Abstract
A new technique is proposed for measuring 15N isotope fractionation during N fixation that obviates some of the possible disadvantages of existing methods. Accurate calculation of N fixation by legumes using the 15N natural abundance technique requires a value for the isotopic composition of fixed N as an input. Isotopic fractionation in fixed N in legumes has usually been measured using N- free solution culture but results can vary with Rhizobium strain and growth conditions. The proposed method avoids these problems and can be used as an integral part of a field experiment for evaluating N fixation.
The technique is essentially a process of adjusting values of δ15 NFootnote 1 for fixed N until % N fixation calculated by the 15N natural abundance method best matches % N fixation estimated by the 15N enrichment method. The use of high % N fixation values improves the sensitivity and reliability of the method.
A field evaluation of this comparison technique using chickpea (Cicer arietinum L.) provided a 15N isotope fractionation factor (−2.37‰) for fixed N close to that obtained by N-free solution culture methods (−2.10‰). The availability of these two independent techniques allowed mutual corroboration of estimates of 15N isotope fractionation during N fixation.
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Notes
In our calculations one \gd15N unit is an enrichment above the natural abundance value equal to 1/1000 of the natural abundance value for atmospheric N. Atmospheric N is taken as 0.3663 atom % 15N. Therefore 1 \gd15N unit expressed in parts per thousand (\%.) equals 0.3663\sx10\t-3 atom % 15N enrichment.
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Doughton, J.A., Vallis, I. & Saffigna, P.G. An indirect method for estimating 15N isotope fractionation during nitrogen fixation by a legume under field conditions. Plant Soil 144, 23–29 (1992). https://doi.org/10.1007/BF00018841
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DOI: https://doi.org/10.1007/BF00018841