| As a new environmental pollutant,microplastic(MP)has been widely concerned in recent years.Although the fate and impact of MP has long been the focus of the study in surface water(ocean,river)environment,its potential harm to soil and groundwater has not been realized until recently.Up to now,the effect of hetero-aggregation of secondary MP with soil minerals and natural organic matter on its transport in the terrestrial environment is still unknown.In addition,agricultural soils often function as a natural reservoir of both microplastic and antibiotic resistance genes(ARGs),so another issue related to MP transport is whether it can transport ARGs in the soil.The purpose of this study was to investigate the effects of natural aging and hetero-aggregation on the downward transport of microplastic in natural soil,to determine the main factors driving the transport process of MP,and to evaluate the potential of aging plastics to spread ARGs.In order to achieve these goals,natural loamy sand(representing the maximum transport potential of the vadose zone)and plastic debris(density lower than water)were collected in the forest land of the university campus and farmland for microplastic leaching test.At the same time,three classes of ARGs(tetracyclines,β-lactams and sulfonamides)widely spread in soil environment were investigated by PCR amplification technology.Twelve different ARG primers were used for PCR amplification of DNAs potentially occurring in MP samples,and the ARGs carried on them were investigated.The results of leaching experiments show that the vertical downward transport of microplastic(MP)occurs in soil environment,and the downward transport of clean microplastic(CMP)whose density is lighter than water can be ignored;the transport capacity of RMP was moderate,with a recorded maximal traveling distance of 3-4 cm;Introduction of humic acid into the system(RMP + HA)significantly enhanced the MP mobility,leading to a maximal traveling distance about 9-10 cm.It was found that although the intrinsic density of MP was lower than water,hetero-aggregation with soil minerals resulting in an increase of MP density could substantially enhanced its downward mobility.This established the hetero-aggregation as a driving mechanism for the downward transport of the light MP.The correlation between the maximum transport distance of CMP,RMP and RMP+HA and zeta potential confirms that surface charge is the dominant factor of MP transport.Only small-sized rod-shaped plastic fibers appeared at the maximum moving distance,suggesting that natural aging,generating plastic fibers,was conducive to plastic transport.Three classes of antibiotic resistance genes were all detected on the plastic surface,confirming MP as a potential way for ARGs to spread downward to the deep soil layer.These findings are important for understanding the transport and concentration distribution of MP and ARGs in soils impacted by agricultural practices. |
