Microplastics and anammox: Unravelling the hidden threats to nitrogen cycling and microbial resilience

Microplastics (MPs) are emerging contaminants that significantly alter soil ecosystems, particularly by disrupting nitrogen (N) cycling processes. This review assessed the impact of MPs on anaerobic ammonia oxidation (AnAOB) in soil, with a focus on their interactions with key soil parameters such as soil texture, pH, organic matter, heavy metals, and biofilm formation. Microplastics influence N removal efficiency (NRE) by modifying microbial habitats, destabilizing biofilms, and altering enzymatic activities. Their hydrophobic and electrostatic properties facilitate the adsorption of heavy metals, which further inhibit activity of anammox bacteria. The anammox bacteria perform optimally in neutral to slightly alkaline conditions (pH 7.5-8.5), where enzymatic activities remain stable, supporting efficient N removal. In acidic soils (pH < 6.5), MPs undergo increased chemical leaching, releasing toxic additives and heavy metals that impair microbial function. However, in alkaline conditions (pH > 8.5), heavy metals remain bound to MPs, reducing their bioavailability and limiting microbial accessibility to essential nutrients. Organic matter-rich soils enhance microbial resilience but can also shift competition toward denitrifies, potentially increasing nitrate accumulation and nitrous oxide emissions. Biofilms formed on MPs can act as microbial hotspots, stabilizing bacterial populations while reducing anammox efficiency by promoting non-anammox N transformations. The presence of MPs not only enhance anammox activity but also increase the abundance of denitrification genes, contributing to enhanced N reduction processes. These findings indicate that MPs not only alter soil microbial dynamics but also disrupt the delicate balance of the N cycle, leading to reduced NRE and increased greenhouse gas emissions, raising concerns about their long-term impact on soil fertility and ecosystem sustainability.