Sustainable management of riverine NO emission baselines.

The riverine NO fluxes are assumed to linearly increase with nitrate loading. However, this linear relationship with a uniform EF is poorly constrained, which impedes the NO estimation and mitigation. Our meta-analysis discovered a universal NO emission baseline (EF = k/[NO ], k = 0.02) for natural rivers. Anthropogenic impacts caused an overall increase in baselines and the emergence of hotspots, which constitute two typical patterns of anthropogenic sources. The k values of agricultural and urban rivers increased to 0.09 and 0.05, respectively, with 11% and 14% of points becoming NO hotspots. Priority control of organic and NH pollution could eliminate hotspots and reduce emissions by 51.6% and 63.7%, respectively. Further restoration of baseline emissions on nitrate removal is a long-term challenge considering population growth and declining unit benefits (ΔN-NO/N-NO ). The discovery of EF lines emphasized the importance of targeting hotspots and managing baseline emissions sustainably to balance social and environmental benefits.