Reference Plant for the Assessment of fixation by N Isotope Dilution Technique

Abstract

Reference plants or ‘standard’ plants play an important role in the assessment of nitrogen fixation by the ¹⁵N isotope dilution technique, in both legumes and non-legume nitrogen fixing plants. The principle of this technique is based on differences in dilution of ¹⁵N isotope between the N₂ fixing plant and the reference plant. In practice both plants receive the same¹⁵-N-labelled fertilizer under the same growing conditions. Throughout growth the reference plant obtains nitrogen from the soil (¹⁴N) and labeled fertilizer (¹⁵N), while the N₂ fixing plant obtains nitrogen from the soil (¹⁴N), labeled fertilizer (¹⁵N) and atmosphere (¹⁴N) throng fixation. There will be dilution of ¹⁵N isotope will be observed in the N₂ fixing plant. From the higher percentage of ¹⁵N in the reference plant relative to the N₂ fixing plant, and the total dry matter and total N content of the N₂ fixing plant, the amounts of N₂ fixed can be calculated. However, the precision of this technique depends to a great extent on the reference plant selected. The evaluation of potential reference plants for particular soil climatic conditions is important for the value and accuracy of the technique.