Misconceptions and practical problems in the use of 15N soil enrichment techniques for estimating N2 fixation

Abstract

The ~SN methods are potentially accurate for measuring N 2 fixation in plants. The only problem with those methods is, how to ensure that the ~SN/14N ratio in the plant accurately reflects the integrated tSN/14N ratio (R) in soil which is variable in time and with soil depth. However, the consequences of using an inappropriate reference plant vary with the level of N 2 fixation and the conditions under which the study was made. For example, the errors introduced into the values of N 2 fixation are higher at low levels of fixation, and decrease with increasing rates of fixation. At very high N 2 fixation rates, the errors are often insignificant. Also, the magnitude of error is proportional to the rate of decline of the 15N/14N ratio with time. Since N 2 fixation in most plants would be expected to below 60%, the question of how 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, and in the same pattern with time, etc. In addition, we have discussed ~SN labelling materials and methods that are likely to minimize any errors even when the fixing and reference plants don't match well in certain important criteria. The use of slow release ~SN fertilizer or 15N labelled plant materials results in slow changes in the ~SN/~4N ratio of soil, and is strongly recommended. Where L~N inorganic fertilizers are used, the application of the fertilizer in small splits at various intervals is recommended over a one-time application. The problem with the reference crop, which has sometimes discouraged potential users of the 15N methods, is surmountable, as discussed in this paper. * Figures in parenthesis represent relative ranking made using each method.

Short-term Nitrogen Fixation by Legume Seedlings and Resprouts After Fire in Mediterranean Old-fields

“…In the present study, two independent estimates for N derived from non-atmospheric sources gave similar %Ndfa estimates. In addition, the narrow range of %Ndfa's which were calculated independently, using different non-legumes, corroborate the robustness of the isotope dilution method to reference plant selection in non-uniformly labelled soil when N 2 -fixation is strong (Danso et al 1993;Busse 2000). The cumulative net N mineralisation during the post-fire nine months (2.6 g Nm À2 in the top 5 cm of soil) was comparable with amounts reported for tallgrass prairies after fire (Ojima et al 1994;Blair 1997).…”

Symbiotic nitrogen fixation in eight Acacia senegal provenances in dryland clays of the Blue Nile Sudan estimated by the 15N natural abundance method

“…These two N sources often differ in 15 N abundance whereby an N-fixing plant, which depends on both soil N and symbiotic N 2 fixation, is less abundant in 15 N than a non-N 2 -fixing plant grown at the same site (Danso et al, 1993;Evans, 2001;Handley and Scrimgeour, 1997;Shearer and Kohl, 1986). Plants dependent on soil N have positive d 15 N values that are close to those of soil nitrogen.…”

Nitrogen use efficiency of 15N-labelled sheep manure and mineral fertiliser applied to microplots in long-term organic and conventional cropping systems

“…At harvests a part of root biomass of winter wheat and soybean was collected down to about 10 cm and for maize down to 18 cm using a small spade to loosen the soil such that roots could be removed. Due to differences in absorption rate and translocation of N, 15 Nenrichment is known to differ in different plant parts, which may limit accurate determination of 15 N excess of the whole plant (Danso et al 1993). Therefore, winter wheat was divided into stem, leaves, chaff, grains and roots, soybean into stem, leaves, pod, grains and roots, and maize into stem, leaves, tassel, silk, husk, cob, grains and roots.…”