Biostimulants in Rice: Advancing Nitrogen Management and Sustainability—A Comprehensive Review

Rice is one of the most nitrogen-demanding staple crops, yet a large proportion of applied nitrogen fertilizer is lost via leaching, volatilization, and denitrification, resulting in low nitrogen use efficiency, increased production costs, and environmental pollution. This situation highlights the urgent need for sustainable strategies that improve nitrogen management and enhance stress resilience in rice systems. This review provides a comprehensive synthesis of current knowledge on the role of plant biostimulants in advancing nitrogen management and sustainability in rice cultivation. We first clarify the definition of plant biostimulants in light of recent international regulations and scientific consensus, emphasizing that they are not direct nutrient sources but substances and microorganisms that stimulate natural plant processes to improve nutrient uptake and stress tolerance. Their agronomic and environmental benefits are discussed, with emphasis on improved root system architecture, enhanced nitrogen uptake, assimilation and remobilization, increased photosynthetic efficiency, and the mitigation of nitrogen losses, which collectively contribute to a reduced environmental footprint. A central part of this review summarizes recent evidence on the application of non-microbial biostimulants for improving nitrogen metabolism and abiotic stress tolerance, microbial biostimulants for biological nitrogen fixation and nutrient solubilization, synergistic effects of combined microbial inoculants, and promising alternative and emerging biostimulant materials in rice. Safety considerations and regulatory frameworks governing the use of these products are also addressed. Overall, the literature indicates that biostimulants represent a practical and environmentally sound approach to support nitrogen management, improve stress resilience, and promote sustainability in rice production. Future research should focus on optimizing formulation, dose, and application timing, and on integrating biostimulants with precision nitrogen fertilization under diverse field conditions.