Impacts of Different Sizes and Concentrations of Polyethylene on Chemical, Physical, Mechanical, and Biological Properties in a Loess Soil

Microplastics (MPs) are greatly released into soils in different ways, specifically through mulching practices in irrigated loess soils in northern Iran as the fertile and susceptible soils to water erosion. This study was conducted to examine the effects of Polyethylene (PE) (a common kind of MPs used in mulching farming in loess soils of north Iran), in a Loess soil under an experimental study. A loess soil was collected from 0- 30 cm surface and incubated at two levels of PE (2.5 and 5 % by weight) in two sizes (1-2 mm and >2mm) for 60 days, at 70% of field capacity moisture and temperature of 25-28 ° C. Soils samples were collected in 1, 15, 30, 45 and 60 days after incubation and different soil chemical, physical, mechanical and biological properties were measured. The results showed that soil pH was significantly (P<0.05) reduced by timing (time after incubation), and application rate of MPs. Inclusion of MPs led to increasing bulk density significantly (P<0.05) and also led to reducing aggregation and mean weight diameter (MWD) compared to control soil. The rate of MWD decreasing varied from 79 % to 87 % in different rates of MPs application. Among the mechanical attributes, liquid limit, plastic limit, and plastic index, all were reduced significantly (P<0.05) after mixing up and incubation with MPs. Reducing aggregation and aeration in treated soils compared to control soils, led to lowering soil microbial respiration, especially with timing. As our results about the effects of MPs in loess soils in a few cases were contrary to other soils studied worldwide, further studies are recommended to perform in loess soils in northern Iran, where receive considerable MPs by farming and open landfill practices in the studied region.