(1) Foliar nutrient relations of non-N2-fixing and potentially N2-fixing (species found to form root nodules elsewhere) deciduous trees at five savanna woodland sites in Tanzania were compared with data on soil nutrients. The study comprised forty-eight species-site combinations. (2) Nodulation and nitrogenase activity were confirmed in mature specimens of twelve of the fourteen potentially N2-fixing legume species studied. The twelve species belong to the genera Acacia, Dalbergia, Dichrostachys, Entada, Pericopsis, Pterocarpus and Xeroderris. (3) Non-N2-fixing species had, on average, a 0.0007 atom% higher 15N abundance in their leaves than potentially N2-fixing species, which had a N isotope composition close to the atmospheric. The result supported the 15N natural abundance method as an indicator of N2-fixation only in principle, as the method did not allow statements about single species. (4) The N concentration in leaves at the end of the rainy season was, on average, substantially higher in potentially N2-fixing species than in non-N2-fixing species at all sites (2.71% as compared to 1*63% of dry mass). There were no differences in the concentrations of K, Mg, Ca, S, Mn and B between the two groups at any of the sites. (5) Phosphorus in leaves of non-N2-fixing species increased considerably in relation to N, concurrently with a similar change in the soils at four sites with miombo (Brachystegia-Julbernardia) woodland vegetation, while there were no differences between sites in potentially N2-fixing species. (6) The concentrations of nutrients in leaves and the relations between them indicate that N limits growth in non-N2-fixing species at most sites, while P is limiting in N2-fixing species. Comparisons of the relation between P and N in non-N2-fixing and N2-fixing species is proposed as an indicator of the availability of these nutrients in the soil, provided that no other nutrient is limiting.
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Peter Högberg (1986) studied this question.