A bacterial method for the nitrogen isotopic analysis of nitrate in seawater and freshwater

We report a new method for measurement of the isotopic composition of nitra te (NO3-) at the natural-abundance level in both seawater and freshwater. T he method is based on the isotopic analysis of nitrous oxide (N2O) generate d from nitrate by denitrifying bacteria that lack N2O-reductase activity. T he isotopic composition of both nitrogen and oxygen from nitrate are access ible in this way. In this first of two companion manuscripts, we describe t he basic protocol and results for the nitrogen isotopes. The precision of t he method is better than 0.2 parts per thousand (1 SD) at concentrations of nitrate down to 1 muM, and the nitrogen isotopic differences among various standards and samples are accurately reproduced. For samples with 1 muM ni trate or more, the blank of the method is less than 10% of the signal size, and various approaches may reduce it further.