Differential responses of humic acid in different soil aggregates under microplastic stress

The electron transfer capacity (ETC) of humic substances (HSs) can reflect environmental conditions and the biogeochemical processes of various substances. However, microplastic (MPs) pollution affects the properties of soil, resulting in the structural transformation of HSs. To investigate the impact of MPs concentration (0.25 % low, 2 % medium, 7 % high, dry soil weight), this study analyzed the variation in HSs' electron transfer capacity (ETC) across a gradient of soil aggregate sizes. The results showed that under MP exposure in soil, the changes and mechanisms of HSs' ETC in soil aggregates of different particle sizes were significantly heterogeneous, showing an overall characteristic of "higher in smaller-sized aggregates (clay < 2 μm, silt 2-53 μm)". The effect of MPs is regulated by the concentration-particle size interaction, with distinct patterns across different soil components: clay and fine aggregates exhibit promotion of humification at low concentrations but inhibition at high concentrations; silt suffers the most significant inhibition at medium concentrations; coarse aggregates show promotion of humification at low concentrations, inhibition at medium concentrations, and a slight recovery at high concentrations; for bulk soil, ETC decreases continuously as MPs concentration increases (CK: 1287.73 μmol e⁻/g soil; high concentration: 832.37 μmol e⁻/g soil). The core of ETC is determined by the integrity of aromatic ring conjugation and active sites of HSs. This study systematically reveals for the first time the response mechanisms of the ETC of HA to MPs in soil aggregates with different particle sizes. These research results can help establish the potential link between the ETC of HSs and the particle size of soil aggregates, which not only provides a new dimension for understanding the migration and transformation mechanisms of elements and pollutants in soil, but also offers an important basis for revealing the complex dynamic evolution of soil health and functions.