| Soil cadmium(Cd)pollution has attracted much attention in recent years.The bioavailability and mobility of soil Cd are closely related to its fractions.It is helpful to master the contents of total soil Cd and its fractions for accurately revealing the accumulation and transportation of Cd in the soil profile.Based on the data of 169 soil profiles(0～100cm)and the background values of soil Cd content measured in the 1980 s,this study used classical statistics and geostatistical methods to analyze the characteristics of Cd accumulation,fraction distribution and spatial distribution in soil profile of Chengdu plain,and to explore effects of soil parent material and agricultural land use on total soil Cd and its fractions.The result showed,(1)In the study area,the contents of total soil Cd were 0.38,0.23,0.20 and 0.20mg/kg in the 0～20,20～40,40～60 and 60～100 cm soil layers,respectively.Compared with the background values measured in the 1980 s,soil Cd accumulation was found in the soil profile and most of the Cd accumulated in the surface soil.The soil Cd contents in soils formed from the three parent materials were related to the soil depth.In the 0～20 cm soil layer,the total Cd content of soils formed from Q3 old alluvium was higher than that of soils formed from Q4 grey alluvium.There was no significant difference between the three parent materials in the 20～40cm soil layer.In the 40～100cm soil layer,the total Cd content of soils formed from Q4 grey alluvium was higher than that of soils formed from Q3 old alluvium.In addition,there was no significant difference in soil Cd content under different agricultural land use patterns.(2)There were obvious differences in Cd fraction distributions in different soil depths.The Cd fraction contents were significantly higher than that in the underlying three layers.Acid soluble Cd(F1)and reducible Cd(F4)were the main fractions in the topsoil(0～20cm),which contributed 31.13% and 30.83% of the total Cd respectively;while F4 accounted for the largest proportions in the subsoils(20～100 cm).The results of semi-variance analysis showed that the nugget/sill ratio of soil Cd fractions ranged from24.64% to 49.03%,which suggested that the spatial dependence of soil Cd fractions were moderate.The nugget/sill ratio of soil Cd fractions decreased with increasing soil depth.The different fractions showed distinct spatial distributions.F1 in the surface soil was generally high in the north and low in the south,and reducible Cd(F2)in the surface soil was generally high in the south and low in the west.The area with high oxidizable Cd(F3)content was mainly distributed in the northeast,and the spatial distribution pattern of F4 was opposite to F1.(3)F1,F2 and F3 showed significant positive correlations with soil organic matter and total nitrogen,and a significant negative correlation with the bulk density in the soil profile.The correlations between Cd fractions and total phosphorus,p H,clay and sand decreased with the increasing depth.F1 was significantly positively correlated with total phosphorus,F2 and F3 were significantly positively correlated with p H and clay,and negatively correlated with sand in the 0～40 cm soil layer.However,the correlations between the Cd fractions and the above physicochemical properties was weak in the 40 cm soil layer.(4)The Cd fractions was higher in the surface soil than in the underlying layer,and the Cd fraction distributions varied across different parent materials.F1 and F4 accounted for the main part of the surface of soils formed from Q3 old alluvium,F1 accounted for the main part of soils formed from Q4 gray alluvium,and F4 was the main fraction in the underlying soil for the two parent materials;in the 0～60 cm soil layer,F2 was the main fraction for soils formed from Q4 gray-brown alluvium,while F4 was the main fraction in the bottom.The content of Cd fractions in different parent materials was significantly different.The F1 of soils formed from Q4 gray alluvium and Q4 gray-brown alluvium were significantly higher than the Q3 old alluvium;F2 in the soils formed from Q4gray-brown alluvium was significantly higher than that formed from the gray alluvium in the 0～40cm soil layer;F3 in the soils formed from Q4 gray-brown alluvium was significantly higher than that in the Q3 old alluvium and Q4 gray alluvium,and F4 in the soils formed from Q4 gray alluvium is significantly higher than that in the gray-brown alluvium.(5)The contents of soil Cd fractions in afforested land were significant different with other two agricultural land uses.F2 in the afforested land was significantly lower than that of rice-wheat/rapeseed rotation land in the 0～40cm layer,while F3 was significantly lower than that of rice-vegetable rotation land in the 20～60cm layer.The effects of agricultural land use on Cd fractions in the profile were mainly dependent on the parent material,and the contents of soil Cd fractions for each agricultural land use were different in different parent materials. |
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