Quantity and distribution of microplastics in film mulching farmland soil of Northwest China

In this study,81 soil samples from the 0～30 cm layer were collected from 27 mulch farmlands in 9 counties of Gansu Province and north Shaanxi.The density floatation method using distilled water and heating identification was combined with optical microscope scanning to extract light-density microplastics（MiPs）from the soil samples,and count the number of pieces and sizes of these MiPs.The results showed that the MiPs were highly abundant in film mulched farmland soils in Northwest China.Their concentrations ranged from5.8×102～1.189×104 pieces·kg-1,averaging at（5.09±1.21）×103 pieces·kg-1.The MiPs covered an area of 82～4155 mm2·kg-1,with a mean of（1.04±0.20）×103mm2·kg-1.The MiPs were sized 0.19 mm2·piece-1on average,the biggest size being 7.11 mm2·piece-1.The MiPs were classified into 5 groups by size,of which in the size 0～0.05 and 0.05～0.3 mm2·piece-1the proportion of abundance and size respectively accounted for 38.8%and 43.6%,8.7%and 36.5%,however in the size 0.3～0.6 and 0.6～1.0 and>1.0 mm2·piece-1were 10.5%,4.6%and2.5%,22.1%,16.8%and 15.9%,respectively,implying that the abundance of the MiPs increased with the reduction in size.The percentage of MiPs sized 0～0.05 mm2·piece-1in the plastic film mulch increased in abundance（by 44.9%）and area（85.2%）over a time period of 5～28 years.In contrast,the corresponding values for the MiPs sized>0.3 mm2·piece-1decreased significantly.These results clearly demonstrate that over the long term,the sizes of MiP pieces in film mulch farmland are reducing,whereas their abundances are increasing,leading to worsening pollution.