Screening Of The Methods For The Determination Of The Available Lead (Pb) And Cadmium (Cd) And Their Distribution Characteristics In Soil Aggregates Of Farmland

The accumulation of heavy metals in crops is largely dependent on the availability of heavy metals in soils.Due to the differences of soil types and pollution characteristics,there is no widely recognized method for the determination of the bio-available heavy metals in soils.It is difficult to compare and evaluate the availability of heavy metals in different regions and different types of soils.At the same time,a large number of studies have discussed the soil as a whole,and the research on the impact of soil structure on the bioavailability of heavy metals in soil is relatively scarce.After the heavy metals enter the soil environment,the distribution of their effective state in different particle size aggregates is still less.In order to screen and establish suitable methods,the extractable abilities of five extractants(0.01mol·L^-1Ca Cl₂,1mol·L^-1NH₄OAc,0.1mol·L^-1HCl,0.05mol·L^-1EDTA,and0.005 mol·L^-1DTPA)and Gradient Diffusion Film Technology（DGT）on four typical farmland soils with very different properties in Chongqing（acid purple soil,neutral purple soil,calcareous yellow soil and calcareous purple soil）were systematically compared.Simultaneously,pot experiments were carried out with Lolium perenne L.as an indicator plant to explore the relationship between the extractable soil Pb and Cd and their accumulation in plants.The feasibility of the tested methods were evaluated based on the extractability and co-relationships.At the same time,the soil aggregates were screened,and the distribution of available forms and forms of Pb and Cd in different particle size aggregates were discussed,which provided a basis for establishing a unified soil environmental quality assessment method based on the content of available forms.The main conclusions obtained are as follows:（1）The extractability of the tested extractants for soil Pb and Cd varied a lot.HCl showed highest extractability on Pb in acid purple soil and calcareous yellow soil,while EDTA did the best in neutral purple soil and calcareous purple soil;HCl showed best extractability in all soils except calcareous purple soil,where EDTA was the best.（2）Considering the correlation between the extraction amount and uptake by plant,all the extraction methods could be applied for the evaluation of the bioavailability of soils Pb and Cd except Ca Cl₂ for Pb due to its weak extractability for Pb for a specific soil type.For the comparison of heavy bioavailability in different soil types,EDTA-extractable Pb and DGT-extractable Cd were recommended due to their well co-relationships between extractable amount in soil and uptake by plant（Lolium perenne L.）as indicated by the correlation coefficients of 0.941 and 0.919,respectively.HCl was relatively suitable as Cd extractant compared to others if DGT method could not be used.（3）In 4 different types of soils,comprehensively considering the quality and heavy metal content of each particle size aggregate,it is found that when exogenous heavy metals enter the soil environment,the distribution of Pb and Cd in different types of soils with different particle size aggregate particles is different.Among them,the total amount of Pb（T-Pb）and the total amount of Cd（T-Cd）in the silt clay aggregate particles（<0.053mm）of the purple soil have the highest distribution,accounting for 26.93%-54.69%of the soil T-Pb,15.33%-33.58%of soil T-Cd.The highest concentration of T-Pb and T-Cd in calcareous yellow soil appeared in the 0.5～1mm middle aggregate particles,accounting for 29.93%of its T-Pb and 18.51%of its T-Cd.The total amount of heavy metals has a great impact on the bioavailability of heavy metals.The Pb and Cd in the three types of purple soils are mainly concentrated in aggregate particles with a particle size of<0.053mm.The aggregate particles of this size can control their Pb The bioavailability of Cd has a great correlation.（4）The ratio of the effective state to the total amount of heavy metals in the particle size（effective proportion）can reflect the relative activity of the heavy metals in each particle size.The main physical components of the four types of soils that control the effectiveness of soil Pb and Cd are different.When exogenous heavy metals enter the soil,the distribution law of the effective Cd proportion in the four soils is significantly different from that of Pb.After the external source of Pb is added,the effective Pb ratio of the 0.5-1mm agglomerate particles in acid purple soil is higher than that of other particle size agglomerate particles in this type of soil,accounting for 34.31%,and the neutral purple soil is 0.053-0.25 mm micro aggregates accounted for the largest proportion of effective Pb,which was 34.52%,while calcareous yellow soil and calcareous purple soil had the largest proportion of effective Pb in large aggregate particles with a particle size of 1-2 mm,which were 26.18%and 34.76%.When exogenous Cd enters the soil,acid purple soil has the highest proportion of effective Cd in aggregate particles of 0.053-0.25 mm,up to 52.05%,and the highest value of effective Cd in neutral purple soil occurs in large agglomerations of 1-2 mm.In calcareous yellow soil,it accounts for 46.23%.There is no significant difference in the proportion of effective Cd in aggregate particles of various sizes in calcareous yellow soil.For calcareous purple soil,the proportion of powder and clay aggregate particles with a particle size of<0.053mm The highest,but only 12.54%.Among them,the effective Cd in the micro aggregate particles of 0.053-0.25 mm in the acid purple soil is very high,which is closely related to the high Cd bioavailability,while the Cd activity in the aggregate particles of each size in the calcareous yellow soil is not The obvious difference is one of the reasons why the Lolium perenne L.grown in it has a small Cd concentration and low bioavailability.（5）The morphology of the heavy metals Pb and Cd in the agglomerates with different particle sizes is obviously different.After exogenous Pb enters the soil,the exchangeable Pb content in the silty clay aggregates with a particle size of less than 0.053 mm in the acid purple soil accounts for the highest proportion of the total soil Pb,which is 33.03%.The Lolium perenne L.in the acid purple soil is enriched in Pb.The ability of powder and clay agglomerates is closely related to the level of bioavailability of Pb.The content of exchangeable Pb in neutral purple soil is significantly reduced compared to the background soil,while the content of iron-manganese-bound Pb in aggregate particles of different sizes has increased significantly,and in aggregate particles with a particle size of<0.053 mm,the content of Pb in the iron-manganese bound state is the highest.At this time,the Pb in the neutral purple soil is transformed from the more active available state to the lower active potential available state,which also makes the Pb bioavailability significantly lower than the acidic state.Pb in purple soil.The most obvious change in calcareous yellow soil occurred in the iron-manganese-bound Pb,and the increase range was as high as 27%to38%.Compared with the other three soils,the change of calcareous purple soil is obviously different in that the proportion of carbonate-bound Pb has increased significantly,resulting in an overall increase in the proportion of available Pb content.It grows on calcium.The enrichment capacity of Lolium perenne L.in purple soil is only lower than that of acid purple soil,which is directly related to its higher content of available Pb.After the exogenous heavy metal Cd enters the soil,the most available Cd distribution of the four contaminated soils is in the large aggregate particles with a particle size of 1-2 mm that contain more organic matter.The available Cd in the aggregate particles of each particle size in the acid purple soil is higher than that of the other three soils.The bioavailability of Cd is strong,which is consistent with the results of the Lolium perenne L.pot experiment.The Lolium perenne L.grown in the acid purple soil The ability to accumulate Cd is obviously stronger than that of the other three soils.