MISCONCEPTIONS AND PRACTICAL PROBLEMS IN THE USE OF N-15 SOIL ENRICHMENT TECHNIQUES FOR ESTIMATING N2 FIXATION

The N-15 methods are potentially accurate for measuring N2 fixation in plants. The only problem with those methods is, how to ensure that th e N-15/N-14 ratio in the plant accurately reflects the integrated N-15 /N-14 ratio (R) in soil which is variable in time and with soil depth. However, the consequences of using an inappropriate reference plant v ary with the level of N2 fixation and the conditions under which the s tudy was made. For example, the errors introduced into the values of N 2 fixation are higher at low levels of fixation, and decrease with inc reasing rates of fixation. At very high N2 fixation rates, the errors are often insignificant. Also, the magnitude of error is proportional to the rate of decline of the N-15/N-14 ratio with time. Since N2 fixa tion in most plants would be expected to below 60%, the question of ho w to select a good reference plant is still pertinent. In this paper, we have discussed some of the criteria to adopt in selecting reference plants, e.g. how to ensure that the reference plant is not fixing N2, is absorbing most of its N from the same zone as the fixing plant, an d in the same pattern with time, etc. In addition, we have discussed N -15 labelling materials and methods that are likely to minimize any er rors even when the fixing and reference plants don't match well in cer tain important criteria. The use of slow release N-15 fertilizer or N- 15 labelled plant materials results in slow changes in the N-15/N-14 r atio of soil, and is strongly recommended. Where N-15 inorganic fertil izers are used, the application of the fertilizer in small splits at v arious intervals is recommended over a one-time application. The probl em with the reference crop, which has sometimes discouraged potential users of the N-15 methods, is surmountable, as discussed in this paper .