Effects of microplastics on water infiltration in agricultural soil on the Loess Plateau, China

Elucidating the water infiltration of agricultural soil would improve understanding of farmland hydrological systems and their sustainable utilization. Few studies have concentrated on the hydrological process of agricultural soil influenced by microplastics (MPs). Experiments including five MPs (polyethylene) contents (0, 0.1%, 0.2%, 0.4% and 0.6%) were conducted to explore the water infiltration of agricultural soil on the Loess Plateau, and the applicability was assessed for four main infiltration models. The results are as follows: There were nonlinear changes in the depth of the wetting front and cumulative infiltration with the MPs content, and the two parameters first decreased and then increased. Compared with the control treatment, the infiltration rate with 0.2% MPs was significantly reduced by 11.7% (p < 0.05), and the treatment had the lowest cumulative infiltration of 18.45 cm. Interestingly, when the content was 0.1%, 0.4% or 0.6%, there was no significant difference in the soil water infiltration rate and cumulative infiltration compared with the control treatment. When the content reached 0.2%, the downward accumulation of MPs played a role in blocking water infiltration. When the content exceeded 0.2%, macropores were formed through the interaction of MPs and soil particles, promoting preferential flow and water infiltration. The changes in saturated water content (SWC), saturated hydraulic conductivity (Ks) and soil water characteristic curve (SWCC) with concentration also supported the nonlinear effects of MPs on water infiltration. The Lewis equation was the best model to simulate the processes of cumulative infiltration. The wetting front propulsion model had the highest efficiency at simulating the wetting front depth. Further studies are necessary to explore the microscopic interactions between MPs and soil particles to reveal the hydrological process of agricultural soil influenced by MPs. Our results are greatly important for increasing the understanding of soil hydrological mechanisms and improving the management of agricultural land.