Nodulation of nine nitrogen-fixing tree species grown in central Sudan

Seedlings of Casuarina equisetifolia Forst., C. cunninghamiana Miq., C. cristata Miq., C. glauca Sieb. ex Spreng, c. obesa Miq., C. huegeliana Miq., Acacia senegal Wildl., A. seyal Del. var. seyal Brenan and Prosopis chilensis (Molina) Stunz were grown in forest nurseries in the Sudan or in a growth chamber in order to investigate the presence of Frankia and Rhizobium bacteria in the soils from central Sudan. All the tree species have been introduced from other continents to the Sudan, except A. senegal and A. seyal which are native to Africa. The formation of root nodules on A. senegal, A. seyal and P. chilensis was studied for a period of 12 weeks at a forest nursery in Um Hager, Tendelti (13°01′N and 32°53′E). Prosopis chilensis and Acacia spp. rapidly formed root nodules with Rhizobium spp., which occurred naturally in the different soil mixtures of sand, clay and compost. However, nodulation was irregular. Casuarina spp. did not develop nodules attributable to the actinomycete Frankia when grown in sandy soil samples collected from scrub savanna around the towns of Tendelti and Khartoum, in Nile silt, or in the sand collected from a 10-year-old Casuarina stand. While the experimental conditions may have been unfavourable for infection in the nursery experiments, the results suggest that Frankia did not occur naturally in the experimental soils. Adding NPK or PK fertilizers supplemented with trace elements, or peanut compost, did not induce root nodule formation by Frankia on C. equisetifolia or C. cunninghamiana. Nodulation of A. senegal, as well as Casuarina spp. was unsatisfactory in the investigated soils. The reason for the insufficient nodulation in Casuarina could be the absence of Frankia. Inoculation of Casuarina and the woody legumes seems to be worthwhile in the forest nurseries of El fau and Tendelti. © 1992.