<p>Nitrogen contamination of groundwater in the Jianghan Plain (JHP) in the middle reaches of the Yangtze River is widespread, but the sources and fate of nitrate (NO₃⁻) in the transitional area between plains and hills are poorly understood. This paper characterized the distribution, sources and transformation processes of NO₃⁻ in groundwater in the hilly-plain transition area of JHP through hydrogeochemistry and dual isotopes (δ¹⁵N and δ¹⁸O-NO₃⁻). The hydrogeochemical results showed that the NO₃⁻ in the plain and hilly areas is heterogeneous and controlled by topography. From plain to hilly areas in summer, NO₃⁻ in groundwater has a general trend of decreasing with the direction of groundwater flow. The groundwater in the plain area is strongly affected by anthropogenic activities, and the NO₃⁻ in the multi-level aquifer gradually decreases with increasing depth. Changes in redox conditions caused by river erosion may be responsible for higher NO₃⁻ in groundwater along the eroded banks. The isotopic results showed that NO₃⁻ in the hilly area mainly comes from soil organic nitrogen, which is a natural source. While, groundwater NO₃⁻ in the plain area is strongly affected by livestock manure and domestic sewage, which may be owing to the high density of residents and the impact of anthropogenic activities.</p>