Enhancing biological nitrogen fixation in rice paddy ecosystems: challenges, opportunities, and sustainable strategies

Synthetic nitrogen (N) fertilizers have substantially increased rice (<i>Oryza sativa</i>) yields but at the expense of low N use efficiency, significant environmental losses, and deterioration of soil health. Biological N fixation (BNF) offers a sustainable and complementary N source, providing a gradual and plant-synchronized N supply that can partially substitute for synthetic N fertilizers. Enhancing BNF in paddy fields to reduce fertilizer inputs has therefore become a topic of considerable scientific and practical interest. This review synthesizes current knowledge of BNF in rice systems, with emphasis on methods for quantifying BNF rates, the ecological and agronomic factors that regulate its magnitude, and the influence of field management practices. It further highlights key challenges, including the inhibitory effects of synthetic N fertilizers on BNF, that constrain the full realization of BNF potential. And it proposes possible solutions such as straw incorporation, the selection and cultivation of ammonium-tolerant diazotrophs, and the application of genetic engineering to develop ammonium-excreting N-fixing bacteria. Collectively, these insights provide a foundation for advancing low-input and environmentally sustainable rice production systems.