PERSISTENCE AND RECOVERY OF INTRODUCED RHIZOBIUM 10 YEARS AFTER INOCULATION ON LEUCAENA-LEUCOCEPHALA GROWN ON AN ALFISOL IN SOUTHWESTERN NIGERIA

Establishment of Leucaena leucocephala was poor at Ibadan (Transition forest-savanna zone) and Fashola (savanna zone, 70 km north of Ibadan) in southwestern Nigeria as a result of low soil fertility and the pre sence of only a few native rhizobia capable of nodulating it. Inoculat ion with L. leucocephala at these two locations in 1982 resulted in st riking responses with Rhizobium strains IRc 1045 and IRc 1050 isolated from L. leucocephala grown in Nigeria. The persistence of inoculated effective Rhizobium strains after inoculation is desirable since it re moves the need for reinoculation. Because of the perennial nature of L . leucocephala and its use in long-term alley farming experiments, we examined the persistence of inoculated rhizobial strains after inocula tion, and their ability to sustain N2-fixation and biomass production at Ibadan. In 1992, ten years after Rhizobium introduction, uninoculat ed, L. leucocephala fixed about 150 kg N ha-1 yr-1 or about 41 % of to tal plant N compared to 180 kg N ha-1 yr-1 or 43% measured in 1982. Se rological typing of the nodules using the Enzyme-Linked-Immunosorbent Assay (ELISA) and intrinsic resistance to the streptomycin test reveal ed that most of the nodules (96%) formed on L. leucocephala in 1992 we re by Rhizobium strains IRc 1045 and IRc 1050, which were inoculated i n 1982. Nodules were absent on uninoculated L. leucocephala grown on t he adjacent field with no history of L. leucocephala cultivation. We c onclude that the N, fixed by Rhizobilum strains IRc 1045 and IRc 1050 persisted for many years in the absence of L. leucocephala and sustain ed effectively fixed N, which growth and yield of L. leucocephala afte r several years, thus encouraging a possible low-input alley farming s ystem by smallholder farmers in Nigeria.