Microplastics from conventional and biodegradable mulch films alter microbial necromass accumulation and organic carbon sequestration in farmland soils

Microplastics (MPs) are emerging pollutants that may affect soil organic carbon (SOC) and thus influence global change. However, the aging behavior of mulch film-derived MPs exposure to farmland soils and their impact mechanisms on microbial necromass carbon (MNC) formation remain poorly understood. Polyethylene (PE) and biodegradable (Bio, polypropylene carbonate and polybutylene adipate terephthalate synthetic material (PPC + PBAT)) mulch film-derived MPs at 0.2 % (w/w) dosages were mixed into farmland soils, incubated for 120 days and sampled every 30 days. On day 120, compared to MPs before incubation, the relative contents of C-H or C-C on the surfaces of PE-MPs and Bio-MPs decreased by 12.2 % and 3.7 %. In the first 60 days, compared to control soils (CK), the bacterial necromass carbon (BNC) content decreased by 54.0-56.0 % and 23.9-44.8 %, while the MNC decreased by 30.2-53.8 % and 11.8-23.0 % in farmland soils exposed to PE-MPs and Bio-MPs, respectively (p < 0.05). This reduction was attributed to the rapid turnover of necromass. During 90-120 days, under PE-MPs and Bio-MPs exposure, the contents of microbial biomass carbon (MBC) (66-76 % and 10-18 %), dissolved organic carbon (DOC) (1.8-16.9 % and 12.4-26.4 %), MNC (8.2-29.2 % and 14.0-16.1 %), and SOC (10.7-11.0 % and 6.8-8.9 %) showed significantly higher levels than those in the CK treatment. The PE-MPs and Bio-MPs promoted an increase in MNC and SOC content, which was associated with MPs aging that altered available nutrients and enzyme activities and directly or indirectly promoted an increase in DOC and microbial biomass. In summary, mulch film-derived MPs affect MNC accumulation and SOC sequestration, especially conventional PE-MPs.