Molybdenum as a context-dependent lever for crop nitrogen use efficiency: a review

Molybdenum (Mo), an essential cofactor for nitrate reductase and nitrogenase, has demonstrated variable crop responses across soil conditions, crop species, application methods, and environmental stress factors. We conducted a Scopus-based scoping review and evidence mapping of studies published from 2015 to 2025 that evaluated Mo effects on NUE-related agronomic, physiological, biochemical, and symbiotic outcomes. Rather than pooling effect sizes, we applied context-stratified direction-of-effect coding and quality-sensitivity analyses. Of the 495 records identified, 141 reports were retained, and the cleaned dataset comprised 249 coded observations, including 233 route-assigned observations. Mixed responses represented the predominant pattern across all coded observations (114/249; 45.8%), whereas clearly positive responses accounted for 86/249 (34.5%). This imbalance indicates that Mo responses are strongly conditional rather than reliably positive. Mixed responses also persisted after lower-support evidence was excluded (97/215; 45.1%), and no conventional application route showed consistent superiority. More importantly, nano-Mo did not demonstrate a favorable route-level pattern: none of the 13 route-assigned observations was clearly positive, whereas 11 (84.6%) were categorized as mixed. The evidence is therefore insufficient to support field-scale recommendation of nano-Mo. Mo management should be guided by constraint-based diagnosis, route- and dose-specific calibration, and field validation using yield, N uptake, and standardized NUE-related endpoints.