An indirect method for estimating super(15)N isotope fractionation during nitrogen fixation by a legume under field conditions

• Published in: Plant and soil Vol. 144; no. 1; pp. 23 - 29
• Main Authors: Doughton, JA, Vallis, I, Saffigna, P G
• Format: Journal Article
• Language: English
• Published: 01-01-1992
• Subjects: Cicer arietinum
• ISSN: 0032-079X

A new technique is proposed for measuring super(15)N isotope fractionation during N fixation. Accurate calculation of N fixation by legumes using the super(15)N 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 super(15)N super(a) for fixed N until % N fixation calculated by the super(15)N natural abundance method best matches % N fixation estimated by the super(15)N 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 super(15)N isotope fractionation factor (-2.37 ppt) for fixed N close to that obtained by N-free solution culture methods (-2.10 ppt).