| Soil is an important component of the terrestrial ecosystem and for human survival and sustainable development.With the migration of various chemical elements in nature,soil is not only the destination of heavy metals but also the source of heavy metals to the water and organisms.Due to the different physicochemical properties of soil samples under different agricultural land use types,fractionation distribution of heavy metals will change,which will further affect the migration and enrichment of heavy metals in soil.In addition,by exploring the enrichment characteristics and influencing factors of heavy metals,their potential migration and transformation in soil can be revealed and their potential risk to the ecological environment can be better evaluated.Such studies can also provide a scientific and effective basis for the remediation of heavy metal pollution in soil.Because of this,the research of heavy metals in soil has become the focus of attention.The provenance,distribution characteristics,influential factors and pollution assessment of heavy metals in soil have been reported by some researchers,while research on the influence of land use patterns on the enrichment of heavy metals is lacking,especially for the fraction concentrations and the mechanism of migration and enrichment in different agricultural land use types in Hainan Island,a typical tropical region,which is rarely reported.In this research,soil samples were collected from adjacent paddy field,abandoned cropland and vegetable land in Qionghai and Dingan in the eastern Hainan Island.Through the outdoor investigation and indoor experimental analysis,combined with the cultivation measures and fertilization methods of each type of agricultural land,the spatial distribution and fractions of heavy metals in different agricultural land use types were analyzed,and the effects of different tillage methods on the migration and enrichment of heavy metals were discussed,which provided a theoretical basis for protecting the environment of agricultural land and preventing soil pollution.The main results of the study are as follows:(1)The total concentrations of Cr,Cu,Pb,Zn and Cd in agricultural soils of study area were 2.06-127.17,3.09-86.63,2.12-62.44,0.21-32.57 and 2.40-78.25mg/kg,respectively.There are some differences of heavy metals concentration in different agricultural land use types.The concentrations of Cr,Pb,Zn and Cd in the three types of land use decreased in the following order: abandoned cropland > paddy field > vegetable land.The concentration of Cu decreased in the following order: abandoned cropland > vegetable land > paddy field.(2)In the soil profile of 0-30 cm depth,the concentrations of heavy metals generally increased first and then decreased with increasing soil depth in paddy fields,mainly concentrated at a depth of 10-20 cm.In vegetable land,the concentrations of heavy metals were the highest in the surface layer(0-10cm)and decreased with increasing soil depth.The concentrations of Cr,Cu and Zn in abandoned cropland were the highest in 0-10 cm soil layer,while the concentrations of Pb and Cd were the highest in 10-20 cm soil layer.(3)In the three land-use types,the proportions of the exchangeable fractions differed slightly(7.06-8.51%).The proportion of carbonate-bound fractions under the three land-use patterns declined in the following order: abandoned cropland(12.46%)> vegetable land(9.14%)> paddy field(4.92%).Because the paddy field was slightly acidic,the carbonate-bound fraction is easily released and the paddy field had the lowest proportion of the carbonate-bound fraction.Metals in the Fe-Mn oxide-bound fraction were relatively high(27.03%)in paddy fields;this fraction is easily released when paddy soil is flooded and hypoxic.The proportions of organic-bound fractions in the three different patterns were as follows: paddy field(10.54%)> abandoned cropland(7.36%)> vegetable land(6.92%).In the three land-use types,metals showed the highest proportion of the residual fraction.The residual fractions in vegetable land and paddy field were higher than those in abandoned cropland.This finding may indicate that active biological promoted the mobility of metals,resulting in the relative enrichment of residual metals.(4)The agricultural soil in the study area was slightly acidic with a pH ranging from 4.86 to 7.65.The Eh of paddy field,abandoned cropland and vegetable land were 93.9,67.4 and 58.9mV,respectively.Among the three land-use types,the particulate diameter of abandoned cropland was the largest;the particulate diameter of paddy field and vegetable land decreased with increasing soil depth.The concentration of organic matter ranged from 9.90 to 27.62g/kg and was the largest in paddy field.(5)Soil physicochemical properties such as pH,Eh,particulate diameter and organic matter may affect the fractionation of heavy metals.With a decrease in soil pH,the concentrations of carbonate-bound fraction and Fe-Mn oxide-bound fraction decreases,and the concentrations of exchangeable fraction and residual fraction increases;however,the correlation between Eh and fractions is opposite to pH.When the content of fine particles increases,heavy metal ions are preferentially adsorbed in soil components with large surface areas,the concentration of available heavy metals can be reduced,which can reduce the bioavailability.In the three land-use types,the soil organic matter concentrations were as follows: paddy field(6.15g/kg)> abandoned cropland(2.70g/kg)> vegetable land(1.75g/kg).This result is consistent with the order of proportions of organic-bound fractions.The organic matter showed positive relationships with organic matter-bound fractions of the four heavy metals.It also had inverse relationships with exchangeable fractions and carbonate-bound fractions.An increase in the soil organic matter concentration reduces the exchangeable sites and metal ions under acidic conditions,transforming them into organic matter-bound fractions,indicating that organic matter in soil can promote the formation of organic-bound fractions. |
PDF Full Text Request