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University of Bonn (2017)

Managing seasonal soil nitrogen dynamics in inland

Yameogo Louis

Titre : Managing seasonal soil nitrogen dynamics in inland valleys of the West African savanna zone

Auteur : Yameogo Louis

Université de soutenance : University of Bonn

Grade : Doctoral Thesis 2017

Résumé
Most cropping systems in the Dry Savanna agro-ecological zone of West Africa qualify as low-input systems. The use of mineral fertilizers is among the lowest in the world with an average of <10 kg ha1 year-1. Nitrogen is the most limiting nutrient element for crop production in the area, and the prevailing low-input systems rely mainly on the provision of native soil N. Depending on environmental conditions and management practices, the process of soil N mineralization not only provides N for crop nutrition but can also entail substantial N losses. Alternate soil drying and wetting cycles and seasonal changes in the rainfall intensity and distribution reportedly affect soil N dynamics. Associated with changes in the soil aeration status, nitrate-N can be lost by leaching and denitrification, mainly in the period between the first rains and crops establishment, the so-called “dry-to-wet season transition period” (DWT). Besides such temporal dynamics, soil N in the undulating inland valley landscape of West Africa is also subject to spatial fluxes and translocation of water and nitrate along the toposequence. This is likely to exacerbate the intensity of nitrate dynamics, particularly in the bottomlands adjacent to valley slopes, used for producing rainfed lowland rice. Thus, managing soil native N by avoiding (mainly nitrate-N) losses during DWT is key for crop productivity in the short-term and to maintain soil fertility in long-term. Field experiments were conducted in Burkina Faso and Benin in 2013 and 2014 to quantify the intensity and dynamics and to evaluate options for managing seasonal soil nitrate-N in inland valleys of the Dry Savanna zone of West Africa. In addition, factors modulating seasonal N dynamics such as rainfall intensity, soil tillage, and location effects were assessed. With the onset of the first rains and the rewetting of dry bare soil, and depending on the toposequence position, mineralization processes lead to an accumulation of 20-45 kg nitrate-N ha-1 in the topsoil. Initial vertical leaching and subsequent lateral subsurface flows of water from the slopes contributed an additional 10-15 kg of nitrate N ha-1 to the valley bottom wetland. In the absence of vegetation cover, this in-situ N mineralization and nitrate influxes had little effects on the performance of rainfed lowland rice in the valley bottom, indicating the occurrence of substantial N losses and pointing out the need for management approaches that contribute to conserving native soil N for enhancing rice production. The integration of transition season crops (either the leguminous green manures Mucuna pruriens and Vigna unguiculata or the non-N2-fixing grass Panicum maximum) in the lowland could capture and temporarily immobilize soil N, reducing the extractable soil nitrate content to 8-25 from 50-75 kg N ha-1 in the bare fallow control treatment. The resulting N accumulation in the transition season biomass was 41-70 kg ha-1 in panicum and 76-86 kg ha-1 in the legumes, where biological N2 fixation contributed 30-50%. Nitrate-catching vegetation, and particularly N2-fixing cowpea and mucuna, effectively reduced the build-up of native soil Nmin, thus potentially reducing nitrate-N losses and, upon biomass incorporation, enhanced the productivity of wet season rainfed rice with grain yield increases of 1-2 t ha-1 above the bare fallow control (1.7 t ha-1). The extent of such effects strongly depends on environmental conditions and management practices, affecting soil N mineralization and changes in the moisture regime. Thus, soil tillage tended to increase N mineralization and the extent of the nitrate peak during DWT. While a 30% reduced rainfall during DWT increased the nitrate accumulation, the absence of drastic changes in soil aeration status limited apparent nitrate losses. On the other hand, a 30% increased rainfall during DWT lead to a rapid soil saturation and little nitrate remained once the volumetric soil moisture exceeded 25%. Differences in the N-supplying capacity of soil types did affect the extent of the N mineralization, but neither the temporal dynamics nor the grain yield of rice. The reported finding point to the need for management approaches contributing to conserve native soil N for enhancing lowland rice production, such as nitrate-catching vegetation during DWT. The targeting of such approaches, however, is highly site specific and their relevance and effectiveness depend on the speed of change in soil aeration status during DWT and thus on rainfall, valley slope and management attributes, but also on the projected type of climate change

Mots clés  : Dry Savanna zone, Ion exchange resin, Nitrate, Oryza sativa, West Africa.

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Page publiée le 10 novembre 2017, mise à jour le 10 mars 2020