched in 60-day period and disappeared in 90-day period,suggesting Cd and Pb could stimulate new species with a short appearance.?5?Dehydrogenase,invertase,and urease activity increased significantly with increasing CO₂ levels,while L-asparaginase activity decreased.The activity of ?-glucosidase was higher under 500 ?mol/mol CO₂ than under 700 ?mol/mol CO₂ condition.Dehydrogenase,invertase,?-glucosidase,and urease,activity decreased under Cd or Pb stress alone,while L-asparaginase activity decreased under Cd stress but increased under Pb treatment.The enzyme activities decreased significantly under Cd + Pb stress relative to individual metals.Elevated CO₂ significantly led to an increase in dehydrogenase,invertase,and urease activity in soils under Cd?Pb or Cd + Pb stress,and decreased L-asparaginase activity.?6?The principal component analysis under different treatments reported that total souble sugars,amino acids,phenolic acids,population of bacteria and actinomycetes,FDA,and Cmic-to-Corg are the principle factors in the rhizosphere microenvironment of Robinia pseudoacacia L.seedlings.In conclusion,the increased organic compounds under elevated CO₂ condition was the major factors for relieving the impact of Cd/Pb contamination on the rhizosphere microenvironment of Robinia pseudoacacia L.seedlings.