Researching legume nitrogen fixation module
Measuring legume N fixation
Measuring legume N fixation
Several different methods have been used over the years to measure legume N fixation and they all have certain limitations. The 15N natural abundance method was used for these studies. This method relies on the fact that the N in the atmosphere has a different fingerprint than the N in the soil. This fingerprint is based on the relative abundance of the two naturally-occurring N isotopes, 15N and 14N. The fingerprint of the air is very constant. By comparing the N in a non-fixing plant that has the N fingerprint of the soil to the N in a legume, we can calculate what percentage of the N in the legume came from the soil and what percentage came from the air.
Figure 1. For soybeans, a mutant variety that does not develop nodules was used as the non-fixing plant. Nitrogen fixation is measured by comparing the composition of the N in the non-fixing plant to the N in the fixing plants.
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Figure 3. A) Total soil nitrogen increased and, B) soybean nitrogen fixed decreased with time under organic management. Total soil N is highly correlated with total soil organic matter. |
Oats were used as the non-fixing plants to measure N fixation in the field peas. Grasses are not an ideal non-fixing plant to use because they have different growth and rooting patterns than the legumes. Non-nodulating mutants do not exist for most legumes as they do for soybean.
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