Dry-wet alternation and microplastics particle size effects on and contributions to soil water and soil pore properties

Agricultural soils always contain microplastics (MPs) residues and farmlands often undergo continuous drying-wetting alternations. However, little is known about how the existing MPs and MPs particle size affect soil physical properties under drying-wetting cycles; also their combined influences are not well understood. Hence, we completed measurements of hydraulic parameters and calculations of water characteristics and pore distributions in soil-MPs mixtures subjecting to five drying-wetting cycles and four MPs particle sizes. Quantitative findings indicated that both MPs and drying-wetting cycles reduced saturated conductivity, which firstly decreased and then increased with the increase of MPs particle size and the progress of drying-wetting cycles. The drying-wetting cycles increased field capacity (FC) and permanent wilting coefficient (PWC), but reduced gravity water (GW) and available water content (AWC). Oppositely, the MPs reduced FC and PWC but increased GW and AWC; furthermore, the average FC and PWC overall firstly decreased and then increased with the increase in MPs particle size; however, the average GW and AWC firstly increased and then stabilized. The MPs reduced total porosity and the drying-wetting cycle also reduced it in soil-MPs mixtures, which whereas increased as the drying-wetting cycles proceeded. Factors contribution analyses indicated that the drying-wetting cycle made greater contributions than MPs particle size to the variation of soil physical properties, and their combined effects mainly made great contributions to the variation of soil hydraulic parameters. Our findings provide evidence for MPs influence on soil physical properties, which deserves attention with regard to the developments of sustainable agricultural practical managements in plastic-polluted soil-crop systems.