Natural15N abundance as a method of estimating the contribution of biologically fixed nitrogen to N2-fixing systems: Potential for non-legumes

Abstract

The15N abundance of plants usually closely reflects the15N abundance of their major immediate N source(s); plant-available soil N in the case of non-N2-fixing plants and atmospheric N2 in the case of N2 fixing plants. The15N abundance values of these sources are usually sufficiently different from each other that a significant and systematic difference in the15N abundance between the two kinds of plants can be detected. This difference provides the basis for the natural15N abundance method of estimating the relative contribution of atmospheric N2 to N2-fixing plants growing in natural and agricultural settings. The natural15N abundance method has certain advantages over more conventional methods, particularly in natural ecosystems, since disturbance of the system is not required and the measurements may be made on samples dried in the field. This method has been tested mainly with legumes in agricultural settings. The tests have demonstrated the validity of this method of arriving at semi-quantitative estimates of biological N2-fixation in these settings. More limited tests and applications have been made for legumes in natural ecosystems. An understanding of the limits and utility of this method in these systems is beginning to emerge. Examples of systematic measurements of differences in15N abundance between non-legume N2-fixing systems and neighbouring non-fixing systems are more unusual. In principle, application of the method to estimate N2-fixation by nodulated non-legumes, using the natural15N abundance method, is as feasible as estimating N2-fixation by legumes. Most of the studies involving N2-fixing non-legumes are with this type of system (e.g., Ceanothus, Chamabatia, Eleagnus, Alnus, Myrica, and so forth). Resuls of these studies are described. Applicability for associative N2-fixation is an empirical question, the answer to which probably depends upon the degree to which fixed N goes predominantly to the plant rather than to the soil N pool. The natural15N abundance method is probably not well suited to assessing the contribution of N2-fixation by free-living microorganisms in their natural habitat, particularly soil microorganisms.