The ecology of nitrogen- fixing symbiosis under arid arid conditions of Kenya

Abstract

The rhizobial populations and nodulation status of both indigenous (mainly Acacia species) and some introduced woody legume species were assessed under glasshouse conditions in soils collected from 12 sites located in different ecological zones of Kenya. The soils were collected on 6 dates spanning a period of 3 years (1989-1992) and at least once every year covering wet and dry seasons. The rhizobial populations among the sites, as estimated by the mostprobable- number (MPN) technique, varied from < 3.6 to > 2.3 X 105 cells g-l soil. There were some intra-site variations in population estimates depending on the trap host species and the method used in sampling the soils. Nodulation also varied across the sites with test species showing frequently higher nodulation ability in native soils. Sesbania sesban (L.) Merr. was the most prolific nodulating species while Acacia tonilis (Forsk.) Hayne was very erratic in nodulation. Most species showed interplant and intraspecific variability within a single soil source. Nodule shapes were consistent within each species irrespective of soil source but nodulation pattern on the root systems differed when grown in axenic conditions. A comparison of nodule internal structure between Acacia polyacantha Willd. and Faidherbia albida (Del.) A. Chev. showed differences in the distribution and sizes of infected cells vis-a-vis uninfected cells. Over 480 isolates were recovered from nodules of test plant species in the various soils. The bulk of the isolates (91%) were watery-, milky-translucent and curdled milk types with moderate to copious extracellular polysaccharide slime (EPS) production. The rest were creamy or white opaque with none or moderate EPS production. They showed a wide range of growth rates on yeast mannitol agar (YMA) plates: very fast-growing (34.3%), fast-growing (47%), intermediate between fast- and slow-growing (8.4%) and slow-growing (10%). Characterization by intrinsic antibiotic resistance (IAR) and salt (NaCl) tolerance showed that Bradyrhizobium was generally more sensitive to antibiotics (streptomycin, kanamycin and ampicillin) and NaCI than Rhizobium. Numerical analyses of the JAR and NaCI sensitivity data showed in most cases, clustering of the isolates according to their cultural and morphological characteristics. There was no apparent relationship of isolates within phenons due to site and date of collection, but isolates from S. sesban tended to cluster within the same phenon. Nitrogen fixation potential as determined by acetylene reduction assay (ARA) ranged from 0.2 + 0.01 (F. albida) to 19.1 ± 1.2 (Sesbania sesban) jlmoles planr1 h-1• The woody legumes were vesicular-arbuscular mycorrhizal (VAM). An intraspecific comparison between two provenances of A. tortilis: Marigat and Mwatate in a native soil (Marigat) showed that the native provenance was inferior in nitrogen fixation attributes (i.e. nodulation and acetylene reduction rates) but superior in phosphorus (P) uptake.. The results in this work portray the major influences of site (habitat), host and microsymbiont in the nitrogen fixation symbioses under the Kenyan arid and semi-arid lands (ASALs) conditions. The ecological and practical implications are discussed in each chapter.