Contribution Of Phosphorus And Arbuscular Mycorrhizal Fungi To Phytoremediation Of Heavy Metal Contaminated Soils

In recent years,the situation of soil heavy metal pollution in China is serious.About20% of farmland soil has been polluted by heavy metals,especially cadmium and lead,casuing serious health risks.The remediation of soil heavy metal pollution has gradually become a research hotspot.Among soil heavy metal remediation techniques,conventional physical and chemical techniques have the advantage of high remediation efficiency,but the disadvantages such as the damages to soil structure,soil fertility,and microbial communities.Phytoremediation has gradually attracted increasing attention because of its advantages such as in-situ remediation,environmental friendliness and protection of microbial ecosystem structure.However,in the practical application,phytoremediation often has the shortcomings of lower efficiency and time-consuming,which call for other measures to improve its efficiency.Phosphate has the advantages of high efficiency and stable immobilization effect of heavy metals.Arbuscular mycorrhizal(AM)fungi,as a group of plant symbiotic fungi,have a wide occurrence in heavy metal contaminated soil,which can increase the tolerance of host plants to heavy metal pollution,and improve the accumulation of heavy metals and the efficiency of phytoremediation.However,there are few studies on combined application of phosphate and AM fungi to enhance phytoremediation.Soil pot culture experiments with single pollution of cadmium and combined pollution of cadmium and lead were artificially simulated respectively.Five types of phosphates,i.e.,micron-sized hydroxyapatite(MAP),nanoscale hydroxyapatite(NHAP),diammonium phosphate(MAP),low polymerized ammonium polyphosphate(LAPP)and high polymerized ammonium polyphosphate(HAPP),were applied at 50 mg/kg and 100 mg/kg,and sweet sorghum(and oil sunflower)were planted.Soil remediation indexes,plant physiological and biochemical indexes,phytoremediation indexes,and AM indexes were measured to the effects and mechanisms of different phophates and AM fungi in phytoremediation of heavy metal contaminated soils.The main results of the study are as follows:(1)It was found that AM fungi significantly reduced available Cd concentration in soil and increased Cd concentration and uptake in sweet sorghum in Cd-contaminated soil,indicating that AM fungi reduced Cd availability in soil and improved the remediation ability of sweet sorghum in Cd-contaminated soil.AM fungi increased the concentrations of Zn,P,Mg,Ca,K and Mn in sweet sorghum,and promoted the absorption of nutrients in sweet sorghum.MAP and 100 mg/kg LAPP significantly reduced available Cd concentration in soil and improved Cd immobilization.LAPP and HAPP increased the concentration of soluble protein and POD activity,and enhanced the anti-oxidative stress ability of sweet sorghum in Cd-contaminated soil.MAP,LAPP and HAPP increased the uptake of P in sweet sorghum,with certain concentration-dependent effect,and improved P nutrition in sweet sorghum.There were significant interactions between AM fungi and phosphate type and concentration.Firstly,AM fungi enhanced the growth promotion effects of five phosphates on sweet sorghum.Secondly,AM fungi and low concentration of MHAP further immobilizaed Cd in soil.Finally,AM fungi combined with MHAP or NHAP promoted the extraction of Cd by sweet sorghum,especially the combination of NHAP and AM fungi increased the phytoextraction of Cd by 66.7%.Moreover,the tolerance of sweet sorghum to Cd was further enhanced and mineral nutrient of sweet sorghum was improved.(2)In Cd-Pb contaminated soil,AM fungi increased Pb and Cd concentrations and uptake of Pb and Cd in the roots of sweet sorghum and oil sunflower,while translocation factor-decreased,indicating that AM fungi fix heavy metals in the root system to reduce the damage of heavy metals in the shoot parts.At the same time,carbohydrate,glutathione and plant hormones were significantly enriched in the anabolic pathways,indicating that AM fungi can improve antioxidant stress ability of sweet sorghum.Different concentrations of phosphate had different immobilization effects on different heavy metals.For example,low concentration of NHAP reduced the bioavailability of soil Cd,while high concentration of MAP reduced the bioavailability of soil Pb.The five phosphates all increased the activities of different antioxidant enzymes and regulated the relative abundance of hormone metabolites(e.g.,salicylic acid upregulation)in sweet sorghum plants.The antioxidant stress ability of the two plants was both improved,and the defense system of sweet sorghum was activated.MAP,LAPP and HAPP significantly increased P concentration in the whole sorghum plants and oil sunflower shoots.There were significant interactions between AM fungi and phosphate type and concentration.AM fungi and phosphate further increased SOD activity in sweet sorghum.Compared with the other three types of phosphates,AM fungi and MHAP or NHAP further increased phytoextraction of Pb and Cd by oil sunflower.In conclusion,AM fungi could immobilize heavy metals in soil or plant roots,reduce the content of heavy metals in shoots,and contribute to the passivation of heavy metals.MAP,LAPP and HAPP increased P nutrition of plants and enhanced the ability of sweet sorghum to resist oxidative stress.AM fungi and MHAP or NHAP synergically promoted heavy metal phytoextraction by sweet sorghum under single cadmium contamination,and by sweet sorghum and oil sunflower under cadmium lead compound contamination,indicating they are a potential combination for promoting phytoremediation.