Biofertilizers for Enhanced Nitrogen Use Efficiency: Mechanisms, Innovations, and Challenges

The extensive use of synthetic nitrogen fertilizers has sustained global food production for more than a century but at high environmental and energetic costs. Improving ni-trogen use efficiency (NUE) has therefore become a key objective to maintain produc-tivity while reducing the ecological footprint of agriculture. This review synthesizes current knowledge on the biological foundations of NUE enhancement, focusing on the role of microbial biofertilizers and biostimulants. The main mechanisms through which plant-associated microorganisms contribute to nitrogen acquisition and assimi-lation are analyzed. In parallel, advances in genomics, biotechnology, and formulation science are highlighted as major drivers for the development of next-generation mi-crobial consortia and bio-based products. Particular attention is given to the current landscape of commercial biofertilizers and biostimulants, summarizing the principal nitrogen-fixing and plant growth–promoting products available on the market and their agronomic performance. Moreover, major implementation challenges are dis-cussed, including formulation stability and variability in field results. Overall, this re-view provides an integrated perspective on how biological innovations, market evolu-tion, and agronomic optimization can jointly contribute to more sustainable nitrogen management and reduce dependence on synthetic fertilizers in modern agriculture.