AN INDIRECT METHOD FOR ESTIMATING N-15 ISOTOPE FRACTIONATION DURING NITROGEN-FIXATION BY A LEGUME UNDER FIELD CONDITIONS

A new technique is proposed for measuring N-15 isotope fractionation during N fixation that obviates some of the possible disadvantages of existing methods. Accurate calculation of N fixation by legumes using the N-15 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 delta-N-15a for fixed N until % N fixation calculated by the N-15 natural abundance method best matches % N fixation estimated by the N-15 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 N-15 isotope fractionation factor (-2.37 parts per thousand) for fixed N close to that obtained by N-free solution culture methods (-2.10 parts per thousand). The availability of these two independent techniques allowed mutual corroboration of estimates of N-15 isotope fractionation during N fixation.