Dynamics of residual film mass and microplastic abundance in long-term plastic-mulched cotton fields

This study investigated the residual plastic film and microplastic (MP) dynamics in cotton fields of Xinjiang after 5–30 years of mulching. Long-term mulching not only caused continuous accumulation of residual films and MPs but also triggered nonlinear accumulation dynamics. When residual film mass exceeded the critical threshold of 160–200 kg·ha–1, the MP generation rate increased significantly (by 85%), a phenomenon termed the critical effect. Residual films (manually collected) and MPs (extracted by density separation) had cumulative increases with mulching duration, reaching 127, 85.8 and 67.9 kg·ha–1 for films, and 10.8 × 103, 9.75 × 103 and 6.34 × 103 fragments kg−1 for MPs at depths of 0–1, 10–20, and 20–30 cm after 30 years. MPs had surface enrichment but also migrated downward, with < 1 mm fragments increasing from 7.9% to 22.6% to depth, while > 2 mm fragments declined from 49.2% to 13.8%. A strong linear correlation (R2 = 0.85–0.94) confirmed residual films as the primary MP source. Beyond the 200 kg·ha–1 threshold, MP accumulation rates accelerated sharply, highlighting fragmentation risks and vertical migration in arid soils. Timely residue removal before reaching critical thresholds is crucial to mitigate soil MP pollution. These findings provide actionable strategies for managing plastic-intensive agroecosystems, emphasizing proactive intervention to disrupt the critical effect and its cascading environmental impacts.