| Cultivated land plays a significant role as the carrier of agricultural production,and the environmentalquality of cultivated land concerns not only the security ofagricultural products,but alsothe security of local ecology.Pollution assessment of is a crucial prerequisite for preventing and curing cultivated lands fromsoil heavy metals.Meanwhile,it is also an important procedure for assuring cultivated land quality and for arranging cultivated land resources,in an effort to achieve the sustainable use of land resources.The black soil region in central Jilin Province is an importantpart of the black soil region in Northeast China,also an important commodity grain base in the country,which includes Changchun,Songyuan and Siping.Furthermore,according to the provenance difference,the region can be divided into Liaohe Basin and Songhuajiang Basin.The thesis selected the black soil region in central Jilin Province as the study area,and it took the biotransformation and bioaccumulation of heavy metals in soil-crop system as the main research object.On basis of field survey,soil sampling and chemical analyzes,the thesis aimed toperform statistical analysis on the total concentration,bioavailable concentration and bioaccumulated concentration,and further use Fi and Ai indexes to determine the biotransformation and bioaccumulationstatus of heavy metals in central Jilin Province and their influencing factors by using GIS-based methods.Additionally,on basis of a modified pollution load index,we proposed an assessment method that considers not only the total concentration,but also the bioavailable concentration and bioaccumulated concentration of heavy metals.Finally,an early-warning zoning was conducted based onbioavailability factors and bioaccumulation factorsby using PCA,catastrophe theory and Thiessen polygon.The thesis discussed the constraints of biotransformation and bioaccumulation of soil heavy metals on cultivated land use in the black soil region.The results are as follow,(1)Characteristics of heavy metals in soil-crop system in the black soil region of central Jilin ProvinceApart from Zn,the concentrations of As,Cd,Cr,Cu,Ni and Pb in soils from both Liaohe and Songhua River Basin exceeded their background values.Overall,the bioavailable heavy metals were of moderate to strong variability in both quantity and space,in comparison to the total concentration.It indicated that the biotransformation of heavy metals was under lots of external influences besides the total concentration.However,the heavy metal concentrationin corn seedsdid not exceed theupper limit,and it was of relatively lower spatial variability.(2)Characteristics of biotransformation and bioaccumulation of heavy metalsCd in the Songhua River Basin in the study area were bio-transformed to a high level(Fi?10%),Cu and Ni were moderately bio-transformed(10%>Fi?5%),however,biotransformation of As,Cr,Zn and Pbwere rather weak(5 %>Fi?0.1%).Among all samples collected in spring,the biotransformation of Cr and Ni were provided with higher variability,and the spatial variation of other elements were moderate.For all samples collected in autumn,As and Znwere with higher variability.In the Liaohe Basin,Cd in both the spring and autumn werestrongly bio-transformed(Fi?10%),Cu and Ni weremoderate(10%>Fi?5%),and As and Cr werebio-transformed to a lower level(5%>Fi?0.1%).The rest of elements were different in biotransformation between spring and autumn.In spring,As,Cr,Ni and Zn in the Liaohe Basin were provided with higher variability,and the rest were moderate.In autumn,Cr,Cu,Ni,Pb were provided with higher variability,Cu was moderate,while Cd inthe Liaohe Basinwasprovided with higher variability but was moderate in the Songhuajiang Basin.The bioaccumulation of As,Cd,Cr,Cu,Ni and Zn in corn seeds in the black soil region of central Jilin Province were allextremely weak(Ai?10).Among all,the bioaccumulation of As,Cr,Ni,Zn were provided with higher variability,Cu was moderate,while Cd was provided with higher variability in the Liaohe Basin but was moderate in the Songhuajiang Basin.A spatial mismatch between soil total concentration,soil bioavailable concentration and bioaccumulated concentration in corn seeds was detected.It thus indicated that the influence that heavy metal exerts on soil environments and crop growth should not merely be determined by soil total concentration,it also needs to consider the biotransformation and the bioaccumulation as well.(3)Influencing factors of biotransformation and bioaccumulationof soil heavy metalsThe natural influencing factor analysisforsoil heavy metal biotransformation and bioaccumulationshowed that soil pH and soil organic matterwere the major influencing factors.In the Songhuajiang Basin,for acid soil,the bioavailable concentration of Cd and Ni increased with soil pH.For neutral soil,the bioavailable concentration of Cr and Zn increasedwith soil pH.The bioavailable concentration of Cuand Pb increased with soil pH in alkaline soil.However,in LiaoheBasin,the bioavailable concentration of Crand Ni increased with soil pH in neutral soil.In addition,the bioavailable concentrationof Cr,Cu,and Pbincreasedwith the decrease of soil organic matter.In both spring and autumn,a synergy could be found between the biotransformation of Cdand Ni,Crand Ni,Cuand Pb in Songhuajiang Basin.Moreover,the synergy could also be foundbetween the biotransformation of Crand Ni in Liaohe Basin.Along with the increase of biotransformation of P,B,Mnand Fe,some of the heavy metals also increased.It was noteworthy that the bioaccumulation was generally against the biotransformation of heavy metals.No significant land use influencing factors in the study area was found toaffect the biotransformation and bioaccumulationof heavy metals.However,as the buffer distance increased,the biotransformation and bioaccumulation of heavy metal showed a fluctuated tendency.The regression equation indicated that,in spring,the biotransformation of heavy metals was determined by provenance,especially Mg;in autumn,it was mainly determined by soil pH and the biotransformation of other heavy metals.The RDA results indicated that bioaccumulation of heavy metalswas determined by soil pH,along with the biotransformation of P,Mn,Mg and other heavy metals.(4)Comprehensive assessmentof heavy metal pollution in the black soil region of central Jilin ProvinceA modifiedpollution load index was used to assessthe pollution of heavy metals in regional soil-crop system.First of all,soil total concentrations suggested a low pollution level.However,the assessment based of bioavailable concentration suggested no to low level of pollution(0.999 and 1.025),indicating that most of the cultivated soil were clean.Other than that,the bioaccumulation pollution index(0.586)indicated that the crops were also clean.The comprehensive index(0.826)also indicated that there was no pollution.(5)Risk warning of heavy metal biotransformation and bioaccumulationin the black soil regionof central Jilin ProvinceAfter selecting indexes by using PCA,the thesis performed a risk warning method based on the catastrophe theory.The results indicated that two samples in spring and five samples in autumn were of higher risk for heavy metal biotransformation.Meanwhile,6.45% of all samples were of higher risks ofbioaccumulation.Moreover,on basis of Thiessen polygon,an early-warning zoning indicated that most of the study area were provided with moderate risk(0.9707).(6)Constraints of biotransformation and bioaccumulationrisk of heavy metal on cultivated land useIt could be concluded that,during soil improvement,besides soil fertility and crop growth,soil pH,soil organic matter and interactions between elements should nevertheless be considered when assessing the potential risks of soil heavy metals.On the other hand,the increase of biotransformation and bioaccumulation hindered the practical use and functional transition of cultivated lands.An early-warning for soil heavy metal pollution contributes to the sustainable use of cultivated land resources. |
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