SUMMARY'^N labelling and variations in natural '''N abundance offer possibilities to quantify tbe importance of mycorrhizas in plant N uptake, Wben atom "" "'X or '"X natural abundance of mycorrhizal and non-mycorrhizal plants are compared in experiments, erne, however, has to account for dilution effects of seed N and the interaction bi'twe-en changes in labfllinE of the available pool and X uptake. This has been considered in studies of N,,-fixation by diazotrophs, but has not befn explicitly corrected for in studies of X uptake by mycorrbizal plants. An example is given, whcrt-ectomycorrhizal plants obtained an atom "o ^''X different {P < 001) from thai of non-mycorrhizal piants, bur where this could be ascribed to dilution effects of seed X and differences in N uptake rates interacting with changes in '"'X labelling, rather than to variations in use of N .sources or to isotope fractionation during uptake. Careful X and '''X budgets based on seed N and sequential har\ests. as well as frequent measurements of atom "" or ahundance of '"X in available pools of X, are thus necessary in studies of this kind. Regressions of '•'N on total X are a useful alternative to comparisons of means of atom ",, only.

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A field experiment was performed in eastern Kenya to estimate N 2 fixation by Sesbania sesban over an 18-month period using the 15 N dilution method. The influence of three reference species, Senna spectabilis, Eucalyptus saligna and Grevillea robusta, on the estimates of N 2 fixation was also assessed. Percentage Ndfa (nitrogen derived from the atmosphere) was calculated based on foliar atom excess (FAE), above-ground atom excess (AAE) or whole tree atom excess (WAE) data. The differences in atom% 15 N excess values between species and plant parts are presented and discussed. We recommend the use of several reference species for estimating %Ndfa and that the different results obtained should be carefully considered in relation to the issues being addressed. In this study, Senna was the most suitable of the three reference species because its N uptake pattern and phenology were very similar to those of Sesbania. When well established, the amount of N fixed by Sesbania accounts for more than 80% of its total N content, according to FAE-based estimates. We estimated the Ndfa by Sesbania after 18 months to between 500 and 600 kg ha À1 , depending on whether FAE, AAE or WAE data were used and on the choice of reference species. The substantial accumulation of N in planted Sesbania highlighted its potential to increase the sustainability of crop production on N-limited soils. We consider the 15 N dilution method to be appropriate for quantifying N 2 fixation in improved fallows in studies, similar to this one, of young trees with high N 2 -fixing ability.

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