The Responses Of Phenolic Acids In The Rhizosphere Soil Of Robinia Pseudoacacia L.seedlings To The Combination Of Elevated Atmospheric CO₂ And Cd Exposure

Phenolic acids in rhizosphere soils generally derive from root exudates,litters,and decomposition of soil humus,which are important for formation and stability of organic and organic-inorganic composite colloids and activation and transformation of heavy metals in soils,have allelopathic effect on rhizosphere microbial,and influence function of rhizosphere microecosystem.Environmental stresses such as elevated atmospheric CO₂ concentration and soil Cd pollution may affect the secretion characteristics of roots by regulating the synthesis of plant phenolic acids,thereby affecting the accumulation of phenolic acids in rhizosphere soil.At present,some progress has been made in the influence of elevated CO₂ concentration and soil Cd pollution on the synthesis of phenolic acids in plants,but the influence on the accumulation of phenolic acids in plant rhizosphere soil is still rare.Although there are sporadic reports on the accumulation of total phenolic acids,there are few studies on the accumulation characteristics of phenolic acids and the main sources of phenolic acids.Exploring the accumulation of phenolic acids in rhizosphere soil is of great significance for a comprehensive understanding and evaluation of global changes on the speciation,migration characteristics and bioavailability of heavy metals in plant rhizosphere soil.Therefore,in this study,Robinia pseudoacacia L.seedlings were taken as the research object,and the accumulation characteristics of total phenolic acids and several phenolic CO₂[（750±31）umol·L^-1]and Cd pollution(0,0.45,and 4.5 mg·kg^-1 dry weight soil),the accumulation characteristics of total phenolic acids and several phenolic acid monomers in rhizosphere soil and roots of seedlings were analyzed by HPLC,and the main sources of phenolic acids in rhizosphere soil were discussed.The results show that:（1）Effects of CO₂,Cd,and seedling growth time and their interaction on the accumulation of phenolic acids in roots and rhizosphere soil of Robinia pseudoacacia L.seedlings were significant.（2）In the seedling roots,with the increase of single CO₂ concentration,the content of phenolic acid monomer except ferulic acid decreased significantly（p<0.05）at 45 days of seedling growth,and increased significantly（p<0.05）at 90 days of seedling growth.The content of total phenolic acid,chlorogenic acid,p-hydroxybenzoic acid and caffeic acid decreased significantly（p<0.05）at 135 days of seedling growth,and the content of gallic acid,p-coumaric acid and ferulic acid increased significantly（p<0.05）.Under single Cd treatment,except total phenolic acids at 45 days of seedling growth,ferulic acid and p-coumaric acid at 90 days of seedling growth,chlorogenic acid and p-coumaric acid at 135days of seedling growth,the accumulation of total phenolic acids and phenolic acid monomers under high concentration Cd treatment(4.5 mg·kg^-1 dry weight soil)was significantly（p<0.05）lower than that under low concentration Cd treatment(0.45 mg·kg^-1dry weight soil).The accumulation of phenolic acids in roots increased first and then decreased with the increase of soil Cd concentration.From 90 days to 135 days of seedling growth,CO₂ increased the total phenolic acid content in roots under Cd pollution,but at 135days of seedling growth,CO₂ decreased the accumulation of chlorogenic acid and caffeic acid under Cd pollution.（3）In the rhizosphere soil,the increase of single CO₂ concentration significantly increased the accumulation of p-coumaric acid（p<0.05）,while the contents of gallic acid,p-hydroxybenzoic acid and caffeic acid decreased after 90 days of seedling growth.The accumulation of total phenolic acid increased only after 90 days of seedling growth（p<0.05）.Under single Cd treatment,the accumulation of total phenolic acids and phenolic acid monomers in rhizosphere soil of high concentration Cd treatment was significantly lower than that of low concentration Cd treatment at 45 days of seedling growth（p<0.05）,and significantly increased at 135 days of seedling growth（p<0.05）.After 90 days of seedling growth,the total phenolic acid content increased significantly（p<0.05）,while the contents of p-hydroxybenzoic acid,caffeic acid and p-coumaric acid decreased significantly（p<0.05）under low Cd concentration.Compared with single Cd pollution,the coupling of elevated CO₂ and soil Cd pollution increased the accumulation of p-hydroxybenzoic acid,caffeic acid and p-coumaric acid in rhizosphere soil,and the accumulation of three phenolic acids increased with the extension of seedling growth time.The accumulation characteristics of phenolic acids in rhizosphere soil and roots were similar under the coupling effect of elevated CO₂ and Cd pollution.However,chlorogenic acid and ferulic acid detected in roots were not detected in soil,and elevated CO₂ reduced the accumulation of gallic acid in rhizosphere soil under Cd pollution,indicating that these phenolic acids were less transported from roots to soil and were decomposed and utilized by soil microorganisms.（4）Pearson correlation analysis showed that total phenolic acid and caffeic acid were positively correlated（p<0.05）and gallic acid was negatively correlated（p<0.05）in rhizosphere soil and seedling roots,indicating that phenolic acid in rhizosphere soil was mainly derived from the secretion of Robinia pseudoacacia L.seedling roots,while gallic acid,p-hydroxybenzoic acid and p-coumaric acid were derived from the decomposition of humus by rhizosphere soil microorganisms.（5）Redundancy analysis showed that root Cd content,Cd accumulation,biomass,C content and soil water-soluble C,water-soluble C/N ratio,C,N,C/N ratio,and p H were significant influencing factors for phenolic acid accumulation in root microdomain soil,and root biomass（37.2%）,Cd accumulation（19.5%）,soil water-soluble C/N ratio（16.4%）,and p H（15.6%）had rates for phenolic acid accumulation.