Accumulation Of Different Forms Of Heavy Metals In Root System Of Urban Road Forest Belts

With the rapid development of the social economy,the construction of traffic infrastructure in the city is becoming more and more perfect,which has brought great convenience to the the transportation industry and travel.But at the same time,it has been accompanied by new environmental problems,namely heavy metal pollution in road soil,which poses a great threat to the environment and human health through biochemical and the geochemical cycle.Road forest belt,as an excessive zone between roads and other areas,has a certain buffer effect on environmental pollutants,and plays a positive role in the protection and purification of heavy metals in the city.Taking green plants' root of three configuration that Sophora japonica are the main tree species as objects in the viatic forest belt on the East Bank of the Jing Mi diversion canal in the Landianchang road,Haidian District,Beijing City,the concentration of Cr,Mn,Cu,Ni,Zn,Cd and Pb in different plant roots and the concentration of the total amount of heavy metals(T)as well as four forms,namely,acid soluble(B1),reducible(B2),oxidizable(B3),residue(B4)was analyzed.At the same time,the root uptake and accumulation characteristics of different forms of heavy metals under different configurations was compared,aimed to discuss the enrichment and absorption characteristics of different diameter(fine-sized:<2mm;smallsized:2-5mm;medium-sized:5-10mm;large-sized: > 10mm)root of Sophora japonica in different configuration and the accumulation characteristics of heavy metals in plant roots under different configurations,to provide theoretical support for the plant configuration mode of road forest belt.The results of the study show that:(1)The concentration distributions of the heavy metal element were that Mn was the highest and the Cd was the lowest.The trend of the heavy metal concentration in the different root diameters of different plant roots was Ophiopogon japonicas > fine-sized root > Lonicera maackii > small-sized Root > Pyrus > Rosa multiflora > medium-sized root> large-sized root.Mn,Zn,Cd had a higher biological activity;Cr in the absorption roots and Pb in the roots,the biological effectiveness was very low.Whereas,the bioavailability of Cr was higher in the root system outside the root of the absorption root;the potential bioavailability of Cu in the root of arbors and shrubs was higher,but in the root of herbs,the bioavailability was high;Ni in the different root had a large difference in the bioavailability among plants in different configurations.With the increase of the diameter,the proportion of B1 state in Mn,Zn,and Cd gradually decreased,while the proportion of other forms,especially B2 state,gradually increased;The proportion of Cr B1 in the root systems at different diameter did not change significantly,The proportion of B2 states showed an increasing trend with the increase of root diameter,while the B3 and B4 states decreased;The proportions of B2 and B4 states of Cu did not change much with the increase of the diameter of the root,the proportion of B1 states increased,while that of B3 showed a decreasing trend;The proportion of Ni B1 and B3 states increased from fine-sized roots to small-sized roots,decreased in the medium-sized roots and large-sized roots,and the B2 state did not change much in the root system.The proportion of B4 state concentration decreased from fine-sized roots to small-sized roots,and then in the medium-sized root and the large-sized roots increased.The proportions of Pb B1 and B4 states increased with the increase of root diameter,while the B2 and B3 states decreased with the increase of root diameter.(2)The ability of roots to enrich heavy metals varies with the heavy metal elements,forms,and plant species,Ophiopogon japonicus had the strongest root-enrichment ability.As the root diameter of Sophora japonica increased,its enrichment ability gradually decreases.In terms of the enrichment coefficient of the total amount(T),the root system had the highest enrichment ability for Cu,Cr and Cd,followed by Mn,Zn and Ni,and the minimum was Pb.The root system had the strongest enrichment ability for Cr,Cd and Pb in the B2 state.And the B1 state enrichment ability of Mn,Ni and Zn was the strongest,the B3 state enrichment ability of Cu was the strongest.And the root system had the weakest enrichment ability for each heavy metal in the B4 state.The enrichment ability of root to accumulate heavy metals was different at different growth stages.After defoliation,the B1 state enrichment ability of Cr,Ni,Cu,Cd,Pb,the B2 state enrichment ability of Cr,Cu,Cd,Pb and the B4 state of Cr,Mn,Ni,Cu,Zn,Pb was the strongest,while during the growth period the B3 state of Cr,Mn,Ni,Cu,Zn in the root system was strongest.(3)Root-system transport capacity for heavy metals in different diameters was in the trend: medium-sized root> large-sized root> small-sized root;The maximal transport capacity of different forms of Cr,Mn,Cu,Ni,Pb in all diameter root was mostly in the periods of before and after defoliation,while the minimum was at the initial stage of growth and growth.The Zn and Cd transport capacities of various forms were mostly in the early stages of growth and growth,while the minimum is in the periods of before and after falling leaves.The order of the retention capacity of each heavy metal was Cu>Mn>Ni>Pb>Cr>Zn>Cd.The retention capacity of different heavy metals in different root systems was different.The ability of Cr,Mn,Ni,Zn,and Pb to reside in the root system was fine-sized root> medium-sized root> small-sized root,while Cu and Cd were fine-sized root > small-sized root>medium-sized root.The root system had the largest retention capacity of Cr,Cu,and Ni in the B3 state,the highest B1 state retention capacity for Mn,Zn,and Cd,and the highest B2 state retention capacity for Pb.The retention rate at each stage was also different due to the different types of heavy metal elements.The maximum retention capacity of Zn was in the period of before deciduous,and the others was in the period of growth stage.The minimum retention capacity of Cr,Cu,and Ni was in the period of before defoliation,Mn and Zn was in the period of After defoliation,Cd is in the early stages of growth.(4)Pb,Cu,Ni,Zn and Cd may have synergistic effects in rhizosphere soil,and they may also have the same source of pollution.The active state of Cr may be interfered by three elements of Mn,Cd and Pb,and there may be mutual promotive effect between the active states of Mn,Cu,Ni and Zn.Heavy metals in B3 and B4 states are not easy to enrich and absorb plants;some elements exist to promote the absorption of other elements,while others will be hindered,and the total concentration of Mn in rhizosphere soil has a certain effect on the heavy metals in root system.The interaction between heavy metals and rhizosphere soil before and in the early stage of deciduous and early growth is strong,and the change between root and rhizosphere soil before deciduous leaves is strongly B2 state,while the other growth period is B1 state.Heavy metals had a strong correlation among different diameter root of Sophora japonica.In the initial stage of growth,the heavy metals migrate more radically in roots.