Adaptation of soil nitrifiers to very low nitrogen level jeopardizes the efficiency of chemical fertilization in west african moist savannas

The moist savanna zone covers 0.5 x 10(6) km(2) in West Africa and is characterized by very low soil N levels limiting primary production, but the ecology of nitrifiers in these (agro) ecosystems is largely unknown. We compared the effects of six agricultural practices on nitrifier activity, abundance and diversity at nine sites in central Ivory Coast. Treatments, including repeated fertilization with ammonium and urea, had no effect on nitrification and crop N status after 3 to 5 crop cycles. Nitrification was actually higher at low than medium ammonium level. The nitrifying community was always dominated by ammonia oxidizing archaea and Nitrospira. However, the abundances of ammonia oxidizing bacteria, AOB, and Nitrobacter increased with fertilization after 5 crop cycles. Several AOB populations, some affiliated to Nitrosospira strains with urease activity or adapted to fluctuating ammonium levels, emerged in fertilized plots, which was correlated to nitrifying community ability to benefit from fertilization. In these soils, dominant nitrifiers adapted to very low ammonium levels have to be replaced by high-N nitrifiers before fertilization can stimulate nitrification. Our results show that the delay required for this replacement is much longer than ever observed for other terrestrial ecosystems, i.e. >5 crop cycles, and demonstrate for the first time that nitrifier characteristics jeopardize the efficiency of fertilization in moist savanna soils

Two Novel Non-Conventional Seed Oil Extracts with Antioxidant and Antimicrobial Activities

Purpose: To investigate the seed oils of Corchorus olitorius and Hibiscus sabdariffa for their antioxidant and antimicrobial potential. Methods: The physicochemical characteristics of oil of the plant seeds were determined using standard methods while their volatile compounds were analyzed by gas chromatography coupled with mass spectrometery (GC-MS). The antioxidant activity of these oils was determined using 2,2-Diphenyl-1- picrylhydrazyl (DPPH) free radical scavenging method, and their antimicrobial effect was determined by agar dilution method. Results: Phospholipids, carotenoids and phenols contents of the studied seed oils were approximately 2.0 %, 100 mg/100g and 6.7 mg/100g, respectively. GC-MS analysis of the volatile compounds showed the presence of various aromatic and aliphatic organic compounds while the antioxidant activity of the oils was in the range of 0 to 5 g/l, which was higher than that of ascorbic acid. The seed oils also showed antimicrobial activity against Staphylococcus aureus , Aspergillus fumigatus and Trichophyton mentagrophytes with minimal inhibitory concentrations (MIC) of 250.0 and 62.5 mg/ml for C. olitorius and H. sabdariffa seed oils, respectively. Conclusion: C. olitorius and H. sabdariffa seed oils showed pronounced antioxidant and antimicrobial activities. These characteristics should be exploited for possible applications in the food supplement, pharmaceutical and cosmetic industries