An appraisal of recent field measurements of symbiotic N-2 fixation by annual legumes

In this paper we review both the methods for estimating N-2 fixation and the quantity of N-2 fixed by annual crop legumes. Available data on the N difference method for estimating N-2 fixation reveal it to be unreliable. Although we advocate the use of isotope-based techniques their limitations necessitate the simultaneous collection of supporting data in the form of nodulation status, soil mineral N availability, and N yields of non-N-2 fixers. To improve confidence in isotopic techniques for estimating N-2 fixation differences in the form and function of plant roots are highlighted as critical areas for future research, as is a greater understanding of the total N contained in legume roots. A survey of the quantities of N-2 fixed reveals the principal crop legumes to be ranked in the following descending order: soybean, lupin, field pea, faba bean, common bean, lentil, and chickpea. (C) 2000 Elsevier Science B.V. All rights reserved.