Microplastics and nanoplastics in soil–plant systems: mechanistic evidence, agronomic relevance, and limits of inference

Microplastics and nanoplastics (MPs/NPs) are increasingly reported in agricultural soils, yet their significance for crop production, soil function, contaminant transfer, and food safety remains difficult to evaluate. The central challenge is not merely documenting particle occurrence, but determining when mechanistic observations support defensible agronomic inference. Here, we combine bibliometric mapping of Scopus-indexed literature published between 2015 and 2025 (n = 1815), a structured critical synthesis of representative empirical evidence, and a claim-specific interpretive framework for MP/NP research in soil–plant systems. Bibliometric analysis characterizes the field's structure and thematic development, whereas empirical synthesis examines how evidence supports biological, functional, transfer, and agronomic claims. Bibliometric patterns indicate rapid expansion of the literature, but limited integration of crop-cycle performance, edible-compartment measurements, multitemporal field evidence, and integrated soil–microbiome–plant endpoints. Across the representative evidence synthesized here, responses to MPs/NPs are strongly context dependent and are modulated by polymer identity, particle size, morphology, aging status, soil matrix, exposure metric, exposure duration, crop species, and co-contaminants. Physiological markers, microbial taxonomic shifts, sorption assays, tissue-associated particle signals, and short-term responses are mechanistically informative, but they do not by themselves establish field-level soil degradation, productivity loss, contaminant-vector effects, edible-tissue relevance, or food-safety implications. The proposed framework separates particle occurrence, validated exposure, mechanistic response, microbial functional response, crop performance, contaminant transfer, edible-compartment relevance, and agronomic inference according to the evidence required for each claim. Under this framework, the representative evidence primarily supports mechanistic plausibility and bounded soil–plant interpretation, whereas generalized field-level agronomic claims require stronger analytical validation, exposure realism, endpoint alignment, temporal structure, material realism, and claim-specific evidence. Rather than classifying MPs/NPs as uniformly harmful or harmless, this review defines how evidence should be configured, interpreted, and limited to support transparent, defensible inference in agricultural systems.