An overview of contemporary advances in the usage of 15N natural abundance (δ15N) as a tracer of agro-ecosystem N cycle processes that impact the environment

During the past 20 years there have been major advances in the application of N-15 natural abundance (NA) measurements to trace the pathways and magnitudes of N fluxes in the soil-plant-atmosphere continuum. However, estimates are often not quantitative due to the unknown extent of isotopic fractionation during a particular N transformation under study, when other processes compete simultaneously for substrate. Examples are the estimation of N fertilizer use efficiency or the transfer of biologically-fixed N-2 to non-fixing companion species in intercrops or crop sequences. In some cases it has been possible to identify a particular process or source leading to a change in the relative isotopic composition (delta N-15 signature) of a system component, by innovative selection of experimental conditions that isolate the source or process from confounding factors. Nevertheless, there are examples where significant contemporary advances have occurred in the application of NA as a quantitative tracer, such as in the estimation of the symbiotic dependence of a range of N-2 fixing plants. The key is the estimation of isotopic fractionation during N-2 fixation and assimilation, and new knowledge has been obtained on factors contributing to variation, and new approaches devised to obtain more accurate estimates of fractionation. A second example is the innovative application of isotopomer measurements of the potent greenhouse gas N2O that enable presumptive identification of the biological and chemical processes resulting in its production under various agricultural scenarios.