Residual mulch-film characteristics affect heavy metal migration of different soil layers in the subtropical croplands of China

In recent years, as the abundance of residual mulch film (RMF) in agricultural soil continues to increase, whether the adsorption capacity of its surface affects the migration of heavy metals is a topic of current interest for scholars. Herein, this study investigated the distribution of RMF abundance and metal concentration in different soil layers of 75 plastic-mulching croplands in subtropical China; meanwhile, we also explored the associations of RMF characteristics with metal concentration. The results showed that land type, film mulching amount, and film mulching time were the main factors affecting RMF abundance, distribution, and particle size composition. The highest abundance of RMF was found in the garden soils (910 n·kg) with more than 15 years mulching period and more than 19.5 kg hm of annual mulch amount. The lowest abundance of RMF was occurred in the group of field and conservation agricultural land (237 n·kg). Moreover, the concentrations of metals in soil, especially Cd, Cr, Cu, and Pb, were closely related to the extent of RMF contamination in the soil environment. In the 0-10 cm and 10-20 cm soil layers, microplastic abundance exhibited a negative correlation with Cr and Cu concentrations and a positive correlation with Pb concentration. Based on the above findings, it is demonstrated that RMF significantly influences the mobility of metals in soil via adsorption processes, with potential synergistic effects between RMF and heavy metals posing a heightened risk to the soil environment.