Mechanisms of soil structure degradation and water movement impediment induced by residual plastic film under long-term mulched cotton cultivation

Plastic mulching is widely applied in agricultural production to improve crop water use efficiency and yield. However, with prolonged use, the accumulation of residual plastic film (RPF) in soil has become increasingly evident, potentially exerting significant impacts on soil structure and water transport processes. This study examined mulched drip-irrigated cotton fields with different cultivation durations in the Manas River Irrigation District, Xinjiang, China. Through a combination of field sampling and undisturbed soil column experiments, we systematically analyzed the spatiotemporal distribution of RPF and its effects on soil physical structure and water movement. Results showed that RPF content in the 0–40 cm soil layer increased significantly with cultivation duration, rising from 31.68 kg·ha −1 (RPF1, 4 years) to 530.12 kg·ha −1 (RPF7, 26 years), with a strong positive correlation to cultivation duration (R 2 > 0.95). RPF accumulation markedly degraded soil structure: compared with non-mulched control fields (CK), bulk density (BD) in 26-year cotton fields increased by 8.95%, the total porosity (TP) decreased by 4.85%, the proportion of large water-stable aggregates (WSA) declined by 33.94%, steady-state soil infiltration rate (SIR) decreased by 61.67%, and cumulative soil evaporation (CSE) was reduced by 38.16%. Structural equation modeling (SEM) further revealed that RPF indirectly suppressed soil infiltration and evaporation by reducing TP, increasing BD, and destabilizing aggregate structure. Sensitivity index (SI) analysis identified a critical RPF range of 264.28–393.47 kg·ha⁻¹, within which soil structure and hydraulic function showed a progressive deterioration. Strengthening post-harvest RPF recovery, improving mechanized film removal, and promoting biodegradable mulches are therefore essential to mitigate long-term soil degradation in arid irrigated systems. • Residual plastic film in 0–40 cm increased 16.7-fold over 26 years. • Residual plastic film alters water flow via porosity, density, and aggregates. • Soil degradation entered a critical stage at 264.28–393.47 kg·ha −1 of film residue.