Nitrogen isotopes have been applied to trace the flows and fate of N since their discovery at the beginning of the twentieth century. The research study by Norman and Werkman (1943) exemplifies their first agronomic application (Hauck et al. 1994). Of all the different N isotopes, only the stable 15N and 14N, and the radioactive 13N, have been used as tracers in research. While 15N and 14N have been used in routine measurements, radioactive 13N have been restricted to specific environmental and basic biological studies. In addition to its radioactivity, thus requiring operator protection and safe disposal, the half-life of 13N is very short and the isotope must be produced close to the experimental site. However, since its detection limit is much smaller than 15N, the use of 13N is fundamental in some research activities and can lead to the elucidation of processes that otherwise could not have been investigated (Hauck et al. 1994; Knowles and Blackburn 1993).
In this chapter, only the use of stable N isotopes is discussed, since their respective detection limit is low enough to monitor almost all the processes of an agricultural interest, and in particular to study fertilizer N as in the MESCOSAGR project.
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