Nitrate Stable Isotopes: Tools For Determining Nitrate Sources and Patterns Among Sites With Different Land Uses in the Mississippi River Basin

Water samples were collected from 24 sites in the Mississippi River Basin to determine whether NO3- stable isotopes, d15N and d18O, could provide information about NO3- sources and processes at sites with different land uses, beyond what is possible using discharge and concentration measurements alone. There were five land use categories: large river basins (>34,590 km2) and smaller basins whose predominant land use was either urban, undeveloped, just crops, or crops and animals. At the basin scale, the isotopic signatures of sites with different land uses separated into distinct fields on plots of d15N and d18O. It was also possible to discern NO3- sources and changes in composition through time and as a function of discharge, even at sites with consistently low NO3- concentrations (<2ppm). However, isotopes were less useful in demonstrating microbially transformed nitrate, in part because the NO3- in the rivers was diluted by the large reservoir of soil derived nitrate. d18O analyses were critical in showing abrupt changes in NO3- composition with discharge. Though denitrification is an important process in saturated soils and in smaller, shallow streams, there was no isotopic evidence of denitrification occurring in these larger rivers.