Improving biological nitrogen fixation through legume nodulating bacteria (LNB) inoculation requires knowledge on the abundance and effectiveness of indigenous population in the ferralsols. The Most Probable Number method (MPN) was used to determine the naturally occurring LNB population in 64 representative soils of four sites in the humid forest zone of Cameroon. Nodulation Macroptilium atropurpureum was examined under tubes experiment in four location sites: Bertoua, Ebolowa, Bokito and Yaounde, and within each of the locations, in four land use systems (LUSs) of different levels of disturbance: mixed farm; fallows; cocoa plantation and forest. The LNB population size varied from 0.78 to 5.25 log units of soil depending on the land use. The most disturbed LUSs (farms and fallows) exhibiting the highest number of LNB on average. Undisturbed ones (cocoa plantation and forest) showed the lowest number of LNB. The percentage of the need for inoculation was more frequent in the forests (94%) and plantations (75 %). It is low in the in the fallows (25%) and the farms (44 %). In total 60 % of the soils analysed need inoculation. These results are important for future investigations on alternative inoculant strains for improving legume production in Cameroon.
The plant rhizosphere microorganisms having the phosphate solubilizing capacity can convert the insoluble soil organic and inorganic phosphates into a soluble form and make the phosphorus (P) available to the plant. With the objective of evaluating the phosphate solubilizing microorganism populations under the rice rhizosphere, soils samples were collected in three locations of two agro-ecological zones of Cameroon and analyzed for their PSMs diversity. Isolation of microorganisms was made on non selective nutrient agar plates and the phosphate solubilizing activity of isolates was tested on National Botanical Research Institute's Phosphate growth medium (NBRIP) amended with sparingly soluble rock phosphates of different origins. The morphological description of isolates allowed evaluating the phosphate solubilizing microorganism's diversity under the rice rhizospheric soil. The most probable number of PSMs ranged between 22 and 53% with an average of 48%, 52.80% and 22.44% for Nkolbisson, Nyokon and Santchou respectively. The 65 isolates obtained from all locations were distributed in four diverse groups. The index of solubilization ranged between 2.70 and 7.24 depending on isolate. From a total of 65 isolates obtained from the three sampling sites, 46 were phosphate solubilizing isolates among which: 20 were of low solubilization, 16 of medium and 10 of high solubilization. This is the first work reporting phosphate solubilizing microorganisms on rice rhizosphere in Cameroon. However, the selection of phosphate solubilizing microorganisms as possible inoculation tools for phosphate-deficient soils should focus on the integral interpretation of laboratory assays, greenhouse experiments as well as field trials.
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