Figure 4.50  Basis of the natural abundance method for assessing the contribution of N₂ fixation to legume nutrition. This method entails measuring plant ¹⁵N/¹⁴N ratio by mass spectrometry. Natural differences in ¹⁵N/¹⁴N ratio between soil and atmospheric nitrogen are exploited. Legumes to the left and right of the figure each have a unique source of nitrogen, while a test plant in the middle relies on both fixed nitrogen and soil inorganic nitrogen. Plants (left) denied a source of inorganic nitrogen (e.g. nitrate) fix atmospheric nitrogen and therefore have low ¹⁵N/¹⁴N ratios. Plants without nodules (right) take up only soil-derived nitrogen and are enriched with ¹⁵N (high ¹⁵N/¹⁴N ratios). ¹⁵N 'signatures' of these two sets of plants can be used to estimate the relative contributions of soil and atmospheric nitrogen as nitrogen sources in the test plant, and therefore to assess the significance of N₂ fixation. (Based on Peoples et al. 1989; reproduced with permission of ACIAR)