Application de la méthode isotopique à l'étude des bilans azotés en zone tropicale sèche

Abstract : In upland tropical sandy soils of West Africa, the real nitrogen recovery for cereals is much less than we expected because the crops such as pearl millet and maize use respectively 25 and 30 % of applied fertilizer nitrogen according to the farmers' methods recommendation. Nitrogen fertilizer application to the soil increases the soil nitrogen uptake by the crop (this term is named by the author "available nitrogen pool") probably due to the root system extension and the prime effect. A large part of the apptied fertilizer to the soil (about 30 to 50 %) is immobilized by microflora. Ammonia volatilization from urea is studied in greenhouse experiment (simulation of field conditions). It could reach 40 % of the broadcasted N-urea in sandy soil (this technics is the one recommanded for the farmers of Senegal). The nitrogen fixed by groundnut or soybeans represents 20 to 80 % of the N2 uptake by the crops.Changes in the magnitude of this process are important. Certains major factors contributing to the fixation such as the presence of effective specific rhizobium strains, the mineraI soil N, the varieties, the fertilizers N, the manure and liming, were studied. From those results, the author have recommended methods which can improve the nitrogen crop nutrition by controlling the nitrogen balance at the level of the agricultural system. The principles of these methods were tested in situ. The results also show that the nitrogen balance in the upland tropical soil is negative in most cases which often explain the reduction of the total soil nitrogen pool during years of cultivation. Ali these aspects have shown that the use of grain legume is becoming the key of maintaining the nitrogen balance of the upland tropical soil of West Africa.
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Francis Ganry. Application de la méthode isotopique à l'étude des bilans azotés en zone tropicale sèche. Sciences de la Terre. Université Henri Poincaré - Nancy 1, 1990. Français. ⟨NNT : 1990NAN10027⟩. ⟨tel-01747177⟩

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