Search

Low soil fertility is one of the main constraints to crop production in the West African savanna. However, the response of major cereals to fertilizer applications is often far below the potential yields. Low fertilizer efficiency, inadequacy of current fertilizer recommendations, and the ignorance of nutrients other than N, P, and K may limit crop production. Nutrient limitations to maize production were identified in on-farm trials in Togo and in several long-term experiments in Nigeria and Benin. Maize ear leaf samples were analyzed for macro and micro-nutrients, and the Diagnosis and Recommendation Integrated Systems (DRIS) was applied to rank nutrients according to their degree of limitation to maize. In the on-farm trials, both yield and DRIS results indicated that, when N is supplied, P limited maize production in all fields, reducing yields by 31% on average. Sulfur was limiting in 81% of the fields and was responsible for an average yield reduction of 20%. In the long-term experiments where N, P, and K had been annually applied, Ca and Mg indices were strongly negative, indicative of deficiency. Zn indices were negative in all trials. Despite N-fertilizer additions, N indices remained negative in some of the long-term experiments, pointing to low efficiency of applied fertilizers. There was a direct link between DRIS indices and the management imposed in the different experiments, indicating that DRIS is a useful approach to reveal nutrient deficiencies or imbalances in maize in the region. [ABSTRACT FROM AUTHOR]

Crop response, tree biomass production and changes in soil fertility characteristics were monitored in a long-term (1986-2002) alley-cropping trial in Ibadan, Nigeria. The systems included two alley cropping systems withLeucaena leucocephalaandSenna siameaon the one hand and a control (no-trees) system on the other hand, all cropped annually with a maize-cowpea rotation. All systems had a plus and minus fertilizer treatment. Over the years, the annual biomass return through tree prunings declined steadily, but more drastically forLeucaenathan forSenna. In 2002, the nitrogen contribution fromLeucaenaresidues stabilized at about 200 kg N/ha/year, while the corresponding value forSennawas about 160 kg N/ha/year. On average, the fourLeucaenaprunings were more equal in biomass as well as in amounts of N, P and cations, while the firstSennapruning was always contributing up to 60% of the annual biomass or nutrient return. Maize crop yields declined steadily in all treatments, but the least so in theSenna+ fertilizer treatment where in 2002 still 2.2 tonnes/ha of maize were obtained. Nitrogen fertilizer use efficiency was usually higher in theSennatreatment compared to the control or theLeucaenatreatment. Added benefits due to the combined use of fertilizer N and organic matter additions were observed only for theSennatreatment and only in the last 6 years. At all other times, they remained absent or were even negative in theLeucaenatreatments for the first 3 years. Most chemical soil fertility parameters decreased in all the treatments, but less so in the alley cropping systems. The presence of trees had a positive effect on remaining carbon stocks, while they were reduced compared to the 1986 data. Trees had a positive effect on the maintenance of exchangeable cations in the top soil. Exchangeable Ca, Mg and K - and hence ECEC - were only slightly reduced after 16 years of cropping in the tree-based systems, and even increased in theSennatreatments. In the control treatments, values for all these parameters reduced to 50% or less of the original values after 16 years. All the above points to theSenna-based alley system with fertilizers as the more resilient one. This is reflected in all soil fertility parameters, in added benefits due to the combined use of fertilizer nitrogen and organic residue application and in a more stable maize yield over the years, averaging 2.8 tonnes/ha with maximal deviations from the average not exceeding 21%. [ABSTRACT FROM AUTHOR]

Scanty information on long-term soil organic carbon (SOC) dynamics hampers validation of SOC models in the tropics. We observed SOC content changes in a 16-year continuously cropped agroforestry experiment in Ibadan, south-western Nigeria. SOC levels declined in all treatments. The decline was most pronounced in the no-tree control treatments with continuous maize and cowpea cropping, where SOC levels dropped from the initial 15.4 to 7.3–8.0 Mg C ha-1 in the 0–12 cm topsoil in 16 years. In the two continuously cropped alley cropping (AC) systems, one with Leucaena leucocephala and one with Senna siamea trees, SOC levels dropped to 10.7–13.2 Mg C ha-1. Compared to the no-tree control treatments, an annual application of an additional 8.5 Mg ha-1 (dry matter) of plant residues, mainly tree prunings, led to an extra 3.5 Mg C ha-1 (∼0.2% C) in the 0–12 cm top soil after 11 years, and 4.1 Mg C ha-1 after 16 years. The addition of NPK fertilizer had little effect on the quantities of above-ground plant residues returned to the soil, and there was no evidence that the fertilizer affected the rate of SOC decomposition. The fact that both C3 and C4 plants returned organic matter to the soil in all cropping systems, but in contrasting proportions, led to clear contrasts in the 13C abundance in the SOC. This 13C information, together with the measured SOC contents, was used to test the ROTHC model. Decomposition was very fast, illustrated by the fact that we had to double all decomposition rate constants in the model in order to simulate the measured contrasts in SOC contents and δ13C between the AC treatments and the no-tree controls. We hypothesized (1) that the pruning materials from the legume trees and/or the extra rhizodeposition from the tree roots in the AC treatments accelerated the decomposition of the SOC present at the start of the experiment (true C-priming), and/or (2) that the physical protection of microbial biomass and metabolites by the clay fraction on this site, having a sandy top soil in which clay minerals are mainly of the 1:1 type, is lower than assumed by the model. [Copyright &y& Elsevier]

Cassava, forming starch-rich, tuberous roots, is an important staple crop in smallholder farming systems in sub-Saharan Africa. Its relatively good tolerance to drought and nutrient-poor soils may be partly attributed to the crop's association with arbuscular mycorrhiza fungi (AMF). Yet insights into AMF-community composition and richness of cassava, and knowledge of its environmental drivers are still limited. Here, we sampled 60 cassava fields across three major cassava-growing agro-ecological zones in Nigeria and used a DNA meta-barcoding approach to quantify large-scale spatial variation and evaluate the effects of soil characteristics and common agricultural practices on AMF community composition, richness and Shannon diversity. We identified 515 AMF operational taxonomic units (OTUs), dominated by Glomus, with large variation across agro-ecological zones, and with soil pH explaining most of the variation in AMF community composition. High levels of soil available phosphorus reduced OTU richness without affecting Shannon diversity. Long fallow periods (> 5 years) reduced AMF richness compared with short fallows, whereas both zero tillage and tractor tillage reduced AMF diversity compared with hoe tillage. This study reveals that the symbiotic relationship between cassava and AMF is strongly influenced by soil characteristics and agricultural management and that it is possible to adjust cassava cultivation practices to modify AMF diversity and community structure., (© 2021. The Author(s).)

Increasing use of herbaceous legumes such as mucuna ( Mucuna pruriens var. utilis [Wright] Bruck) and lablab ( Lablab purpureus [L.] Sweet) in the derived savannas of West Africa can be attributed to their potential to fix atmospheric nitrogen (N2). The effects of management practices on N2 fixation in mucuna and lablab were examined using 15N isotope dilution technique. Dry matter yield of both legumes at 12 weeks was two to five times more in in situ mulch (IM) than live mulch (LM) systems. Land Equivalent Ratios, however, showed 8 to 30% more efficient utilization of resources required for biomass production under LM than IM systems. Live mulching reduced nodule numbers in the legumes by one third compared to values in the IM systems. Similarly, nodule mass was reduced by 34 to 58% under LM compared to the IM systems. The proportion of fixed N2 in the legumes was 18% higher in LM than IM systems. Except for inoculated mucuna, the amounts of N fixed by both legumes were greater in IM than LM systems. Rhizobia inoculation of the legumes did not significantly increase N2 fixation compared to uninoculated plots. Application of N fertilizer reduced N2 fixed in the legumes by 36 to 51% compared to inoculated or uninoculated systems. The implications of cover cropping, N fertilization, and rhizobia inoculation on N contributions of legumes into tropical low-input systems were discussed.