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 (delta(15)N and 8180), 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 km(2)) 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. delta(18)O 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 delta(15)N values characteristic of manure, and urban sites tended to have high delta(18)O values characteristic of atmospheric nitrate.