Abstract 15 N 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 15 N-enrichment or 15 N-natural abundance have assumed a dominant role in quantifying N cycle processes involving BFN. However, it is only through the direct 15 N 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 15 N 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 15 N 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 15 N 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 N 2 O requires further investigation.