An assessment of nitrogen fixation capability of leguminous trees in Uruguay.
A survey of the nodulating ability of 17 Uruguayan native and sub-spontaneous tree legumes has added Inga uragüensis to the list of legumes known to fix N2. Nodulation and N2 fixation capability of the following species were confirmed: Acacia caven, Calliandra tweediei, Parapiptadenia rigida, Lonchocarpus nitidus, Erythrina crista-galli. Whereas A. caven bore many nodules in all soil types observed, Prosopis nigra and P. affinis were always poorly nodulated. Seedlings of E. crista-galli bore aerial nodules at the base of their stem when grown in waterlogged conditions. The morphology, structure and nitrogenase activity (ARA) of the aerial nodules were similar to root nodules. Nodulation ability of Geoffroea decorticans was not confirmed. The species belonging to the Caesalpinioideae sub-family did not nodulate and were unable to do so even when they were inoculated with a mixture of rhizobia strains, thus confirming previous observations. Nodule morphology was described according to Corby's classification. Anatomical studies showed the presence of spaces in the inner zone of A. caven nodules. E. crista-galli and Enterolobium contortisiliquum nodules exhibited several infected zones, probably related to the coralloid morphology of the nodules. The acetylene-reducing activity of the nodules of the different species was generally low. Rhizobial strains were isolated from the most active nodules for future investigations.