An inferential ceiling in nanomaterial-assisted phytoremediation studies: Insights from a semi-systematic review of functional evidence in soil microbial communities

Nano-assisted phytoremediation is increasingly investigated as a strategy to enhance soil remediation, yet the functional role of the soil microbiome in these systems remains largely unresolved. This semi-systematic review, guided by PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) guidelines, critically evaluates how microbial communities are measured and interpreted in nano-assisted phytoremediation studies. Out of 446 screened records, only 24 independent studies simultaneously addressed soil-based systems, plant-microbiome interactions, contaminant stress, and nanomaterial application. We classified the selected studies according to the level of microbial functional evidence provided, the conceptual role attributed to the microbiome, and the integration between microbial data and soil functional measurements. Our synthesis reveals a systematic mismatch between data and interpretation: microbial communities are predominantly characterized through taxonomic profiling or predictive functional tools, while direct validation of microbial processes is almost entirely absent. Despite this, the microbiome is frequently framed as a functional actor in remediation outcomes, based largely on associative or inferred evidence. This recurrent pattern defines an inferential ceiling, understood here as the tendency to derive functional interpretations from taxonomic or proxy-based data without direct validation of microbial processes, thereby limiting mechanistic understanding of microbiome-mediated processes in nano-assisted phytoremediation. We argue that overcoming this limitation requires experimental designs explicitly capable of testing microbial necessity and process-level function, rather than further reliance on compositional or predictive proxies.