Data are presented which establish the reproducibility of 15N:14N mass spectrometer ratio measurements at approximately ± 1 unit of 816N, the per mill 16N enrichment. This degree of reproducibility has been obtained even when the concentration of large volumes of water is required or with nitrogen derived from soil samples. The procedures we follow to achieve this precision are also detailed. A series of shelf chemicals had a range of values of 816N, probably reflecting isotopic fractionation during their manufacture. Commercial fertilizers had values of 816N close to zero and uniformly lower than sources of nitrogen of recent biological origin. While the 816N value of the products of the mobilization of soil N (i.e., NH4+ and NO2-) will probably most often be of interest rather than the values of the total nitrogen, some values of the latter are reported. The 816N of the total nitrogen of soils sampled near Cerro Gordo in the Illinois Corn Belt had values higher than the common commercial fertilizers. Some soils sampled in other locations had significantly lower values than those found in central Illinois.
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