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

Abstract

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 of ω(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 of φ(silt) and φ(medium coarse sand) are determining factors for heavy metal concentrations in fine and coarse particles of aeolian sandy soil. 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.