Legacy herbicide bound to PLA-PBAT and LDPE plastic mulch film fragments in soil can induce phytotoxicity to subsequent crops

Agricultural herbicides are essential for maximizing yields through effective weed control; however, their interaction with plastic residues in soil, particularly from mulch films, remains poorly understood. This study investigated the absorption, persistence and phytotoxic effects of the herbicide trifluralin (2,6-dinitro-N,N-dipropyl-4-(trifluoromethyl) benzenamine) when combined with macro- (2 × 2 cm) and microplastics (63–500 µm) of either conventional low-density polyethylene (LDPE) or biodegradable poly(lactic acid) – poly(butylene adipate-co-terephthalate) (PLA-PBAT) plastic film. Plastics were incorporated into soil at realistic field application rates (400 kg ha−1) and their effects on maize (Zea mays L.) growth, soil nutrient dynamics, and plant elemental composition were assessed over 45 days. PLA-PBAT plastic absorbed approximately four times more 14C-trifluralin than LDPE, while the 14C-trifluralin mineralisation rate remained low for both PLA-PBAT (ca. 4%) and LDPE (<1%). In the absence of trifluralin, the soil addition of micro- or macro-plastics (orientated vertically or horizontally) had minimal effect on plant growth. However, leaching of trifluralin from contaminated LDPE and PLA-PBAT reduced maize biomass by up to 87%, particularly in the microplastic treatments. The inhibition of root growth from the trifluralin-contaminated plastics led to major changes in soil chemistry (increased NO3-) due to reduced nutrient uptake. These findings suggest that plastic particle size can influence environmental impact, with microplastics posing higher risks as herbicide vectors than larger fragments. Biodegradable plastics may also act as more effective vectors for herbicide absorption than conventional plastics, leading to enhanced phytotoxicity when plastic residues contaminated with herbicides are incorporated into agricultural soils.