Nitrate stable isotopes: Tools for determining nitrate sources among different land uses in the Mississippi River Basin

A study was conducted to determine whether NO₃^– stable isotopes (δ¹⁵N and δ¹⁸O), at natural abundance levels, could discriminate among NO₃^– sources from sites with different land uses at the basin scale. Water samples were collected from 24 sites in the Mississippi River Basin from five land-use categories: (1) large river basins (>34 590 km²) draining multiple land uses and smaller basins in which the predominant land use was (2) urban, (3) undeveloped, (4) crops, or (5) crops and livestock. Our data suggest that riverine nitrates from different land uses have overlapping but moderately distinct isotopic signatures. δ¹⁸O data were critical in showing abrupt changes in NO₃^– source with discharge. The isotopic values of large rivers resembled crop sites, sites with livestock tended to have δ¹⁵N values characteristic of manure, and urban sites tended to have high δ¹⁸O values characteristic of atmospheric nitrate.