Effects Of Phytoremediation On The Stability Of Soil Aggregates And Distribution Characteristics Of Heavy Metals In Lead-Zinc Mining Area

Heavy metal pollution not only destroys the structure and properties of soil,but also inhibits the growth of plants.As a basic structural unit of soil,aggregates are the important places for soil physical,chemical and biological reactions,and are closely related to the distribution of heavy metals and their bioavailability in contaminated soil.At present,the effects of phytoremediation on heavy metal and bioavailability contents in response to soil aggregate are still unclear.Therefore,in this study,we evaluated the impacts of different phytoremediation?control,ryegrass,artemisia,silphuim and cole?on the stability of aggregates and soil organic carbon?SOC?and glomalin-related soil protein?GRSP?contents.The distribution and bioavailability of heavy metals in soil aggregates were also investigated in order to find their influencing factor.The results are as follows:?1?Phytoremediation increased the mean weight diameter of dry aggregates?DMWD?and the percentage of>5 mm dry aggregates.It also increased the mean weight diameter of wet aggregates?WMWD?and the stability of wet aggregates?WSA?,while decreased the percentaget of<0.25 mm wet aggregates.The results indicated that phytoremediation enhanced the mechanical stability and water stability of aggregates in contaminated soil.The MWD and WSA were higher in ryegrass treatment than in the control by 63.3%and 44.6%,respectively.?2?Ryegrass increased the SOC content in different aggregate sizes.The highest SOC content was in>2 mm aggregates,which was 1.13-fold higher than the control.Phytoremediation?except cole treatment?increased the contents of total glomalin-related soil protein?T-GRSP?and easily extracted glomalin-related soil protein?EE-GRSP?in different aggregate sizes,and the highest T-GRSP and EE-GRSP contents were in the ryegrass treatment.Additionally,SOC and GRSP contents were significantly positively correlated with large aggregates?>2 mm?and negatively correlated with small aggregates?<0.25 mm?.The results indicated that contents of SOC and GRSP were involved in the consolidation of soil particles and promoted the formation of large aggregates.?3?Phytoporemediation significantly decreased the contents of heavy metals,which were decreased by 14.9⁵3.4%?Cu?,7.1³0.5%?Pb?,11.1³1.9%?Zn?,and 17.3³4.3%?Cd?,respectively.The contents of bioavailability Zn and Cd?CaCl₂-Zn and CaCl₂-Cd?in soil were as follows:control>cole>silphuim>artemisia>ryegrass.The contents of Cd,Zn,Pb and Cu in the aggregates decreased with the increase of particle size,and were the lowest in the ryegrass treatment,which was 0.32⁷2.42%lower than the control.Phytoremediation significantly reduced CaCl₂-Zn and CaCl₂-Cd contents in soil aggregates,and their contents in ryegrass and artemisia treatments were the lowest.?4?Heavy metals are preferentially enriched on small aggregates?<0.25 mm?.As the small aggregates which adsorbed heavy metal particles from large aggregates?>2 mm?in phytoremediation,the mass loading of heavy metals and their bioavailability in<0.25 mm aggregates were reduced compared with the control.And the mass loading of heavy metals and their bioavailability in>2 mm aggregates increased significantly,and were most obvious in ryegrass treatment.?5?The contents of Cd,Zn,and Cu were negatively correlated with SOC and GRSP and stability of soil aggregates.The distribution of heavy metals in the soil was positively correlated with the content of nitrogen and phosphorus,soil bulk density,GRSP and soil aggregate stability.The distribution of heavy metal bioavailability in the soil is mainly affected by soil pH,moisture and soil aggregate stability.Aggregate,especially for large fractions,play the important role in affecting the distribution of heavy metals.In contrast to other treatments,ryegrass significantly increased SOC and GRSP contents and the stability of aggregates,and decreased the toxicity of heavy metals.Therefore ryegrass treatment was more effective than other treatments in improving phytoremediation efficiency.