LEGUME TREES AND SHRUBS IN THE TROPICS - N-2 FIXATION IN PERSPECTIVE

The currently known nodulation status of woody members of the three sub-families of the Leguminosae is summarized. Nodulation potential cannot be assumed, even in the Papilionoideae. Nodulating genera show great diversity in nodulation processes, from infection through to final nodule structure. Estimates of N-2 fixation in natural ecosystems are difficult to achieve, but generally suggest low rates. This may relate to limitation of nutrients other than combined N: N-2 fixation should be considered in conjunction with other constraints on growth. Mycorrhizas and cluster roots may be important components in N acquisition, supplementing N-2 fixation as well as enabling uptake of other nutrients, including water. The papilionoid tribe Mirbelieae is used as an example of how Australian legumes appear to be particularly versatile in methods of nutrient acquisition. Great diversity in both host and rhizobia is illustrated in Acacia symbioses and suggests considerable scope for selection and improvement. Before long-term sustainable systems involving nodulated woody legumes can be managed, more information is needed on how natural systems operate.