| Urban soil is the source and sink of urban pollutants and is closely related to the quality of urban ecological environment and human health.With the continuous progress of industrialization and urbanization,human activities have significantly changed the nature of urban soils,causing soil pollution,destroying environmental quality,and threatening human health.The determination of soil magnetic index has the advantages of rapid,sensitive,economic,non-destructive and large amount of information.It analyzes different magnetic parameters and their ratios to quantitatively describe the scientific information of the content,particle size and type of soil magnetic minerals.At the same time,human activities have led to the heterogeneity of the spatial structure of the urban soil magnetic field.Therefore,the development of urban soil magnetic characteristics and their spatial variation and spatial distribution are of great significance for the precise restoration of soil pollution.In this study,Shanghai urban sample zone soil was used as the research object,using classical statistical analysis,geostatistics,multifractal theory and other methods to analyze the soil magnetic characteristics,spatial variability characteristics and the characteristics of the three scales(sample zone,Minhang district,industrial zone).With multi-fractal features,the following conclusions are drawn:(1)The variation degree of soil magnetic parameters in the urban sample zone of Shanghai is medium,and the variation rate of soil magnetic susceptibility ?lf is 12.13?262.62 × 10-8m3kg-1,the average value is 45.17 × 10-8m3kg-1,and the variation range of SIRM is 760.17?40776 × 10-5Am2kg-1,the average value is 6338.0 × 10-5Am2kg-1,which is higher than the Shanghai soil background value.The increase of the magnetic properties of the soil samples in the study area may be related to the input of anthropogenic sources.The correlation coefficient between ?lf and SIRM(R2=0.8986)is greater than the correlation coefficient between ?lf and HIRM(R2=0.3048),indicating that the surface soil ferromagnetic minerals are mainly contributed by ferrimagnetic minerals.About 2/3 of the soil samples in the study area have a ?fd of less than 2%,and 94%of the remaining 1/3 soil samples have a ?ld variation between 2%and 10%.The average ?ARM/?lf value is 3.567 and the average ?ARM/SIRM value It is 27.20 × 10-5mA-1,and the topsoil magnetic mineral grains are mainly composed of pseudo-monodomain and multidomain particles.(2)The determination coefficient of the magnetic parameters of the soil in the sample zone ranges from 0.643 to 0.966,indicating that the variation function model fits well,and the fitting model can describe the spatial variation structure of the magnetic parameters well.?f and S-300mT conform to the Gaussian model,?fd and ?ARM conform to the exponential model,and SIRM and ?ARM/SIRM conform to the spherical model.There is a strong spatial autocorrelation of ?fd.Structural factors play a leading role in the spatial variability of ?fd in the study area.The spatial variability of the remaining magnetic parameters is dominated by random factors and structural factors.(3)The fitting results of the semi-variance function model show that with the decrease of the research scale,the nugget coefficients of ?lf??fd and SIRM first decrease and then increase,the nugget coefficient of ?ARM continuously decreases,and the nugget coefficient of S-300mT increased first and then decreased,and the nugget coefficient of?ARM/SIRM continued to increase.Except for the spatial structure of ?lf?SIRM??ARM/SIRM in the industrial zone with pure nugget effect,the soil magnetic parameters at all other scales have a certain spatial autocorrelation.The spatial distribution results show that the spatial distribution of the soil magnetic mineral content decreases from the outer suburbs to the suburbs to the city center and then increases.(4)The generalized fractal dimension spectrum shape of soil magnetic parameters?lf??fd??ARM?SIRM and ?ARM/SIRM in the sample zone are all horizontal "S" shapes,and D(q)decreases with increasing q value.It shows obvious multifractal characteristics,while the generalized dimension spectrum shape of S-300mT is approximately a straight line,and the multifractal characteristics are not obvious.The generalized fractal dimension curve of SIRM is the steepest,and the spatial heterogeneity is the largest.The multi-fractal spectra of the six soil magnetic parameters are all "bell-shaped" or a quadratic parabola with a similar opening downward,which shows an asymmetrically convex curve.The multifractal spectrum width ?? value of soil magnetic parameters from large to small is:SIRM>?fd>?ARM>?lf>?ARM/SIRM>S-300mT.The spatial variability of soil SIRM is slightly stronger than other parameters.(5)The generalized fractal dimension of six soil magnetic parameters in Minhang District decreases with the increase of mass probability statistical moment order,the decreasing trend of S-300mT is relatively weak,and the spatial distribution pattern of topsoil magnetism has multiple Fractal features can be used to study the spatial heterogeneity of multi-fractal methods.The information dimension D1 of the six soil magnetic parameters is relatively large,indicating that the spatial variability of soil magnetic minerals in Minhang is dominated by short-distance variability such as human activities.The ?Dq and ?? values of ?fd in Minhang District are the largest,indicating that the spatial structure of soil superparamagnetic SP particles is more complicated and the spatial distribution is uneven.It may be that human activities such as exhaust emissions and industrial pollution have destroyed their original The spatial structure causes the irregular characteristics of its spatial distribution to be more obvious.The shape of the multifractal spectrum curve of soil ?fd is right hooked,?f<0,that is,the spatial variability of ?fd is dominated by low-value data.(6)The ?Dq of ?fd and ?ARM/SIRM in the industrial zone is the largest,and the slope of the curve is larger in the generalized fractal dimension diagram,indicating that the multifractal characteristics of ?fd and ?ARM/SIRM are more obvious.The D1/D0 values of soil magnetic parameters are all less than or equal to 1,from large to small as S-300mT(1.000)??lf(0.940)??ARM(0.927)?SIRM(0.908)??fd(0.881)??ARM/SIRM(0.809),indicating that the content of ferromagnetic minerals in the topsoil of industrial areas is mainly concentrated in a dense area.The multifractal singular spectrum curve of ?lf and S-300mT is left hooked,and ?f>0,indicating that the spatial distribution of ?if and S-300mT is dominated by high-value data,that is,the spatial variability of ?if is caused by human activities leading.The multifractal spectrum of ?fd has a certain symmetry,?f=0,indicating that the spatial variability of ?fd is affected by both human and natural causes.(7)With the decrease of the research scale,the ?? value of each magnetic parameter keeps increasing,and the intensity of spatial variation keeps increasing.In addition to affecting the intensity of spatial variation of soil magnetism,the change of sampling scale also affected the dominant factor of spatial variation of soil magnetism in the study area.When the research scale is a small scale,the spatial magnetic variation of soil is dominated by high magnetic mineral concentration;when the research scale is a large scale,the spatial magnetic variation of soil is dominated by coarse-grained magnetic particles.Multifractal theory can accurately reveal the spatial structure variation of soil magnetic parameters,and point out the effect of scale effect on soil magnetic spatial variation. |
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