Analyse du foncionnement d'une parcele de riz irrigué sur sol alcalin. Application à la gestion intégrée de la fertilisation azotée et du calendrier cultural dans le delta intérieur du fleuve Niger (Mali)

摘要

The irrigated perimeters of the “Office du Niger” (Mali), is characterized by a tendency of soils toward alkalization and the variable yield performances of irrigated rice, as influenced by nitrogen fertilization. The objective of this study is to analyze the interactions between the in situ physical/chemical properties of soil and the modes of application of cultural practices such as nitrogen fertilization, rice transplanting and water management, and their influence on the nitrogen use efficiency by the rice plant, rice growth and yield. The operational objective is to identify the rice growing practices that can improve the nitrogen use efficiency in the specific context of alkaline soils. The method combines an agronomic diagnosis in other to identify factors that influence the actual situation of farmer plots and laboratory and field experiments to analyse the identified factors.The agronomic diagnosis done in the field of ten peasants permitted to classify the plots in three groups according to the nature of soils and the floodwater management. If the alkali nature of soil and/or the bad floodwater management practices determined the measured outputs, great yield variability within the same group remained nevertheless influenced by cultural practices. The respect of nitrogen fertilization plan and/or the age of the transplanted rice plants appear to be factors that can explain this variability.The results of laboratory and pot experiments permitted to determine the effects of soil alkalinity and floodwater level on the nitrogen dynamics following urea topdressing. The volatilization process of ammonia is more important on alkali soil and in presence of a floodwater. This process concerns only 20% of the applied nitrogen, while denitrification appears potentially more important in the absence of a floodwater and on a less or non alkali soil. These observations are probably the results of favourable conditions for nitrification in the absence of floodwater and a more microflore activity on non alkaline soil. Without flooding, rice growth in pot is more affected on alkaline than non alkaline soil, depending on the availability of nitrogen and the in situ soil pH.The results of field experiments confirmed the important effect of nitrogen application, soil type, water management and their interactions on N use efficiency and rice yield. The nitrogen use efficiency rate was much higher and resulted in more vegetative growth on alkaline soil, showing less nitrogen losses since irrigation water was well managed. Independently of N nutrition, rice reproductive organs and grain production are affected on alkaline soils. Short draining periods during fertilizer applications does not show significant effects on N use efficiency and yield. In contrast to the working hypothesis, transplanting of old rice plants does not show negative effect on rice production; even though tillering is affected, plants seem to adjust the unfavourable conditions of their new environment.Yield improvement strategies must be sought through nitrogen management practices (reduction in N losses during applications) in non alkaline soils, and reduction in the effects of unfavourable pH on grain production in alkaline soils. In non alkaline soil, the benefit of short drainage during the period of fertilizer application is questionable since N loses are mainly related to nitrification and denitrification processes. It seems then preferable to take into account the high reducing conditions when applying N fertilizer. On alkaline soils, lowering the in situ pH and maintaining flood water at optimal level should be preferred. Cultural practices based on knowledge of soil pH and redox potential would be an ideal solution. A deeper analysis and modelling of soil-water-plant system seem necessary to improve management of the alkali soils of the “Office du Niger”.