A new technique is proposed for measuring ~SN isotope fractionation during N fixation that obviates some of the possible disadvantages of existing methods. Accurate calculation of N fixation by legumes using the ~SN 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 6 ~N a for fixed N until % N fixation calculated by the 15N natural abundance method best matches % N fixation estimated by the ~SN 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 ~SN isotope fractionation factor (-2.37%0) 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 ~~N isotope fractionation during N fixation.