Objective Extreme climate events have a significant impact on groundwater quality. However, the sources of the "three nitrogen species" (ammonia nitrogen, nitrite nitrogen, and nitrate nitrogen） and their responses to hydrogeochemical processes under such conditions remain insufficiently understood. Particularly under extreme precipitation and drought conditions, the spatiotemporal distribution characteristics and source apportionment of the "three nitrogen species" have not been sufficiently explored.

Methods This study focuses on the groundwater in the downstream area and the southern branch region of the Ganjiang River Basin in Jiangxi Province. Samples were collected to analyze the hydrogeochemical characteristics under extreme climatic conditions. The Positive Matrix Factorization (PMF) model was employed to identify the sources of the "three nitrogen species". The study compared the concentration changes, spatial-temporal distribution, source contributions, and response mechanisms of the "three nitrogen species" under extreme precipitation and drought conditions.

Results The results indicate that extreme climate events significantly impact the groundwater levels and redox environments in the lower Ganjiang River. During extreme drought, groundwater levels decline substantially and Eh values decrease, indicating a more reduced environment. The groundwater is characterized by an HCO₃－Ca type, with enhanced variability of different ions during both extreme precipitation and drought periods. During extreme precipitation, \mathrmNH_4^+\text-\mathrmN concentrations decrease, while \mathrmNO_3^-\text-\mathrmN concentrations increase in the downstream direction. In contrast, during extreme drought, \mathrmNH_4^+\text-\mathrmN and \mathrmNO_2^-\text-\mathrmN concentrations rise significantly. Source apportionment using Positive Matrix Factorization (PMF) reveals that dissolution of salts (e.g., gypsum, magnesium salts), soil nitrogen transformation (e.g., nitrification and denitrification）, and human activities (e.g., agricultural practices, domestic sewage discharge, and industrial wastewater emissions) are the primary sources of \mathrmNH_4^+\text-\mathrmN , \mathrmNO_2^-\text-\mathrmN , and \mathrmNO_3^-\text-\mathrmN , respectively. Extreme climate events significantly alter the contribution rates of these sources.

Conclusion Under extreme climate conditions, the variations in nitrogen compounds in groundwater are closely related to hydrogeochemical processes. During extreme drought, enhanced evaporation and concentration lead to the accumulation of \mathrmNH_4^+\text-\mathrmN . In contrast, during extreme precipitation, dilution by precipitation recharge causes an increase in \mathrmNO_3^-\text-\mathrmN and a decrease in \mathrmNO_2^-\text-\mathrmN . This study elucidates the sources of the “three nitrogen” in groundwater and their hydrogeochemical response mechanisms under extreme climate conditions, providing theoretical support for regional groundwater pollution control and water resource management.