The use of variation in the natural abundance of 15N to assess symbiotic nitrogen fixation by woody plants

Deeply rooted woody plants capable of symbiotic N2 fixation are often dominant components of plant communities. The importance of symbiotic N2 fixation to the N economy of these plants and to overall ecosystem productivity is poorly understood. This is a consequence of technical difficulties in detecting and/or measuring N2 fixation under field conditions using conventional approaches (i.e., acetylene reduction assay). The utility of the natural 15N abundance approach to assess symbiotic N2 fixation is discussed by Shearer and Kohl in Chapter 20 of this volume. This approach is analogous to isotopic dilution methods widely used in agricultural studies except that it takes advantage of small deviations in the natural 15N abundance of soil from that of the atmosphere and, therefore, does not require isotope application to the soil.