Nitrogen pollution in wetland ecosystems has become a pressing global issue in recent years. The effects of redox potential on the environmental behavior of nitrogen remain elusive. In the present work, both field experiments and laboratory analyses have been carried out to characterize the spatial-temporal changes of dissolved inorganic nitrogen (NO₃⁻, NO₂⁻, NH₄⁺) and redox potential in the riparian zones of West Dongting Lake Wetland, China. The stability field of groundwater for inorganic nitrogen was represented in an Eh-pH diagram. It is noted that NH₄⁺ is the most abundant species in groundwater when −500 mV < Eh < +300 mV at pH < 9.23; while under oxidizing conditions (Eh > 400 mV), nitrogen is generally in the form of NO₃⁻. The range of both Eh and pH scales was from −381 to 193 mV and from 6.84 to 7.32, respectively, which involved two types of reduction, dissimilatory nitrate reduction to ammonia and denitrification, and the inhibition of the oxidation pathway by nitrification. In groundwater, NH₄⁺ accounted for 50%–90% of the total concentration of dissolved inorganic nitrogen, in accordance with the Eh-pH diagram. This study reveals the severe contamination of groundwater by NH₄⁺ in the riparian zones of Li and Yuan. It provided a theoretical basis to predict nitrogen pollution preliminarily and facilitate management and protection of wetlands.