Estimation of legume symbiotic dependence: an evaluation of techniques based on 15N dilution

An 15N isotope dilution technique, first proposed more than 40 years ago, is now widely used to estimate of the proportion of legume N derived from biological N2 fixation. The proposed attributes of the method are critically examined. The problems associated with the non-uniform distribution of applied isotope and the traditional use of a non-fixing reference plant are elaborated. The use of models which do not require a reference plant is a relatively recent development in the evolution of the methodology. The characteristics and efficacy of the modelling approach are reviewed.