Measurement of N₂ fixation by ¹⁵N natural abundance in the management of legume crops: roles and precautions

Abstract

The improved utilization of N₂ fixing components in agricultural production systems is a goal of agricultural research in many countries. Other goals, for example, relate to conservation of biological resources in forests or fragile ecosystems. Evaluation of success in achieving these goals will be aided by more accurate quantitative assessment of the levels of N₂ fixation actually achieved in the field. There are at least 6 reasons to be given in support of this (1). The following relate especially to evaluating N₂ fixation by nodulated legumes, (i) Knowing the magnitude of the input of fixed N [usually as kg N ha^-1 (growth period)^-1] is a basic component of evaluating a production system. (ii) Ecological considerations demand understanding of the relative importance of N₂ fixation in the maintenance of N balance. (iii) Understanding the amount of N₂ fixed by legumes in various cropping sequences allows development of more efficient systems, (iv) When the user is satisfied that nodule development is adequate, measurement of N₂ fixation establishes whether a legume is achieving its potential or establishes success of improvement programs. (v) Measurement allows more reliable assessment of whether N₂ fixation rates are adequate for or a limitation upon crop yield. (vi) Measurement allows proper assessment of the benefits of fixed N (as distinct from indigenous soil N retained during growth of a leguminous crops) for growth of subsequent crops.