15N2 as a tracer of biological N2 fixation: A 75-year retrospective

N-15(2) has played a crucial role in fundamental studies of biological N-2 fixation. However, due to operational constraints, it has more often served as a qualitative rather than a quantitative tracer of biologically-fixed N (BFN). Therefore indirect methods based either on N-15-enrichment or N-15-natural abundance have assumed a dominant role in quantifying N cycle processes involving BFN. However, it is only through the direct N-15(2) approach that biological N-2 fixation can be traced through the various components of the soil-plant system. Technological advances in the automated control of the chamber environment have made the N-15(2) technique more attractive to long-term studies. Thus the need to enclose plants in a chamber and maintain conditions conducive to plant growth should no longer be seen as a major obstacle to the use of N-15(2). The way is now open to evaluate the efficacy of indirect methods used to estimate the contribution of BFN to the N economies of crop and pasture systems, and the dynamics of BFN in agroecosystems. In addition, new applications of N-15(2) such as stable isotope probing are emerging, which have the potential to characterize non-cultivated diazotrophs in a range of environments. The role of biological N-2 fixation in the formation of reactive N in the environment and its relationship with the emission of the greenhouse gas N2O requires further investigation. (C) 2016 Elsevier Ltd. All rights reserved.