The Mechanisms Of Enhanced Cd Remediation In Polluted Soils By Intercropping Oilseed Rape With Sedum Alfredii

Heavy metal contamination in soils has attracted increasing attention in recent years.Phytoextraction is an emerging technique with advantages of being environmentally friendly,cost-effective,and an in situ remediation technique.However,phytoextraction is facing a certain bottleneck,the practical application of hyperaccumulator is still limited due to its small biomass,slow growth and large manpower.Therefore,seeking the plant with large biomass,easy cultivation and fast growth is the point of penetration.Studies have found that Brassica juncea has a higher ability of Cd uptake and toleration.Oilseed rape,as one of the staple oil crops in South China,also belongs to Brassica.Therefore,screening oilseed rape cultivars has broad application prospects.On this basis,analysis of the differences in Cd accumulation,distribution and transportation in oilseed rape,as well as the influence of soil properties provide the basis on safe production and phytoremdiation of metal contaminated soil.At present,the phytoextraction cycle is relatively long,and other measures should be combined to improve the efficiency of phytoextraction.In addition,considering that the farmers still need to cultivate on the slightly and moderately polluted soil to ensure the economic income,intercropping of hyperaccumulator and cash crops is a relatively desirable way of phytoremediation.However,there are few studies focused on the intercropping between hyerperaccumulator and accumulating cash crops in HMs contaminated soils.In addition,as important executors of soil functions,microorganisms mediate most biogeochemical cycling processes,soil nutrient status,speciation and bioavailability of HMs,thus substantially affecting absorption and accumulation of heavy metals by plants.Therefore,it is of great significance to study the composition of soil microbial community and their linkage with HMs accumulation in the intercropping system of hyperaccumulator Sedum alfredii and high accumulating oilseed rape,as it provides theoretical basis of“phytoremediation coupled with agro-production”.The main results of this study are as follows:（1）A field experiment was conducted to evaluate 39 oilseed rape cultivars（Brassica napus L.）for their ability to accumulate Cd.The results showed that seed yield ranged from 1260 to 3498 kg ha^-1,and shoot Cd concentration ranged from 1.58to 3.17 mg kg^-1.However,the Cd concentration in seeds was lower than the national standard of 0.20 mg kg^-1.The Cd accumulation in shoot could reach 162.9μg plant^-1,and the translocation factors（TFs）for Cd were higher than 1.0.We have established a screening system for high Cd accumulating cultivars using comprehensive evaluation indexes including seed yield,food safety,translocation factor and Cd removal ability of aboveground parts.According to the criteria,R-1,R-16 and R-25were selected as high Cd accumulating cultivars which have the potential to remediate slightly and moderately Cd contaminated soils.（2）Four field experiments were conducted simultaneously to evaluate uptake and distribution of Cd in 20 oilseed rape cultivars at four agricultural locations with different Cd contamination levels,as well as the influence of soil characteristics together with soil total and bioavailable Cd concentration on Cd uptake and distribution in oilseed rape.The results showed that concentrations of Cd in shoot of oilseed rape among four sites ranged from 0.14-3.53 mg kg^-1.For most cultivars,the concentration of Cd in root or shoot were higher than that in pod,and the lowest in seeds.The results of stepwise multiple linear regression analysis showed that the Cd concentration in oilseed rape was related to OM,CEC,AP,AK,TCd and ACd（R²=0.993-0.999,P<0.05）.The above results indicate that the effect of soil physical and chemical properties on Cd absorption and accumulation in oilseed rape was greater than that of cultivars,and the heavy metal concentration was largely affected by soil physical and chemical properties.（3）Five representative soil types in China,including black soil,fluvo soil,paddy soil,red soil and purple soil,were selected to investigate the effects of intercropping system between high Cd accumulating oilseed rape cultivar and hyperaccumulator S.alfredii on the Cd phytoremediation efficiency in pot experiments.Intercropping significantly increased the biomass,grain yield and total Cd accumulation by oilseed rape.The removal amount of Cd in the intercropping system per pot was in the range of 217-2270μg,which was the highest in red soil,followed by paddy soil and the lowest in black soil.（4）Rhizobox experiments were conducted to investigate the influence of intercropping on the rhizosphere bacterial microbiota and their linkages with the Cd phytoextraction by Zhongyouza 19（B.napus）,Xikou Huazi（B.juncea）and S.alfredii using 16S r RNA gene sequencing.The Cd accumulation in shoots of B.napus and B.juncea grown under intercropping were enhanced by 370%and 27.8%,respectively,as compared to monoculture.Intercropping significantly enriched the abundance of Actinobacteria,Bacilli,Deltaproteobacteria,while depleting that of Acidobacteria in rhizosphere.Intercropping with S.alfredii influenced more on microbial composition of B.napus rhizosphere than that of B.juncea.The change in rhizosphere bacterial communities was related to metal availability,soil properties（P<0.001）,and plant parameters.The enriched families of Pedosphaeraceae,Ruminococcaceae,Chitinophagaceae,Gemmatimonadaceae,Nitrosomonadaceae,and Parachlamydiaceae were positively correlated with metal concentration in plants.The results showed that oilseed rape and S.alfredii intercropping regulated the complex plant-microbe-HMs interactions,changed the structure of rhizosphere functional bacterial community structure and increased the bioavailability of HMs and plant Cd accumulation.（5）Field experiments were conducted to evaluate the phytoextraction benefit,nutrient benefit and economic benefit under high Cd accumulating oilseed rape cultivar and hyperaccumulator S.alfredii intercropping system.The results showed that the shoot biomass of oilseed rape and S.alfredii under intercropping were increased,and accumulations of Cd in oilseed rape and S.alfredii under intercropping were increased by 4.65%and 20.6%,respectively.As compared to unplanted soil and monoculture,intercropping significantly increased available P while decreased available K in rhizosphere soil.The comprehensive benefits evaluation index of intercropping system was the highest than that of S.alfredii monoculture and oilseed rape monoculture,indicating that the oilseed rape and S.alfredii intercropping system had obvious advantages.