How is measurement of nitrogen fixation by the acetylene reduction assay (ARA) by gas chromatograph done?
The method you refer to can be used as a qualitative measure for nitrogen fixation only. It has long been abandoned to arrive at quantitative estimates.
For details, I refer to an online publication you can download for free:
Unkovich M, Herridge D, Peoples M, Cadisch G, Boddey B, Giller K, Alves B, Chalk P. 2008. Measuring plant-associated nitrogen fixation in agricultural systems: Australian Centre for International Agricultural Research (ACIAR).
The method you refer to can be used as a qualitative measure for nitrogen fixation only. It has long been abandoned to arrive at quantitative estimates.
For details, I refer to an online publication you can download for free:
Unkovich M, Herridge D, Peoples M, Cadisch G, Boddey B, Giller K, Alves B, Chalk P. 2008. Measuring plant-associated nitrogen fixation in agricultural systems: Australian Centre for International Agricultural Research (ACIAR).
It depends on how much accuracy in attribution you are aiming at. There are whole system measurements that take into consideration the nitrogen flow between atmosphere and plant and soil system (nitrogen balance) and those that only look at such as the nodule counting. I would prefer to isolate a part of the field for a closed nitrogen balance measurements for accuracy.
Isotope dilution technique is the method of choice that provides accurate quantitative assay of N fixation (same for fertilizer labeled N15 uptake). It is based on the relative abundance of N15:N14 ratios in the air compared to that in the soil. To improve the sensitivity of measurement, the soil N15 abundance can be enriched.
As mentioned by Dr Lambers, acetylene provides an indicative and qualitative assay of biological fixation. As mentioned by others, it is also recommended to work out the N balance (often included in similar N studies) to ascertain the accuracy of isotope dilution technique by mass balance which implicitly assumes that there is no discrimination in isotopic N uptake by the plant. But one should be realized that the form of N (NO3, NH4, R-NH2) affects plant "preference" which adds an artifact in isotope assay as a result of in which forms N15 and N14 are present predominantly in thesoil.
15N2-feeding technique in my opinion is the best way to accurately assessing the amount of BNF.But the problem is the price of 15N2 and the control system of CO2, temp and O2 in the chamber. However, unlike ARA and N balance, the results is absolutely non-questionable. See my paper.
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