The 15N natural abundance method for measuring biological nitrogen fixation: Practicalities and possibilities

The principle and some of the sources of error of the natural 15N abundance method of estimating biological N2 fixation (BNF) are briefly described. It is concluded that care must be exercised in selecting reference plants which have phenology and rooting habits similar to those of the N2 fixing plants. A sampling strategy must be designed which allows determination of within-site variability of the 15N abundance of both reference and N2 fixing plants. It is also concluded that, once the suitability of a site for application of the method is established, the method is capable of providing estimates of BNF that compare favourably with other methods. However, in most cases the natural abundance method is not sensitive enough to trace biologically fixed N from the N2 fixing plant to other N pools. While short term isotopic discrimination presents a certain degree of difficulty in the application of the natural 15N abundance method, long term discrimination not only provides the essential basis for the method, but is also capable of providing additional information about the site of interest. (author). 30 refs, 2 figs, 3 tabs