Residual plastic film exerts dual effects of blocking and preferential flow on soil water movement

Plastic-film mulching (PFM) significantly improves farmland productivity in arid and semi-arid regions. However, the residual plastic film (RPF), also named “white pollution”, has become a serious concern. The effect of RPF on soil water flow is unclear, but it has negatively impacted the use of PFM. This study evaluated the effects of RPF on soil physical and hydraulic properties. The impact on infiltration uniformity and preferential flow was examined based on dye tracer technology. The experiments were carried out with 5 RPF levels (0, 150, 300, 450, and 600 kg ha−1), and 4 infiltration amounts (20, 40, 60, and 80 mm). The results showed that increasing RPF amounts decreased soil bulk density and saturated hydraulic conductivity, but increased soil porosity and saturated soil water content in the RPF layer (0–30 cm). RPF accumulation enhanced the blocking effect in the RPF layer promoting preferential flow in the deeper soil layers. In general, with more RPF in soil, dye-stained coverage (DC) and uniform infiltration depth (UniFr) decreased, while the coefficient of variation (CVd), preferential flow fraction (PFF), dye-stained depth (MDD), length index (LI), and the difference of soil water storage between before and after dyeing (∆SWS) increased, especially in the deeper soil layers with high infiltration amount. 450 kg ha−1 RPF produced the strongest preferential flow with a maximum MDD 32.9 cm, PFF 39.2 %, LI 136.8, CVd 0.73 %, and minimum UniFr 8.8 cm. Furthermore, the phenomenon became prominent if higher infiltration amounts were applied; under 450 kg ha−1 RPF, the average CV was 8.0 % higher for 80 mm infiltration compared with 20 mm infiltration. Finally, correlation analysis shows that the RPF was the main driving factor of preferential flow.