Longitudinal Nitrate Dynamics in a Large Agricultural Watershed Inferred from δ ¹⁵ N, δ ¹⁸ O, ³⁶ Cl/Cl, ⁸⁷ Sr/ ⁸⁶ Sr, 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 δ18ONO3, 36Cl/Cl, 87Sr/86Sr, and solute concentrations measured from the headwaters to the river mouth. Results show considerable variation in δ15N and δ18ONO3 with increasing distance from the headwaters, while variations in NO3- concentration are only present in the Upper Wabash River. Source apportionment using δ15N and δ18ONO3 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.