Biochar mitigates microplastic- and nanoplastic-induced soil degradation and plant stress: a global meta-analysis.

Microplastic (MP) and nanoplastic (NP) contamination threatens soil functioning and plant performance, yet the effectiveness of biochar in mitigating these impacts varies substantially among experimental conditions. We conducted a meta-analysis of 59 peer-reviewed studies comprising 1418 effect-size estimates to quantify biochar-mediated responses across the soil-plant continuum. Effect sizes were calculated as the natural logarithm of the response ratio (lnRR) and analyzed using multilevel random-effects models. Subgroup analyses examined the effects of growth medium, plant functional group, plastic type, biochar feedstock, application rate, treatment duration, pyrolysis temperature, and experimental system, while meta-regression identified continuous moderators of response magnitude. Biochar produced positive pooled responses in soil properties (lnRR = 0.130), soil biological properties (0.186), biomass yield (0.218), photosynthetic performance (0.326), and plant nutrient content (0.599). Oxidative-damage indicators declined, including malondialdehyde (lnRR = - 0.154) and H2O2 (- 0.150), while plant microplastic accumulation showed a pronounced negative response (lnRR = - 0.811). The magnitude of these responses varied significantly among environmental and management contexts. Biochar application rate was the only significant continuous moderator, whereas pyrolysis temperature and treatment duration were not significant linear predictors of overall effect magnitude. Stronger responses in controlled systems than in more environmentally complex systems further indicated a potential gap between experimental efficacy and field-scale performance. Overall, the available evidence indicates that biochar is associated with improvements across multiple soil and plant response domains under MP/NP stress, although the magnitude of these responses is strongly context dependent. These findings provide a quantitative basis for developing context-specific biochar strategies while highlighting the need for geographically broader and long-term field validation.