Impact of long-term conventional and biodegradable film mulching on microplastic abundance, soil structure and organic carbon in a cotton field

Biodegradable film mulching has attracted considerable attention as an alternative to conventional plastic film mulching. However, biodegradable films generate transitory microplastics during the film degradation. How much of this transitory microplastics is being formed and their impact on soil health during long-term use of biodegradable plastic film are not known. Here, we quantified the amounts of microplastics (0.1-5 mm in size) in the topsoil (0-20 cm) of two cotton fields with different mulching cultivations: (1) continuous use of conventional (polyethylene, PE) film for 23 years (Plot 1), and (2) 15 years use of conventional film followed by 8 years of biodegradable (polybutylene adipate-co-terephthalate, PBAT) film (Plot 2). We further assessed the impacts of the microplastics on selected soil health parameters, with a focus on soil carbon contents and fluxes. The total amount of microplastics was larger in Plot 2 (8507 particles kg^-1) than in Plot 1 (6767 particles kg^-1). The microplastics (0.1-1 mm) were identified as derived from PBAT and PE in Plot 2; while in Plot 1, the microplastics were identified as PE. Microplastics > 1 mm were exclusively identified as PE in both plots. Soil organic carbon was higher (27 vs. 30 g C kg^-1 soil) but dissolved organic carbon (120 vs. 74 mg C kg^-1 soil) and microbial biomass carbon were lower (413 vs. 246 mg C kg^-1 soil) in Plot 2 compared to the Plot 1. Based on ¹³C natural abundance, we found that in Plot 2, carbon flow was dominated from micro- (<0.25 mm) to macroaggregates (0.25-2 and >2 mm), whereas in Plot 1, carbon flow occurred between large and small macroaggregates, and from micro-to macroaggregates. Thus, long-term application of biodegradable film changed the abundance of microplastics, and organic carbon accumulation compared to conventional polyethylene film mulching.