Distribution characteristics of residual film over a cotton field under long-term film mulching and drip irrigation in an oasis agroecosystem

Without an efficient mulch film recovery, the residual plastic film (RPF) causes pollution hazards to agronomic systems and natural environments. Here, we examined the distribution characteristics of RPF in the topsoils (0-40 cm) in an oasis agroecosystem in Northwest China. After cotton harvest in 2016, we collected 2304 soil samples from six cotton fields with different years of continuous mulching management (5, 9, 11, 13, 15 and 19 a). A total of 2471 pieces of RPF were separated and weighted. The weight of each RPF fragment was calibrated by its size measured in digital images. Our results showed that the amount of RPF ranged from 121.85 to 352.38 kg ha(-1) across the six fields, remarkably exceeding the national standard of China (75 kg ha(-1)). The occurrence frequency of RPF fragments maximized at 5-15 cm depth. With the increase in soil depth, RPF fragments became smaller, and their total mass decreased linearly. After the constant mulching practice, RPF accumulated at an annual rate of 15.69 kg ha(-1), mainly occurring at 0-30 cm depth. The accumulation of the larger RPF fragments (> 25 mg per piece) primarily took place from 5 to 15 years after mulching started. It took over 15 years for the larger RPF fragments to degrade into smaller ones (< 25 mg per piece), which accounted for 65% of the total number of RPF fragments in the field with 19-year of mulching. RPF fragments with a weight larger than 100 mg mostly concentrated at the surface (0-10 cm), whereas, the other smaller RPF fragments showed an evident downward migration. That said, with the constant mulching management, RPF pieces became more fragmented and distributed deeper, making mulch film recovery more challenging. To the best of our knowledge, this is the first time that the dynamics of RPF distribution in the soil profile after long-term mulching have been characterized. If the current mulching method continues, the accumulated RPF is going to cause severe soil pollution and risk the sustainability of the oasis agroecosystem.