Longitudinal Nitrate Dynamics in a Large Agricultural Watershed Inferred from δ¹⁵N, δ¹⁸O, ³⁶Cl/Cl, ⁸⁷Sr/⁸⁶Sr, and Geochemical Tracers

Abstract. Nitrogen (N) dynamics in large agricultural, mixed land-use watersheds are rarely investigated. These dynamics are complex due to spatially variable nitrogen sources such as fertilizers, manure, and effluent from wastewater treatment plants (WWTPs). Here, we investigate the longitudinal trends in N sources and processes along the Wabash River, Indiana, USA (watershed area 85,340 km²) using δ¹⁵N and δ¹⁸O_NO3, ³⁶Cl/Cl, ⁸⁷Sr/⁸⁶Sr, and solute concentrations measured from the headwaters to the river mouth. Results show considerable variation in δ¹⁵N and δ¹⁸O_NO3 with increasing distance from the headwaters, while variations in NO₃^- concentration are only present in the Upper Wabash River. Source apportionment using δ¹⁵N and δ¹⁸O_NO3 identifies manure and septic waste as the dominant source of N in the headwaters and at specific locations downstream supported by the presence of poultry and swine farms and WWTPs near the river. A clear denitrification trend is indicated by a strong correlation between δ¹⁵N and δ¹⁸O (R² = 0.9, p = 0.0001) with an O:N fractionation ratio (slope) of 0.7. We infer that denitrification occurs in the river or subsurface rather than in fields or riparian zones. Denitrification is likely obscuring the fertilizer contribution since the δ¹⁵N data are not similar to fertilizers; however, ³⁶Cl/Cl ratios point to an old source of Cl^- such as potash fertilizers. These results show that N dynamics in large agricultural watersheds are complex illustrating the spatial variability in N loading and denitrification processes between the headwaters and river mouth, and that additional studies at large watershed scales are warranted.