Natural 15N abundance as a method of estimating the contribution of biologically fixed nitrogen to N2-fixing systems: potential for non-legumes

• Published in: Plant and soil 1988-08, Vol.110 (2), p.317-327
• Main Authors: Shearer, G. (Washington Univ., St. Louis, MO (USA). Dept. of Biology), Kohl, D.H
• Format: Article
• Language: English
• Subjects: Agricultural soils; Agronomy. Soil science and plant productions; Atmospherics; AZOTE; Biological and medical sciences; CICLO BIOGEOQUIMICO; Conifers; CYCLE BIOGEOCHIMIQUE; CYCLING; Economic plant physiology; Estimation methods; FIJACION DEL NITROGENO; FIXATION DE L'AZOTE; Fractionation; Fundamental and applied biological sciences. Psychology; Legumes; NITROGEN; NITROGEN FIXATION; NITROGENO; Plants; Soil air; Soil biology; Symbiosis (nodules, symbiotic nitrogen fixation, mycorrhiza...)
• ISSN: 0032-079X; 1573-5036

The ¹⁵N abundance of plants usually closely reflects the ¹⁵N 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. The 15 N abundance values of these sources are usually sufficiently different from each other that a significant and systematic difference in the ¹⁵N abundance between the two kinds of plants can be detected. This difference provides the basis for the natural ¹⁵N abundance method of estimating the relative contribution of atmospheric N₂ to N₂-fixing plants growing in natural and agricultural settings. The natural ¹⁵N 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 in ¹⁵N abundance beween 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 natural ¹⁵N 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). Results 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 natural ¹⁵N 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.