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Microplastics increase nitrogen mobility in soils while biochar regulates NH4+ and NO3- transport: A soil column study
Summary
Tiny plastic particles that build up in farm soil can make it harder for soil to hold onto nitrogen fertilizer, causing more of it to wash away into groundwater, which matters because excess nitrogen runoff can contaminate drinking water sources. The good news: adding biochar (a charcoal-like soil additive made from rice husks) helped soil trap nitrogen even when microplastics were present, suggesting a practical way to protect water quality as plastic pollution in farmland continues to rise.
This study investigated the effects of microplastics (MPs) and biochar (BC) on nitrogen (NH4+ and NO3-) leaching and their vertical distribution in a soil column system. A completely randomized design with three replications was conducted using rice husk biochar (0% and 2% w/w) and microplastics (0%, 1%, and 2% w/w) under controlled leaching conditions. Nitrogen was applied as NH4NO3, and leachates were collected weekly over a six-week period. Results showed that microplastic application significantly increased both NH4+ and NO3- losses from the soil system. The highest total nitrogen leaching occurred in the MP2BC0 treatment, indicating that microplastics impaired soil structural stability and enhanced nutrient mobility. In contrast, biochar application consistently reduced nitrogen leaching across all microplastic levels, demonstrating a strong mitigating effect. This reduction was associated with the high cation exchange capacity and porous structure of biochar. Vertical distribution results revealed that NH4+ accumulated more in the 10-20 cm layer under control conditions, while microplastics enhanced ammonium depletion in both 0-10 cm and 10-20 cm layers. Biochar significantly improved NH4+ retention across both depths. Similarly, NO3- showed strong downward mobility, which was intensified by microplastic application but partially restricted by biochar. Overall, the findings demonstrate that microplastics increase nitrogen leaching and vertical mobility in soil, whereas biochar acts as an effective soil amendment for mitigating these losses under microplastic contamination.