Moisture governs diesel biodegradation in sand soil – polystyrene microplastic have a negligible impact

Global plastic production has reached 450 million tons annually, yet less than 15 % is recycled, with nearly one-third entering the environment, often as microplastics. This study investigates how microplastics influence pollutant behavior and soil microbiome function. We hypothesized that microplastics act as competitive sorbents, interacting with humic acids, which function like sponges, regulating the transient availability of xenobiotics through sorption and desorption. This may reduce their immediate toxicity, thereby facilitating biodegradation by the soil microbiome. To test this, we conducted 365-day microcosm experiments using sandy soil and diesel as the sole carbon and energy source. A characterized bacterial community, known for degradation, was added, alongside varying amounts of PS microplastic. Water content was adjusted to reflect permanent wilting point (PWP), field capacity (FC), and saturation (SAT). PS microplastics showed complete resistance to biological degradation, with minimal surface changes after 12 months. Diesel degradation kinetics were most significantly influenced by water availability. Compared to PWP, microbial communities mineralized diesel 2–7 times more effectively under FC conditions and 5–7 times more effectively under SAT conditions. These results challenge the biodegradability of PS and emphasize that unmodified PS does not significantly affect hydrophobic xenobiotic degradation processes. Water availability was the most decisive factor.