Effect Of Rhizosphere Interaction On Cadmium Uptake Of Oilseed Rape And Its Mechanism

Oilseed rape(Brassica napus L.)is one of the most important oil crops in China,which is also one of the most common enrichment plants.Reasonable application of agriculture management is of great significance for improving oilseed rape to repair cadmium(Cd)contaminated soil.In this research,a kind of oilseed rape(Brassica napus L.)named“Xiangzayou 6” was proposed as the research object,intercropping with four intercrop crops to enhance the Cd accumulation of oilseed rape,using three separation treatments(including the no barrier,mesh barrier and solid barrier).There were used to distinguish the influences of rhizosphere effects vs.spatial effects in interspecific facilitation on the biomass of oilseed rape,Cd accumulation,the pH of rhizosphere soil,the available Cd of rhizosphere soil,the organic acid content of rhizosphere soil,and so on.For this study,the study was intended to use daikon(Raphanus sativus L.),white lupin(Lupinus albus L.),faba bean(Vicia faba L),and ryegrass(Lolium perenne L.)as the intercrop crops.To investigate the effects of different crops on the available Cd of oilseed rape rhizosphere soil and the secretion of organic acids(citric and malic acids),the differences between the effects of different types of crops on the accumulation of oilseed rape Cd were compared.The main conclusions are as follows:(1)Different types of crops have no effect on the pH of the oilseed rape rhizosphere soil,and since the initial pH of study soil is 4.5,the three separation treatments have no significant effect on the pH value in the four intercropping systems(P > 0.05).In the oilseed rape-white lupin(OW system),oilseed rape-faba bean(OF system)and oilseed rape-ryegrass(OR system)systems,the water-soluble Cd was increased in the rhizosphere soil of oilseed rape,but in the OD and OF systems,the DTPA-Cd was increased in the rhizosphere soil of oilseed rape with the increasing root contact.(2)The effect of different types of crops on the biomasses of oilseed rape and intercropping crops,in the OD system,daikon is the dominant species,compared with mesh and solid barrier treatments,the no barrier treatment causes the above-ground and below-ground biomass of oilseed rape to decline,and the total biomass of the whole intercropping system: mesh barrier(40.42 g/pot)> solid barrier(39.06 g/pot)> no barrier(38.30 g/pot).In the OW system,oilseed rape is an advantage species in the system,with the increasing root contact the biomass of oilseed rape was increased,the total biomass of the intercropping system: no barrier(32.29 g/pot)> mesh(31.03 g/pot)> solid barrier(24.55)g/pot.In the OF system,oilseed rape also is an advantage species,with the increasing root contact the above-ground biomass of oilseed rape was increased,but the below-ground biomass of oilseed rape was slightly decreased,the total biomass of the intercropping system:no barrier(28.56 g/pot)> solid barrier(28.26 g/pot)> mesh(26.47 g/pot).In the OR system,the competitive relationship between oilseed rape and ryegrass in the spatial effect,the rhizosphere effect increases the oilseed rape above-ground and below-ground biomass,but together the rhizosphere and spatial effect has reduced the above-ground and below-ground biomass of oilseed rape,the total biomass of the whole intercropping system: mesh barrier(34.04 g/pot)> solid barrier(27.33 g/pot)> no barrier(25.24 g/pot).(3)The difference of citric and malic acids in the four systems rhizosphere soil of oilseed rape,the total amount of organic acid was largest(820.3 mg/kg)in the OW system with the no barrier treatment.In terms of each organic acid(citric acid malic acid)which participated in the chelation of Cd,oilseed rape secretes more malic acid as the increasing root contact when intercropping with the white lupin.Additionally,with the increasing root contact with faba bean,the secretion more citric acid in the rhizosphere soil of oilseed rape.(4)The effect of three treatments on oilseed rape’s bioconcentration factor(BCF)(mean above-ground Cd concentration/ the initial concentration of the Cd added in the substrate)and translocation factor(TF)(the concentration of Cd in the oilseed rape and crops above-ground)/ mean the concentration of Cd in oilseed rape and crops below-ground)in the OF and OW systems,the rhizosphere effect and spatial effect increased the BCF of oilseed rape,but between the four systems did not have a significant effect on the TF values of oilseed rape.(5)The effect of three treatments and four intercropping systems on the above-ground and below-ground Cd concentration and total Cd accumulation.The below-ground Cd concentration of oilseed rape was significantly improved in the OD system,and intercropping with the white lupin,faba bean and ryegrass played a positive role in improving the above-ground Cd concentration of oilseed rape.The decrease in the biomass of oilseed rape in the OD system led to a decrease in total Cd accumulation,while the total Cd accumulation of oilseed rape increased significantly with the increasing root contact with white lupin and faba bean,but in the OR system,did no significant change in the total Cd accumulation of oilseed rape.Different intercropping systems and rhizosphere and spatial effects have a significant effect on the physiology and biochemistry of oilseed rape under the Cd stress.Comprehensive considerations of the oilseed rape biomass and total Cd accumulated.The available Cd content in the rhizosphere soil of oilseed rape was increased when intercropping with the white lupin,and the highest Cd accumulation of oilseed rape in the OW system(1205.54 g/pot).Thus,it is implying the dominance of white lupin for improving the phytoextraction Cd content of oilseed rape than the other three crops.Intercropping systems must learn from and apply the experience gained in successful experiments.Likewise,natural organic acids and selection of suitable plant species can also be coupled for phytoextraction of Cd.It is challenging for phytoremediation in the future to a selection of appropriate crop species to further enhance the Cd accumulation of the oilseed rape.