M. Khouma

Impact of irrigation water quality on soil nitrifying and total bacterial communities

Disturbance induced by two contrasting irrigation regimes (groundwater versus urban wastewater) was evaluated on a sandy agricultural soil through chemical and microbial analyses. Contrary to wastewater, groundwater displayed very high nitrate contents but small amounts of ammonium and organic matter. Despite these strong compositional shifts, soil organic carbon and nitrogen, nitrate and ammonium contents were not significantly different in both types of irrigated plot. Moreover, neither microbial biomass nor its activity, determined as fluorescein diacetate hydrolysis activity, was influenced by irrigation regimes. Bacterial community structure, assessed by denaturing gradient gel electrophoresis (DGGE) of 16S ribosomal DNA fragments, was also weakly impacted as molecular fingerprints shared an overall similarity of 85%. Ammonia-oxidizing bacterial community (AOB) was monitored by DGGE of the functional molecular marker amoA gene (alpha subunit of the ammonia monooxygenase). Surprisingly, no amoA signals were obtained from plots irrigated with groundwater, whereas signal intensities were high in all plots under wastewater. Among the last, compositional shifts of the AOB community were weak. Overall, impact of irrigation water quality on soil chemistry could not be evidenced, whereas effects were low on the total bacterial compartment but marked on the AOB community.

Effect of pearl millet–cowpea cropping systems on nitrogen recovery, nitrogen use efficiency and biological fixation using the 15N tracer technique

Abstract Nitrogen is one of the most limiting nutrients for crop production in many areas of Africa. One strategy to improve yields is to choose crops with high nitrogen use efficiency (NUE) that can produce economic yields under limited water supply. Little information is available on the comparative performance of pearl millet (Pennisetum glaucum L. R. Br.) and cowpea (Vigna unguiculata L. Walp.) in sole crops and intercrops systems for the NUE of applied fertilizers. This study was conducted under field conditions at the Senegal Agricultural Research Institute from July to October 2001. Two cropping systems were used and included sole crops of pearl millet and cowpea and a millet–cowpea intercrop. 15N-labeled urea at rates of 20 kg ha−1 (sole and intercrop cowpea) and 41 kg ha−1 (sole millet and intercrop millet) was applied. Sole millet produced 6,041 kg yield ha−1 and accumulated 95.27 kg N ha−1, of which 15.30% was derived from the nitrogen fertilizer and 84.70% from nitrogen mineralized in soil. Therefore, the NUE of the fertilizer was 36.29%. In intercrop millet, sole cowpea and intercrop cowpea, the NUEs were 15.20, 45.33 and 46.00%, respectively, indicating that the use of nitrogen fertilizer significantly decreased in intercrop only for millet. However, no significant difference was observed between sole and intercrop cowpea relative to the nitrogen derived from N2 fixation. The amount of nitrogen immobilized in the soil was significantly higher in sole millet than in sole cowpea and millet–cowpea intercrop. The Land Equivalent Ratio (LER) of grains and total dry matter showed an advantage of the millet–cowpea cropping system over sole crops.