Effects of microplastics exposure on soil inorganic nitrogen: A comprehensive synthesis

Microplastics, a growing environmental concern, impact soil inorganic nitrogen (N) transformation, specifically affecting water-extractable nitrate N (NO₃^--N) and ammonium N (NH₄⁺-N). However, inconsistencies among relevant findings necessitate a systematic analysis. Accordingly, the present meta-analysis addresses these discrepancies by evaluating the effects of microplastics on soil inorganic N and identifying key influencing factors. Our meta-analysis of 216 paired observations from 47 studies demonstrates microplastics exposure causes an overall significant reduction of 7.89% in soil NO₃^--N concentration, but has no significant impact on NH₄⁺-N concentration. Subgroup analysis further revealed effects of microplastics on soil inorganic N were modulated by microplastics characteristics, experimental conditions (exposure time, experimental temperature, plant effects), and soil properties (soil texture, initial soil pH, initial soil organic carbon, soil total N concentration). We found that microplastics exposure above 27 ℃ enhances soil NO₃^--N concentration, a finding linked to specific soil properties and conditions, underscoring the impacts of global warming. Importantly, the microplastics polymer type was the most influential predictor of effects on soil NO₃^--N concentration, while soil NH₄⁺-N concentration was primarily affected by soil texture and microplastics type. These findings illuminate the complex effects of microplastics on soil inorganic N, informing soil management amid increasing microplastics pollution.