Soil aggregation alterations under soil microplastic and biochar addition and aging process

Soil microplastics (MPs) are a substantial threat to soil health, particularly by disrupting soil aggregation. Additionally, MPs undergo aging processes in the soil, which may significantly alter their long-term impacts on soil structure. To investigate these effects, we conducted an eight-month soil incubation experiment, examining the influence of MPs and their aging on soil aggregation. The experiment utilized a factorial design with various combinations of MPs and biochar additions: 1% by weight of 1000-mesh polyethylene and polypropylene MPs, and 5-mm biochar, resulting in six treatment groups: [CK], [PE], [PP], [Biochar], [PE+biochar], and [PP+biochar]. Our findings revealed that both MPs and biochar underwent aging throughout the incubation, evidenced by the formation of oxygen-containing functional groups on their surfaces. Microplastics, particularly polyethylene, primarily affected the 0.5-1 mm and >2 mm aggregate fractions, with average reductions of 21% and 77%, respectively. These adverse effects intensified with the aging of MPs. Contrary to expectations, the addition of biochar was found to exacerbate the negative impacts of MPs on the 0.25-0.5 mm aggregates, with a decrease of 11% associated with PE MPs. The influence of biochar on mitigating the damage caused by MPs to soil aggregation is dependent on aggregate size.