The severity and increase of the Imperata cylindrica constraint as a weed, the decline of the traditional fallow systems as a means of soil fertility management and the lack of inorganic fertilizer appear to have created opportunities for adoption of mucuna (Mucunapruriens) technology by smallholder farmers in some areas in the derived savanna of West Africa. What is not known, however, is the extent to which the establishment and N contribution of mucuna in these areas depend on symbiotic properties such as effective nodulation and mycorrhizal infection. Short term surveys carried out in 34 farmer's arable fields located in four different sites in the derived savanna, southern Benin, West Africa, together with results of greenhouse and field experiments showed that mycorrhizal infection rate of mucuna ranged from 2 to 31% and correlated positively with nodulation and shoot dry matter production. Nodulation occurred in 79% of the fields with numbers of nodules ranging from 0 to 135 plant-1. Mucuna responded both to inoculation and N fertilizer in degraded soils but growth response depended on the rhizobia strains and mucuna varieties. Mucuna accumulated in 12 weeks about 313 kg N ha -1 as either a sole crop or 166 kg N ha -l when mixed/intercropped with maize, respectively. Across all cropping systems it derived an average of 70% of its N from atmospheric N2 (estimates made by the 15N isotope dilution method), representing 167 kg N ha -~ per 12 weeks in the field. Mucuna interplanted with maize obtained a greater proportion of its nitrogen (74%) from fixation than did mucuna grown alone (66%) suggesting that competition for soil N influences the proportion of nitrogen fixed by mucuna. The total amount of N2 fixed per hectare was, however, reduced significantly by intercropping mucuna with maize. A preceding mucuna crop provided a maize yield equivalent to 120 kg N kg ha-1 of inorganic N fertilizer.

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Studies have revealed that some Xylaria species were closely associated with fungus-growing termite nests. However this relationship rarely had been investigated and the host specificity of termite-associated Xylaria was not yet clearly established. Eighteen Xylaria rDNA-ITS sequences were obtained from fungus combs belonging to 11 Macrotermitinae species from eight regions. Low diversity was found between isolates, and nine sequences were retrieved. Termite-associated Xylaria were shown to be monophyletic, with three main clades, all including strains from various termite hosts and geographical localities. This new molecular study shows no species specificity with respect to fungus-growing termites, which suggests that there might be substrate specialization.

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Biological nitrogen fixation (BNF) of 17 soybean cultivars was comparatively estimated by the delta 15N natural abundance technique using two non-nodulation soybeans (Clay and Chippewa) as reference plants. A field study was established on the experimental farm of the University of Abomey-Calavi, Benin on a typical "terre de barre" soil classified by Food and Agriculture Organization-United Nations Educational, Scientific and Cultural Organisation as Rhodic Ferralsol. A nitrogen-free pot trial was also carried out using soil substrate sampled from the Atlantic Ocean beach. In the N-free medium, N content of the whole soybean cultivars ranged from 2.6 to 8.1 mg N per plant compared with an average of 1.8 mg N per plant observed with the non-fixing soybeans. Plant delta 15N of the nodulating soybeans ranged from -2.7756 degrees C (Jupiter) to 0.1951 degrees C (Conquista), while the non-nodulating cultivars Chippewa and Clay had 2.67 degrees C and 9.30 degrees C, respectively. Percentage and amount of N derived from air (Ndfa) were significantly different (P < 0.01) among soybean cultivars, and values depended highly on the selected reference plants. When Clay was used as the reference plant, the average percentage Ndfa was 1.4 times higher than when Chippewa was the reference plant. Both reference plants consistently ranked promiscuous soybean cvs. TGx 1894 3F and TGx 1908 8F as the best cultivars and cv. TGx 1888 29F as the least in percentage Ndfa, suggesting that any of the reference plants could be used in delta 15N method for assessing N-2-fixation. The two identified promiscuous soybean cultivars with greatest capacity to fix N could be included in a soybean extension program for West African farming systems

L'utilisation des champignons mycorhiziens arbusculaires (CMA) améliore durablement la baisse de la fertilité des terres et donc les rendements. Cette étude avait pour but d'évaluer la densité et la diversité des CMA associés à la culture du maïs (Zea mays L.) dans différentes zones agro-écologiques (ZAE) du Bénin. Des échantillons (sols et racines) ont été prélevés dans 38 champs répartis dans sept ZAE. Les propriétés chimiques et la densité des spores des CMA ont été déterminées à partir des échantillons de sols. La diversité des CMA a été déterminée par les indices de diversité. Les échantillons de racines ont servi à déterminer la fréquence et l'intensité de mycorhization. Les résultats obtenus ont montré la présence des CMA dans les ZAE enquêtées. Une variation de la densité en spores des CMA (p<0,05) par zone agro-écologique est observé avec une abondance de spore de 12501,50/100g de sol sec dans la zone cotonnière du nord Bénin (ZAE 2). Quatre genres sont identifiés au niveau de toutes les ZAE. Il s'agit des genres: Glomus, Acaulospora, Gigaspora et Scutellospora avec une dominance des Glomus (52,83%). De plus, il existe une forte corrélation entre les paramètres chimiques et la densité de spore ainsi qu'entre les indices de diversités biologiques. Il ressort de cette étude une abondance et une diversification des CMA associés au maïs dans les différentes ZAE.

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