Aliou Faye

The Exotic Legume Tree Species Acacia holosericea Alters Microbial Soil Functionalities and the Structure of the Arbuscular Mycorrhizal Community

ABSTRACT The response of microbial functional diversity as well as its resistance to stress or disturbances caused by the introduction of an exotic tree species, Acacia holosericea, ectomycorrhized or not with Pisolithus albus, was examined. The results show that this ectomycorrhizal fungus promotes drastically the growth of this fast-growing tree species in field conditions after 7 years of plantation. Compared to the crop soil surrounding the A. holosericea plantation, this exotic tree species, associated or not with the ectomycorrhizal symbiont, induced strong modifications in soil microbial functionalities (assessed by measuring the patterns of in situ catabolic potential of microbial communities) and reduced soil resistance in response to increasing stress or disturbance (salinity, temperature, and freeze-thaw and wet-dry cycles). In addition, A. holosericea strongly modified the structure of arbuscular mycorrhizal fungus communities. These results show clearly that exotic plants may be responsible for important changes in soil microbiota affecting the structure and functions of microbial communities.

Effect of distance and depth on microbial biomass and mineral nitrogen content under Acacia senegal (L.) Willd. trees

The relations between plants and soil biota involve positive and negative feedbacks between soil organisms, their chemical environment, and plants. Then, characterization of microbial community functioning is important to understand these relations. An experiment was conducted in a field system in the north of Senegal for two years (2005 and 2006) in order to investigate the effect of depth and distance from Acacia senegal tree stem on soil microbial biomass and inorganic-N content. Soils were sampled during dry season (April, T(0)) and wet season (August, T(1)) along transects (R(0), foot tree; R(/2,) approximately 0.50 m distance from the stem; and R, approximately 1 m distance from the stem) and at different layers: 0-25 cm, 25-50 cm and 50-75 cm of A. senegal trees rhizosphere. Total microbial biomass and inorganic-N content were negatively correlated to the distance from tree stem and the depth. The highest values of microbial biomass and mineral nitrogen were found at the foot tree (R(0)) and at 0-25 cm layer. Inorganic-N was mostly in nitrate form (NO(3)(-)) during the dry season. In contrast, during the wet season, inorganic-N was dominated by ammoniac form (NH(4)(+)). Soil total microbial biomass and inorganic-N (NH(4)(+)+NO(3)(-)) were negatively correlated. Our results suggest a positive influence of A. senegal rhizosphere on soil microbial biomass and inorganic-N content.

Effect of Inoculation with Rhizobia on the Gum-Arabic Production of 10-Year-Old Acacia senegal Trees

ABSTRACT The aim of our study was to assess if the inoculation of Rhizobium on mature Acacia senegal trees would have an effect on the gum-arabic yield. A plantation located in Rotto (Department of Linguere Senegal) containing 80 similar 10-year-old trees was identified. The distance between each tree planted was about 5 m. Soil analysis showed soil characteristics were not different within the plantation. The plantation was divided into two parts: one part where the 10-year-old trees were inoculated at the beginning of the rainy season (August 2002 and August 2003) with 1 L of a liquid inoculum prepared with dried alginate beads containing a mixture of selected rhizobial strains (CIRADF 300, CIRADF 301 and CIRADF 302). The taping was done at the same intensity on each tree and when the leaves started to fall. The gum-arabic production was measured for every tree during two years (December 2002/April 2003 and December 2003/April 2004). The rainfall the first year was low, about 250 mm. The effect of the inoculation was assessed in terms of percentage of trees producing gum and amount of gum-arabic produced. Results were not significantly different (P ≤ 0.05). Before the second rainy season, mature trees from the inoculated plot were reinoculated and tree taping accomplished. The annual rainfall the second year was around 440 mm. Data suggested that gum-arabic yield of inoculated trees was significantly higher compared to the noninoculated tree gum-arabic yields. We can assume that the inoculation of 10-year old-trees will enhance the gum-arabic yield under natural conditions.

Differing Courses of Genetic Evolution of Bradyrhizobium Inoculants as Revealed by Long-Term Molecular Tracing in Acacia mangium Plantations

ABSTRACT Introducing nitrogen-fixing bacteria as an inoculum in association with legume crops is a common practice in agriculture. However, the question of the evolution of these introduced microorganisms remains crucial, both in terms of microbial ecology and agronomy. We explored this question by analyzing the genetic and symbiotic evolution of two Bradyrhizobium strains inoculated on Acacia mangium in Malaysia and Senegal 15 and 5 years, respectively, after their introduction. Based on typing of several loci, we showed that these two strains, although closely related and originally sampled in Australia, evolved differently. One strain was recovered in soil with the same five loci as the original isolate, whereas the symbiotic cluster of the other strain was detected with no trace of the three housekeeping genes of the original inoculum. Moreover, the nitrogen fixation efficiency was variable among these isolates (either recombinant or not), with significantly high, low, or similar efficiencies compared to the two original strains and no significant difference between recombinant and nonrecombinant isolates. These data suggested that 15 years after their introduction, nitrogen-fixing bacteria remain in the soil but that closely related inoculant strains may not evolve in the same way, either genetically or symbiotically. In a context of increasing agronomical use of microbial inoculants (for biological control, nitrogen fixation, or plant growth promotion), this result feeds the debate on the consequences associated with such practices.

Symbiotic Status of Two Protected Forests of Acacia nilotica Var. tomentosa in the Senegal River Valley

Soil from two protected forests of Acacia nilotica var. tomentosa, located in the Senegal River Valley, was sampled to assess their rhizobial and mycorrhizal status. Forty-one soil samples (0.5 kg each) were collected at 10–15 cm depth in the rhizosphere of Acacia in the two protected forests of Diarra and Lam Nadie (26 and 15 samples, respectively). Soil extracts were used to test rhizobial nodulation of in-vitro growing seedlings of two varieties of A. nilotica, var. tomentosa and var. adansonii. Arbuscular mycorrhizal (AM) fungi soil infectivity was estimated by the most probable number (MPN) method using Acacia nilotica var. tomentosa as test plant. In the Diarra forest, eight soil samples of the 26 tested contained indigenous rhizobial strains nodulating both A. nilotica var. adansonii and A. nilotica var. tomentosa, whereas only one from Lam Nadie contained rhizobial strains. Fifty-six strains were isolated, purified, and their infectivity and efficiency tested on Acacia nilotica var. tomentosa after 60 days of culture under controlled conditions. Strains ND37 from Diarra and NL2 from Lam Nadie are the more effective in terms of shoot and root dry weight. The estimation of the number of viable propagules using the MPN method showed some significant differences between the populations of AMF of Diarra, which was high, and that of Lam Nadie, which was very low. The percentage of colonization of Acacia nilotica var. tomentosa was correlated with the MPN values. These results are discussed with regards to the state of forests.