The dual role of biochar in modulating microplastic effects on soil nitrogen

Microplastics persist in the environment due to their resistance to degradation and pose an increasing concern for terrestrial ecosystems, threatening soil physicochemical properties and nitrogen (N) cycling. Biochar is widely used to improve soil quality and mitigate contamination. However little is known about how biochar modulates microplastic effects on soil N cycling. Therefore, we conducted a soil incubation experiment to investigate the effects of different polyethylene microplastic particle sizes (6.5, 600 and 1700 μm) on soil N dynamics and to evaluate the combined influence of biochar in microplastic-contaminated soils. Although significant differences among individual microplastic-size treatments were observed at some sampling times, no consistent particle-size dependent effect on soil nitrogen pools was observed throughout the incubation period. However, the addition of biochar significantly increased soil total N, nitrate-N, and alkali-hydrolyzable N contents. Microbial community analysis revealed that the 1700 μm polyethylene microplastic combined with biochar resulted in distinct microbial community structures compared with the control treatment, particularly after 45 days of incubation. Microbial community analysis revealed that the combination of polyethylene microplastics and biochar altered the composition of the dominant bacterial genera, with the greatest community shift observed under the 1700 μm microplastic treatment after 45 days of incubation. Overall, both microplastic presence and biochar amendment altered soil microbial community composition. These findings suggest that corn straw biochar may improve soil nitrogen availability under the experimental conditions of this study and provide insight into microbial responses to polyethylene microplastics. Additional studies across different soil types, biochar feedstocks, microplastic polymers, and field conditions are needed to evaluate the broader applicability of these findings.