Evaluation of nitrogen footprint in double-rice production systems in southern China

The food system is the largest contributor to anthropogenic reactive nitrogen emissions, with excessive emissions posing serious threats to the environment and human health. The rice-producing region of southern China, characterized by intensive nitrogen fertilizer application, has become one of the critical zones for reactive nitrogen emissions. Taking the double-rice cropping areas in nine southern provinces of China as the research object, this study systematically analyzes the spatial-temporal distribution and composition of nitrogen footprint of double-rice by combining the rice yield and farm input in China, and applying the life cycle assessment (LCA) method and nitrogen footprint model. The results show that the nitrogen footprint per unit yield of double-rice in southern China is 7.18 - 10.92 kg(Nr-eq)·kg⁻¹, with a declining trend over time and an annual reduction rate between 0.6% and 2.5%. The spatial distribution exhibits marked regional disparities, which are progressively widening. The nitrogen footprint per unit yield in southern China (Guangdong Province, Guangxi Zhuang Autonomous Region) is significantly higher than that in central China (Hubei Province, Hunan Province). In terms of emission composition, agricultural machinery operation and fertilizer application contribute the most to the potential reactive nitrogen emissions, accounting for 43% and 36% of the potential nitrogen footprint of double- rice, respectively. Among reactive nitrogen forms, NH₃ volatilization represents the highest proportion (94.9%), which may be attributed to the warm and humid climate in southern China and the common practice of surface urea application. The contribution rate of reactive nitrogen emissions from nitrogen fertilizers is exhibiting a clear downward trend year by year. Conversely, the percentage attributable to pesticides and diesel is continuously increasing. Promoting mechanized rice production and optimizing nitrogen fertilizer management are important measures for the green and healthy development of double-rice in China.