Francis Ganry

Variation in the biological N2 fixation by tree legumes in three ecological zones from the north to the south of Senegal

The utilization of N2‐fixing trees (NFTs) in agroforestry, in semiarid regions, is often limited by weak N2 fixation. This study was conducted to evaluate the N2‐fixing capacity of trees growing under natural conditions for 10 years with neither chemical fertilizer nor inoculum application. For this purpose, natural isotopic N abundance (δ15N) of leaves harvested from NFTs and reference plants (ref) growing in three agroecological zones of Senegal were analyzed. At each site (one site per zone), average δ45N ref was calculated N contents and δ15N values increased from the north in the Sudano‐Sahelian zone (Bambey and Nioro) to the south in the sub‐Guinean zone (Djibelor). The percentage of fixed N2 (%Ndfa) was low at both Bambey (Ndfa ≤ 22%) and Nioro (Ndfa ≤ 39%) and very high at Djibelor (76% δ≤ Ndfa δ≤. 95%). To explain the low levels of Ndfa observed at Bambey and Nioro, the hypothesis of lack of Bradyrhizobium strains in these soils was made. This hypothesis was confirmed by results obtained from a t...

Management of soil organic matter in semiarid Africa for annual cropping systems

Ensuring sustainable agriculture in semiarid Africa requires the implementation of methods to balance nutrients and to conserve soil organic matter (SOM). There is an urgent need to improve the management of all types of SOM input. In this paper; the authors review a wide range of agricultural practices and discuss their advantages, limitations and feasibility. They distinguish ‘traditional systems’ such as traditional fallow, parkland and manuring from ‘improved systems’ such as ‘improved fallow’, forest fallow, alley cropping, cover crops and application of composted manure. Biomass production (BMP) for ‘improved systems’ is mainly linked: (i) for agroforestry, to the tree species used in forest fallow, to the synchronization of nutrient supply by the soil with the cereal demand in alley cropping, and generally to the efficiency of the root system and its development with the depth; (ii) for cover crops, BMP is mainly linked to the initial soil fertility and to the ecological zone: establishment and management of cover crops are not yet fully mastered under some conditions such as an annual rainfall below 800 mm and/or on very clayey soils; (iii) for manure, BMP is mainly linked to the improvement of fallow in order to ensure sufficient forage resources. Because semiarid Africa is mainly a livestock zone, the authors emphasise manure: constraints, quality indicators and tools used to encourage its production, are analysed. Thus it was concluded that the intensification of manure production and its rational use in semiarid African regions, threatened by drought and malnutrition, is very important: this cannot be separated from the production of plant biomass, whose possibilities have been examined above.

Efficiency of nitrogen fertilizer in the sugarcane-vertical system in Guadeloupe according to growth and ratoon age of the cane

Sugarcane is one of the main economic resources of Guadeloupe (France). Cane grown on the islands vertisols shows nitrogen deficiency which is accentuated with each successive ratoon. This deficiency could partially explain the observed decrease in yield. The present study, based on the isotopic N method applied to different ratoons in the field, was aimed at: (i) diagnosing the problem in the crop environment itself; and (ii) quantifying the fertilizer-N balance. The results indicated that decrease in yield and N absorption by the cane was related to ratoon number. The real utilization coefficient for the fertilizer (RUC%) ranged from 6 and 34%, and a high proportion (30–40%) of fertilizer-N was immobilized in the soil (NiS%) after the annual crop cycle. The N absorbed by the cane was essentially derived from the soil. Rainfall at the beginning of (re)growth determined crop development and N supply to the crop. When the water requirements of the crop are satisfied, nitrogen supply and cane yield can be improved in two ways: (i) by increasing the efficiency of the applied N fertilizer (RUC% and NiS%); and (ii) by maintaining the soils capacity to supply cane with N. This implies maintaining and, if necessary, upgrading the structural state of the vertisols.

Carbon Sequestration in a Savannah Soil in Southwestern Burkina as Affected by Cropping and Cultural Practices

Soil organic matter (SOM) plays a dominant role in soil fertility and in the reduction of greenhouse gas emissions. The way in which land is managed directly influences SOM. The objective of this work was twofold: (1) to evaluate the potential storage of C in a plinthic luvisol in southwest Burkina Faso under three different management methods—natural savannah vegetation, continuous cropping without manure and continuous cropping with manure; and (2) to examine the factors (chemical, physical and management) determining C storage in the soil. The methodology used in the field was characterization of the environment by soil mapping, measurement of bulk density, and soil sampling. In the laboratory, the determination of the soil physical and chemical characteristics and measurement of C and N contents and particle size distribution of the SOM were accomplished. The results show that the C content in the top 30 cm was 61 Mg ha−1 under savannah as compared to 16 Mg ha−1 under continuous cropping, with the C being present mainly in the surface layer. Cow manure applied at 2 Mg DM ha−1 yr−1 for 13 years led to an increase of 9 Mg C ha−1. Particle size analysis showed that: (a) the carbon content was highest in the clay size fraction, and (b) the method of soil management mainly influenced the C content of the coarse fractions (200–2000 μm) and the fine fractions (0–20 μm) at the surface.

Litter Production and Soil Fertility in a Vitellaria paradoxa Parkland in a Catena in Southern Mali

Many authors have expressed concerns about the evolution of soil fertility under cotton and cereal cultivation in southern Mali. Nevertheless, farmers have now cropped some of their fields for more than 30 years and still obtain fair yields. Our hypothesis is that the trees (mainly Vitellaria paradoxa) that farmers keep in their cropped fields help to maintain soil fertility. Consequently, we studied the relationships between the trees associated with crops and soil fertility on a sloped catena. On the lower section, 24 trees ha−1 cover 8% of the surface. They produce annually ∼56.7 kg of leaves per tree. Corresponding mineral recyclings, under the crown, are, per m2 and per year: 19 g N; 1 g P; 19 g K; 29 g Ca; 10 g Mg. The soil contains significantly more C and N under the crown of the trees than outside this area. V. paradoxa trees are less numerous on the middle section (3 ha−1) and on the top section (1.5 ha−1) of the catena than on the lower section, and consequently, their effect on the soil is decreasing from the lowlands to the uplands. On the lower section of the catena, the trees have an impact on the fertility of the whole field. Therefore, farmers intensify the cultivation in this section, which is the more favorable section of the catena.

Alley cropping of maize and Gliricidia sepium in the Sudanese Sahel region: some technical feasibility aspects.

An association in an alley cropping experiment of a short-term maize variety and a tree legume (Gliricidia sepium) adapted to the Sudanese Sahel region was studied from the point of view of the nitrogen balance and plot yields. Isotopic labelling applied in the field enabled the contribution to maize nitrogen nutrition from different nitrogen sources (fertilizer, prunings, and soil) to be quantified. For equal cultivated areas, alley cropping provides a maize yield greater than that of maize grown in pure stand without nitrogen fertilizer. However this yield is only 40% of that obtained in pure stands with fertilizer nitrogen. Alley cropping gives a very favourable Land Equivalent Ratio (LER) of 0.90 and 1.58 compared with N-fertilized and non-N-fertilized plots, respectively. These LERs demonstrate the increased biological efficiency of the cultivated soil in an agroforestry system. The percentage of the total nitrogen in the maize coming from prunings (Nfdp) varies between 30 and 35% and the true coefficient of nitrogen utilization of the prunings (TCUp) varies from 15-25%. In the environment of central Senegal, the percentage of total nitrogen of G. sepium coming from N 2 fixation is quite low (Ndffix = 25%), and consequently, in the maize, the nitrogen coming from N 2 fixation (Ndffix) is only 8%. It is therefore necessary to improve the efficiency of nitrogen fixation of G. sepium in this zone to assure the sustainability of the agroforestry system.

Developing Indicators for Soil and Fertilizer Nitrogen Use Efficiency for Maize Using On-Farm Experiments in Burkina Faso

Measurement of recovery of fertilizer N and soil mineral N by maize must be improved in the western semiarid tropics for maximizing crop yield and reducing environmental pollution. The relevant variables for characterizing this N use are the N uptake by the plant from the applied fertilizer (NdfF) and that from the soil (NdfS). The direct measurement of these two variables requires the use of 15 N labeled fertilizer, a difficult and expensive technique. It would therefore be useful to have sensitive indicators for these variables. This study was conducted to measure the variability in the N use from soils. The experiments were carried out with maize at 24 sites in a 3000 ha area of village land with Luvisols located in the central zone of Burkina Faso. Each site had two treatments with two replicates: 0 and 80 kg N ha −1 applied as urea labeled with 15 N. Statistical analysis of the results identified three groups of fields differing in soil fertility, each corresponding to a certain distance from the farmstead, and provided a relationship between the variables NdfF, NdfS, and easily measured indicators. The most pertinent indicators (r 2 >0.9) were: (a) for NdfS, the total N in plant without fertilizer (Nplt0) and the apparent crop recovery of N fertilizer (ACR %), and (b) for Nplt0, Nav (available N) as measured by the Waring Bremner test, exchangeable K, and (c) the clay content. The coefficients of determination improved when the regressions were calculated for each soil fertility group. These results have two important applications: (a) the possibility of estimating easily the part of N taken up by maize that comes, respectively, from the soil and the fertilizer; and (b) the identification of two factors limiting soil fertility, that is, the mineral N and exchangeable K + content of the soil before cropping.