Nitrogen management to minimize yield-scaled ammonia emission from paddy rice in the Middle and Lower Yangtze River Basin: A meta-analysis.

Paddy fields in China contributed to one third of the global cropland ammonia (NH ₃ ) emission inventory, while rice accounted for half of cereal consumption, necessitating exhaustive considerations of the balance between NH ₃ emissions abatement and food demand. The concept of yield-scaled emission intensity (emissions per unit crop production) has the potential to guide sustainable intensification strategies, yet its application to NH ₃ emissions remains poorly understood. Here, by constructing novel crop-specific models for single rice production and NH ₃ emissions in the Middle and Lower Yangtze River Basin (LYRB) as a case study, the relationships between fertilizer N application and yield-scaled NH ₃ were estimated. Contrary to our hypothesis of a tipping point, our results showed that yield-scaled NH ₃ curves could not directly identify optimal nitrogen (N) rates. However, the benefit of lower N fertilizer rate on NH ₃ abatement consistently outweighed the risk of yield loss. The exponential relationships between yield-scaled NH ₃ and N surplus allowed us to estimate the N surplus criterion as 15.6 kg N ha ^-1 (or 190 kg N ha ^-1 fertilizer N rate) for the LYRB. Under the N surplus criterion, NH ₃ emissions can be reduced by 23-27% without severely impacting rice yield, compared to the N rate required for the highest yield. Moreover, five major controlling factors for yield-scaled NH ₃ were estimated by random forest models, ranked in order of importance as N rate, total N, K rate, mean annual precipitation, and soil organic carbon. Among the agricultural practices (irrigation, tillage, and fertilizer management), deep placement was the most effective measure to reduce yield-scaled NH _3, showing 48% reduction potential, followed by proper N splitting frequency (43%). Overall, this study highlights the efficacy of N application optimization and targeted farm management in mitigating NH ₃ emission while maintaining crop productivity.
Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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