Effects Of CO₂ Enrichment On Lead Absorption And Accumulation Of Trifolium Repens L. And Lead Activation In Rhizosphere Soil

Soil heavy metal pollution and elevated atmospheric CO ₂ concentrations are global environmental problems that directly affect the stability and function of terrestrial ecosystems.The plant root-soil interface is an important part of the plant ecosystem and directly controls the nutrient elements and pollutants from the soil into the plant.In the natural ecosystem,heavy metal pollution in soil and elevated CO ₂ concentrations in the atmosphere often coexist,and their joint action will inevitably have a profound impact on the tolerance and heavy metal enrichment ability of functional plants.However,most studies focus on the effect of a single factor on biomass,and pay less attention to the regulation of rhizosphere interface of functional plants.In-situ absorption of Trifolium repens L.for the remediation of heavy metal contaminated soil is a potential remediation method.In this study,Trifolium repens L.was taken as the research object.Combined with soil culture simulation experiments,root scanning technology,Diffusive Gradients in Thin-films technology（DGT）and Visual MINTEQ model were used to explore the growth,lead accumulation and tolerance of Trifolium repens L.under the combined effect of CO ₂ enrichment and lead pollution,and the response of rhizosphere environment to it.At the same time,the effect of CO ₂ enrichment on the transformation of lead forms at the root-soil interface of Trifolium repens L.was studied,and the effect of CO ₂ enrichment on the release behavior of lead in rhizosphere soil was revealed.This study explained the feedback mechanism of Trifolium repens L.remediation of lead pollution under the condition of CO ₂ enrichment from the perspective of root-soil interface,which provided a scientific basis for the application of CO ₂ enrichment fertilizer in strengthening phytoremediation technology and a reference for evaluating the safety of plant rhizosphere interface system under the conditio n of complex environmental factors.The main results are as follows:（1）A pot experiment was conducted to study effects of the elevated CO₂ enrichment on the biomass,root morphology,antioxidant enzyme activity,chlorophyll content and photosynthetic parameters of Trifolium repens.The results showed that under different Pb stresses（CK,Pb100 and Pb500）,elevated CO₂（800 ppm）could significantly increase th e biomass and root-shoot ratio of white clover,promote the lateral root growth and photosynthesis of Trifolium repens,and reduce the oxidative damage of plants and improve the stress resistance of Trifolium repens by affecting the malondialdehyde content and antioxidant system in Trifolium repens.Compared with the ambient CO₂（460 ppm）,the aboveground and underground biomass of Trifolium repens increased by 62.74%～82.78%and147.91%～179.81%respectively,the MDA content in leaves decreased by 3.12%～15.7 6%,and the net photosynthetic rate increased by 51.9%～65.51%.At the same time,elevated CO₂alleviated the degradation of chlorophyll caused by Pb stress in plants,and the increase in CO₂concentration could compensate for the damage of Pb stress on Trifolium repens to a certain extent.（2）A pot experiment was conducted to study the effects of elevated CO₂ enrichment on the lead content,enrichment coefficient,transfer factor and rhizosphere environment in different parts of Trifolium repens.The results showed that elevated CO₂ enhanced the ability of Trifolium repens to extract Pb from soil and accumulate it in the body by affecting the root growth and photosynthesis of plants,and it also showed that the elevated CO₂ could affect the nutrient content in rhizosphere by affecting plants.Compared with the ambient CO₂（460ppm）,the Pb content in the aboveground and undergrou nd parts of Trifolium repens increased by 37.46%～134.56%and 41.94%～58.45%respectively,the enrichment coefficient increased by 43.61%～189.17%,and the transfer factor increased by 3.80%～58.42%.In addition,under the elevated CO₂,the pH of rhizosphere soil of Trifolium repens decreased significantly,the phosphorus content decreased significantly,the nitrate-ammonium nitrogen content increased significantly,and the CEC content did not change significantly.The above results prove that it is feasible to improve the remediation efficiency of Pb contaminated soil by Trifolium repens by CO₂ fertilization.（3）The concentration and form of heavy metals in soil liquid phase were quantitatively analyzed by Visual MINTEQ model.The results showed that elevated CO₂ promoted the transformation of loosely bound humic acid complex lead（HA1-Pb）into stably bound humic acid complex lead（HA2-Pb）in soil solution.Pb mainly exists in the complex state of organic matter（accounting for more than 90%）in soil,and when the p H decreases,the bound state will dissociate and release metal ions into the soil liquid phase,which has potential biological effectiveness.Under different levels of Pb pollution,the proportion of Pb²⁺in soil solution to total dissolved Pb increased with the increas ing of CO₂ concentration.Elevated CO₂ promoted the activation of Pb in soil through acidification and dissolution,significantly improved the bioavailability of Pb,and promoted the absorption of Pb by Trifolium repens.（4）Diffusive Gradients in Thin-films（DGT）combined with DIFS model was used to study the effect of elevated CO₂ on the solid-liquid redistribution characteristics and release power of lead in soil.The experimental results show that elevated CO₂ could make the unsteady Pb in the soil solid phase be released more quickly,specifically,the characteristic time T_c（s）for the interface system to reach equilibrium after disturbance is significantly reduced,and the adsorption/desorption rate is significantly increased.At the same time,the results of DIFS simulation showed that the replenishment capacity of solid phase to liquid phase in Trifolium repens rhizosphere soil was influenced by CO ₂ concentration,bioavailable metal capacity,adsorption and desorption rate constant and other factors.Under the condition of elevated CO₂,Pb in soil solid phase transfers faster to liquid phase,which can promote the absorption of heavy metals by plants.