不同沙生灌木下土壤颗粒及重金属空间分布特征

Spatial Distribution of Soil Particles and Heavy Metals Under Different Psammophilic Shrubs in the Ulan Buh Desert

以乌兰布和沙漠沙冬青、花棒、猫头刺、白刺和梭梭灌木下及流动沙丘0~100 cm风沙土为研究对象,通过野外分层采样与室内实验,采用多重分形理论探讨土壤粒径分布(particle size distribution,PSD)和ω(Mn)、ω(Zn)、ω(Cr)、ω(Cu)、ω(Pb)、ω(Ni)、ω(As)、ω(Cd)及分布特征并分析其相互关系,反映土壤结构及重金属污染状况.结果表明,α(q)-f(q)图像均左偏,呈左钩状,即不同灌木均可增加粒度级配中细小颗粒子集比例,同时,沙冬青、花棒可显著增加PSD范围、非均匀度及离散度.不同灌木下土壤重金属整体表现为ω(Mn)>ω(Zn)>ω(Cr)>ω(Cu)>ω(Pb)>ω(Ni)>ω(As)>ω(Cd),除ω(Cu)及ω(Cr)外,各灌木下0~10 cm土层重金属含量大多高于流动沙丘;沙冬青下ω(Mn)、ω(Cd)均值分别为354.899、1.429 mg·kg^(-1),远高于流动沙丘,各土层间差异显著(P<0.05),ω(Mn)、ω(Cd)最大值分别出现在>30~40 cm、>10~20cm土层;花棒下ω(Pb)均值居各灌木之首为13.556 mg·kg^(-1),高于沙冬青3.24%,而ω(As)则低于沙冬青0.37%,但均高于流动沙丘.φ(粉粒)与φ(中砂)分别为决定风沙土中重金属含量的细粒及粗粒土壤,8种重金属均与φ(黏粒)、φ(粉粒)正相关,除ω(Cr)外,其余重金属均与φ(中砂)负相关,重金属含量与多重分形参数间的相关系数与φ(黏粒)、φ(粉粒)、φ(中砂)相似,即多重分形维数同样可较好地描述重金属含量与决定性土壤颗粒间的关系.研究显示,沙冬青及花棒可拦滞风沙流中的细小土壤颗粒使其沉降在周围表层土壤,并更好地改善各土层PSD特征,且二者作用后的风沙土中重金属含量高于其余灌木及流动沙丘,故可通过种植二者局部改良土壤并拦滞、原位固控重金属,减轻下风向城市空气中的重金属污染.

Aeolian sandy soil was collected at depths of 0 to 100 cm at intervals of 0-10, 10-20, 20-30, 30-40, 40-60,60-80,and 80-100 cm from under the cover of psammophilous shrubs Ammopiptanthus mongolicus,Hedysarum scoparium,Oxytropis aciphylla,Nitraria tangutorum and Haloxylon ammodendron and from a mobile dune in the Ulan Buh Desert.Multi-fractal theory was used to determine the soil particle size distribution（PSD）,which measured the fractions of ω（Mn）,ω（Zn）,ω（Cr）,ω（Cu）,ω（Pb）,ω（Ni）,ω（As）,and ω（Cd）,as well as their distribution status in the laboratory and analyzed correlations between the PSD and heavy metal content to identify the relationship between soil structure and heavy metal pollution.This result shows that a range of different types of shrubs can increase the distribution of fine soil particles,and specifically Ammopiptanthus mongolicus and Hedysarum scoparium can increase range,heterogeneity and degree of dispersion of the PSD.The heavy metal content in the soil under the different shrubs is ranked as ω（Mn）〉ω（Zn）〉ω（Cr）〉ω（Cu）〉ω（Pb）〉ω（Ni）〉ω（As）〉ω（Cd）.In the first 10 cm of the soil under the shrubs,the concentration of all heavy metals are higher than samples from the mobile dune except for Cr and Cu.Average value ofω（Mn） and ω（Cd） is 354.899 and 1.429 mg·kg^-1 under Ammopiptanthus mongolicus and Hedysarum scoparium respectively.Concentrations of Mn and Cd under Ammopiptanthus mongolicus and Hedysarum scoparium shrubs are significantly higher than those of mobile dune（P〈0.05）,and the concentration of both is significantly various between each soil layers（P〈0.05）.The highest value for ω（Mn） is in soil layer between 30 and 40 cm below the surface and the highest for ω（Cd） appears between 10 and 20 cm below the surface.The average ω（Pb） under Hedysarum scoparium is 13.556 mg·kg^-1 which is the highest among all the shrubs,exceeding 3.24% under Ammopiptanthus mongolicus and higher than samples from mobile dune.However,average ω（As） under Hedysarum scoparium is exceeded 0.37% under Ammopiptanthus mongolicus but still higher than samples from mobile dune.The grain size ofφ（silt） and φ（medium coarse sand） are determining factors for heavy metal concentrations in fine and coarse particles of aeolian sandysoil.Heavy metal measurements all positively correlate with φ（silt） and φ（clay）,however,they negatively correlate with φ（medium coarse sand）,except for ω（Cr）.The correlation coefficients between heavy metal contents and the multifractal parameters are similar for φ（clay）,φ（silt）,and φ（medium coarse sand）.Therefore,multifractal parameters can describe the correlation between heavy metals and their determinant soil grain size.This study shows that,Ammopiptanthus mongolicus and Hedysarum scoparium can entrap and deposit fine particles from the sand flow to surface soil layers around shrubs and improve the condition of PSDs for different soil layers with respect to heavy metals.Hence,in aeolian sandy soil these concentrations are higher than for mobile dunes after planting Ammopiptanthus mongolicus and Hedysarum scoparium.Therefore both should be used to promote good soil structure of particular areas to entrap and in-situ control heavy metals to decrease heavy metal pollution in downwind cities.