| With the increasing frequency and intensity of human activities,especially the rapid development of the economy in recent decades,the global ecological environment is gradually deteriorating,such as a great quantity of heavy metals are released into the ecological environment currently.At present,the soils(such as farmland)in most parts of China is seriously contaminated with heavy metals.More importantly,the pollution triggered by the coexistence of various heavy metals poses a great threat to the sustainable development of China's environmental health and ecological security,especially recruit multiple effects on the physiological and ecological characteristics of plant species and soil microbial community structure.Therefore,it has become one of the major scientific issues in the research field of environmental science to explore and clarify the ecotoxicological effects of heavy metals on the physiological and ecological characteristics of plant species and soil microbial community structure as well as the corresponding mechanism.In addition,nitrogen(N)is one of the most important nutrients for plant's growth performance.Soil N cycle is an important part of soil material cycle,and the process of biological N fixation mainly mediated by soil N-fixing bacterial communities(SNB)plays a key role in the soil N cycle.Triticum aestivum L.is one of the three major cereals in the world,and China is one of the countries with the largest wheat production in the world.The physiological and ecological characteristics of wheat plays a vital role in its growth performance.Meanwhile,SNB in wheat rhizosphere could improve the availability level of soil N which can enhance the growth performance of wheat.However,heavy metal pollution may affect the physiological and ecological characteristics of wheat and SNB in wheat rhizosphere,which in turn affects the growth performance of wheat.Therefore,the present study aims to address the effects of copper(Cu)and lead(Pb),both individually and in combination with a gradient concentration,on the physiological and ecological characteristics of wheat and SNB in wheat rhizosphere by using the metagenomic theory and method.The treatment groups with all combinations of type and level of Cu and Pb were experienced in this present study,specifically,(?)the control with distilled water(CK;0 mg kg-1 soil);(?)the low concentration independent Cu(35 mg kg-1 soil);(?)the high concentration independent Cu(70 mg kg-1 soil);(IV)the low concentration independent Pb(35 mg kg-1 soil);(?)the high concentration independent Pb(70 mg kg-1 soil);(?)the low concentration combined Cu and Pb(35 mg kg-1 soil);and(?)the high concentration combined Cu and Pb(70 mg kg-1 soil).The purpose of this study was to estimate and clarify the ecotoxicological effects of heavy metals on the physiological and ecological characteristics of wheat and SNB in wheat rhizosphere as well as the corresponding mechanism.The results of this study could build a strong theoretical foundation for effective risk assessment of environmental health and ecological security of heavy metal pollution as well as create a strong practical significance for the growth management of wheat under the background of heavy metal pollutionThe main results of the present study are as follows:?.Ecotoxicological effects and the corresponding driving mechanisms of Cu and Pb on physiological and ecological characteristics of wheat(?)The number of tillers,plant height,ground diameter,single-leaf fresh and dry weights,leaf thickness,single-plant fresh and dry weights,leaf chlorophyll and N contents,and plant peroxidase and catalase activities of wheat significantly decreased when treated with Cu and Pb individually regardless of concentration.Thus,Cu and Pb individually significantly reduced the morphological and physiological performance of wheat,especially the power of the productive stem,competitive ability for sunlight acquisition,plant supporting ability,leaf growing ability,leaf photosynthetic capacity,leaf growing ability,plant growing ability,and enzymatic defensive ability under stress of wheat.The significantly reduced plant height and leaf chlorophyll and N contents of wheat under heavy metal stress might be the main reason for the significantly ecotoxicological effects of heavy metals on the morphological and physiological performance of wheat.This may result from a significant reduction in the competitive ability for sunlight acquisition as well as the leaf photosynthetic capacity of wheat leaves.Another possibly equally important reason may be that Cu and Pb significantly reduced the plant peroxidase and catalase activities of wheat,which can significantly inhibit the enzymatic defensive ability under stress conditions.In addition,the ecotoxicological effects of Cu and Pb on the morphological and physiological performance of wheat significantly increased with increasing concentrations,especially impacting the leaf chlorophyll and N contents of wheat.Accordingly,the leaf photosynthetic capacity of wheat may be dramatically reduced with increasing concentrations of Cu and Pb.The possible reason for this is that the contents of Cu and Pb in the soil samples increased markedly with increasing concentrations of Cu and Pb,which can distinctively increase the additional energy cost for the growth performance of wheat.(?)The ground diameter and single-plant fresh weight under the independent Pb treatment were significantly lower than those under the independent Cu treatment with the same concentration.Thus,the independent Pb exerted more toxic effects on the morphological performance of wheat,especially the plant supporting ability and plant growing ability,than the independent Cu.Because Cu is an essential trace element involved in several important physiological processes in plant species,Cu tolerance exists widely in a variety of plant species.Secondly,as a non-essential element for plant growth,Pb can cause distinct toxicity on the growth performance of plant species.(?)The ground diameter,single-leaf dry weight,leaf thickness,single-plant fresh weight,and plant proline content under the combined Cu and Pb treatments were significantly lower than those under the independent Cu or the independent Pb treatments with same concentration.Thus,the combined Cu and Pb can confer more ecotoxicological effects on the morphological and physiological performance of wheat compared with the independent Cu and/or the independent Pb,especially the plant supporting ability,leaf supporting ability,leaf growing ability,plant growing ability,and enzymatic defensive ability under stress of wheat.Therefore,compared with the independent Cu or the independent Pb treatments,the combined Cu and Pb treatments can address a synergistic effect on the morphological and physiological performance of wheat.The possible explanation for the higher ecotoxicological effects of the combined Cu and Pb treatments on the morphological and physiological performance of wheat presumably results from the increased acidification in soil samples under the combined Cu and Pb treatments than that under the independent Cu or the independent Pb treatments,which can pose a positive effect on the accumulation and solubility of Cu and Pb as well as the corresponding toxicity on plant species?.Ecotoxicological effects and the corresponding mechanisms of Cu and Pb on SNB in the wheat rhizosphere(?)Cu and Pb significantly affect the alpha and beta diversity of SNB.Under Cu and Pb individually regardless of concentration,the OTU richness,Chaol index,and ACE index of SNB significantly decreased,which indicated that the number and richness of SNB species significantly decreased under Cu and Pb.This may be due to the considerable soil acidification when treated with Cu and Pb individually regardless of concentration,which further increased the accumulation and dissolution of Cu and Pb and than enhanced their ecotoxicological effects,especially on the composition of soil SNB taxonomic groups by changing the intake and utilization of SNB nutrientand The reason for the significant decrease in the number and richness of soil SNB species may also be attributed to the significant increases in the heavy metal content in the soil under Cu and Pb treatment resulting in the increased energy costs of SNB growth,which can decrease both the availability and utilization of nutrients necessary for the normal growth performance of bacterial communities.The OTU richness and the Chaol index of SNB under the low concentration independent Pb were significantly lower than those under the independent Cu regardless of concentration,which indicated that the independent Pb exerted stronger ecotoxicological effects on the number and richness of SNB species than the independent Cu.This is likely due to greater soil acidification mediated by the independent Pb compared to the independent Cu with same concentration.Since Pb easily forms a complex with alkaline substances in the soil,it is difficult to transfer,resulting in a significant decrease in soil pH.Soil pH reduction can also affect soil microbial communities by altering the levels of different inhibitors and activators in the soil,as well as the effective concentration of the respective substrates.In addition,compared with the combined Cu and Pb treatments,the ACE index of SNB was significantly reduced under the independent Pb.Thus,the combined Cu and Pb treatments noticeably enhanced the richness of SNB species compared with the independent Cu treatment or the independent Pb treatment.Accordingly,compared with the independent Cu or the independent Pb treatments,the combined Cu and Pb treatments can incur an antagonistic effect on the alpha diversity of SNB.Therefore,different types of heavy metal interactions do not always have a synergistic effect on all ecological processes(?)The relative abundance of SNB taxa were significantly altered by Cu and Pb exposure.For example,c_Alphaproteobacteria and o_Sphingomonadales were significantly altered under the high concentration independent Cu,f_Desulfarculaceae under the high concentration Pb,and o_Micrococcales and f_Clostridiales Family XVII Incertae Sedis under the high concentration combined Cu and Pb.The differential shifts in SNB composition in response to Cu and Pb is primarily due to the different degree of heavy metal tolerance of these taxa.Therefore,Cu and Pb may exert selective pressures on different SNB taxa,which respectively leads to a decline or rise in the relative abundance of those taxa. |
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