Distribution and weathering characteristics of microplastics in paddy soils following long-term mulching: A field study in Southwest China

Abstract

Agricultural plastic-film residues have been considered as one of the important sources of microplastics in the agroecosystem. However, limited researches were conducted on the accumulation of microplastics in long-term film-mulched paddy soil. This study aims to investigate the distribution and the weathering characteristics of filmy microplastics in a mulched paddy field (non-mulch, four years of mulched, and ten years of continuous mulched soil were investigated) in Southwest China. More than 50 % of the microplastics in the mulched soil were 1–3 mm, whereas the largest percentage of the microplastics in the non-mulched soil was <1 mm (55.3 %). Microplastic compositions in this field mainly consist of polyester (PES) and polyethylene (PE) (82.1 %). The abundance of microplastics increases with the film mulching time, which were 76.2 ± 18.4, 118.6 ± 44.8, and 159.6 ± 23.5 items kg−1 in soil with non-mulching, four years of mulching, and ten years of continuous mulching, respectively. The filmy microplastics accumulated annually in the plough layer is estimated at 18.1 million items ha−1. Weathering characteristics of filmy microplastics extracted from paddy soil were characterized using FTIR, SEM-EDS, AFM, and contact angle meter. The vinyl, carbonyl, and hydroxyl indices calculated from FTIR results showed that the degradation degree of microplastics incereased as mulching time rose; compared with commercial PE films, the oxygen-containing functional groups of soil-extracted PE films were increased. This study revealed the status of microplastic pollution in paddy soil with long-term mulching. It provided primary data and a scientific basis for further study on environmental behavior and ecological impacts of microplastics in agricultural soils.