Effet de la mise en culture des forêts secondaires sur les peuplements de macroinvertébrés du sol dans la zone de contact forêt-savane de Côte d'Ivoire

L'impact de la mise en culture des forêts sur les peuplements de macroinvertébrés du sol a été étudié en moyenne Côte d'Ivoire avec pour objectif d'identifier les plus sensibles à la perturbation des forêts. Les macroinvertébrés du sol de parcelles de forêt secondaire et de cultures vivrières ont été échantillonnés en utilisant la méthode TSBF. A l'exception de la jachère, la mise en culture de la forêt secondaire a entraîné une réduction de la densité de 76,2 %, 22,1%, 22,1 %, 0,92 %, respectivement dans les champs de maïs, d'arachide, de riz et de bananiers plantain. Une Analyse en Composantes Principales (ACP) a montré que le «type de culture» et «l'effet du feu» sont susceptibles d'être les principaux facteurs qui déterminent l'abondance et la distribution des macroinvertébrés. Par ailleurs, les Fourmis, les Diplopodes, les Vers de terre, et les Coléoptères, se sont avérés comme les macroinvertébrés les plus sensibles à la dégradation des forêts.The impact of cropping systems in the secondary forest area on soil macro-invertebrates was investigated in middle Côte d'Ivoire with the aim of identifying the most sensitive organisms to forest perturbation. Soil macro-invertebrates from different cropping systems were sampled using TSBF methodology and their abundance were compared to the secondary forest referred as control. Except fallow, the conversion of secondary forest has reduced the density of macro-invertebrates at 76.2 %, 22.1 %, 22.1 %, 0.92 %, under maize, groundnut, rice and plantain banana fields respectively. A Principal Component Analysis (PCA) showed that the type of land-use and fire are factors controlling abundance and distribution of soil macro-invertebrates. In addition, ants, diplopoda, earthworms and coleoptera are likely to be the most sensitive organisms to forest perturbation. Keywords: Macroinvertébrés du sol, forêts, cultures vivrières, bio-indicateurs/Soil macro-invertebrates, forests, food crops, bio-indicatorsSciences & Nature Vol. 4 (2) 2007: pp. 197-20

Modeling the contribution of ecological agriculture for climate change mitigation in cote d'Ivoire

The use of crop models is motivated by the prediction of crop production under climate change and for the evaluation of climate risk adaptation strategies. Therefore, in the present study the performance of DSSAT 4.6 was evaluated in a cropping system involving integrated soil fertility management options that are being promoted as ways of adapting agricultural systems to improve both crop yield and carbon sequestration on highly degraded soils encountered throughout middle Côte d’Ivoire. Experimental data encompassed two seasons in the Guinea savanna zone. Residues from the preceding vegetation were left to dry on plots like mulch on an experimental design that comprised the following treatments: (i) herbaceous savanna-maize, (ii)10 year-old of the shrub Chromolaena odorata fallow-maize (iii) 1 or 2 year-old Lalab pupureus stand-rotation, (iv) the legume L. pupureus -maize rotation; (v) continuous maize crop fertilized with urea; (vi) continuous maize crop fertilized with triple superphosphate; (vii) continuous maize crop, fertilized with both urea and triple superphosphate (TSP); (viii) continuous maize cultivation. The model’s sensitivity analysis was run to figure out how uncertainty of stable organic carbon (SOM3) can generate variation in the prediction of soil organic carbon (SOC) dynamics during the monitoring period of two years, within the first soil layer and to estimate the most suitable value. The observed variations were of 0.05 % in total SOC within the short-term and acceptable dynamics of changes were obtained for 0.80% of SOM3. The DSSAT model was calibrated using data from the 2007-2008 season and validated against independent data sets of yield of 2008-2009 to 2011-2012 cropping seasons. After the default values for SOM3 used in the model was substituted by the estimated one from sensitivity analysis, the model predicted average maize yields of 1 454 kg ha-1 across the sites versus an observed average value of 1 736 kg ha-1, R2 of 0.72 and RMSE of 597 kg ha-1. The impact of fallow residues and cropping sequence on maize yield was simulated and compared to conventional fertilizer and control data using historical climate scenarios over 12 years. Improving soil fertility through conservation agriculture cannot maintain grain yield in the same way as conventional urea inputs, although there is better yield stability against high climate variability according to our results

Controls on timescales of soil organic carbon persistence across sub-Saharan Africa

Given the importance of soil for the global carbon cycle, it is essential to understand not only how much carbon soil stores but also how long this carbon persists. Previous studies have shown that the amount and age of soil carbon are strongly affected by the interaction of climate, vegetation, and mineralogy. However, these findings are primarily based on studies from temperate regions and from fine-scale studies, leaving large knowledge gaps for soils from understudied regions such as sub-Saharan Africa. In addition, there is a lack of data to validate modeled soil C dynamics at broad scales. Here, we present insights into organic carbon cycling, based on a new broad-scale radiocarbon and mineral dataset for sub-Saharan Africa. We found that in moderately weathered soils in seasonal climate zones with poorly crystalline and reactive clay minerals, organic carbon persists longer on average (topsoil: 201 ± 130 years; subsoil: 645 ± 385 years) than in highly weathered soils in humid regions (topsoil: 140 ± 46 years; subsoil: 454 ± 247 years) with less reactive minerals. Soils in arid climate zones (topsoil: 396 ± 339 years; subsoil: 963 ± 669 years) store organic carbon for periods more similar to those in seasonal climate zones, likely reflecting climatic constraints on weathering, carbon inputs and microbial decomposition. These insights into the timescales of organic carbon persistence in soils of sub-Saharan Africa suggest that a process-oriented grouping of soils based on pedo-climatic conditions may be useful to improve predictions of soil responses to climate change at broader scales

Stakeholder collaboration in climate-smart agricultural production innovations: insights from the Cocoa industry in Ghana

Although collaboration is vital in addressing global environmental sustainability challenges, research understanding on stakeholder engagement in climate-smart production innovation adoption and implementation, remains limited. In this paper, we advance knowledge about stakeholder collaboration by examining the roles played by stakeholders in scaling up ecological sustainability innovations. Using the illustrative context and case of green cocoa industry in Ghana, the analysis identified three distinctive phases of stakeholder engagement in ecological sustainability innovations implemented from 1960-2017. We highlight defining periods of ecological challenges encompassing the production recovery sustainability initiative phase solely driven by the Ghana Cocoa Board (COCOBOD)–a governmental body responsible for production, processing and marketing of cocoa, coffee and sheanut. During the period, major initiatives were driven by non-governmental organisations in collaboration with COCOBOD to implement the Climate-Smart agriculture scheme in the cocoa sector. The findings have implications for cocoa production research and stakeholder collaboration in environmental innovations adoption

Changes in soil quality after subsequent establishment of Chromolaena odorata fallows in humid savannahs, Ivory Coast

In the buffer zone of the Lamto Reserve, a protected area located in the forest-savannah transitional zone in central Ivory Coast, the exotic shrub Chromolaena odorata has invaded abandoned fields and degraded forests. As a result, they have turned into thickets that are used by local farmers as natural fallows to enhance soil fertility for food production. However, information about their impact on soil is scanty and investigations focusing exclusively on baseline data on C odorata fallows in humid savannahs are still lacking. This study was designed to assess changes in soil physical, chemical and biological properties after the establishment of C. odorata fallows in humid savannahs of Ivory Coast. Sampling sites were selected such that a portion of C odorata fallow was located next to the shrub savannah, the most common natural ecosystem in the area. Results revealed a rise of soil organic carbon (+27.9%), total nitrogen (+36.7%), total phosphorus (+56.8%), extractable calcium (+68.3) and magnesium (+1403%) in the first 10 cm of soil beneath the C odorata fallow relative to the savannah. Furthermore, the fallow was associated with high N-mineral pool as N-NO3- and N-NH4+ content increased at +72.5% and +71.5%, respectively. The infiltration capacity of water under C odorata-based fallow was markedly high and soil macroinvertebrates, mainly earthworms, showed significant increases in density and biomass. The large quantity of good quality standing biomass produced by C odorata is likely the main factor controlling the improvement of soil quality. The results suggest that there are merits for the integration of C odorata fallow in a cropping system for sustainable food production in the buffer zone of Lamto Reserve. This can also help to reduce pressure on forest islands

Gradual enrichment of N-15 with humification of diets in a below-ground food web : relationship between N-15 and diet age determined using C-14

1. Stable nitrogen (N) isotope has been widely used to disentangle food webs and to infer trophic positions of organisms based on an assumption that the stepwise enrichment occurs along trophic levels. The enrichment of N-15 in soil organisms with diet humification has also been reported, but the underlying mechanism has not been fully examined. 2. To examine the effect of diet humification on N-15, we estimated the stable N isotope ratios and diet ages of earthworms and termites. These organisms feed on organic matter with various degrees of humification, ranging from undecomposed plant materials to humified organic matter (soil organic matter), in a gallery forest and a savanna in the Ivory Coast. We defined diet age as the time elapsed since carbon (C) in the diet of earthworms and termites was fixed from atmospheric CO2 by photosynthesis; it was estimated by comparing the radiocarbon (C-14) content of these organisms to atmospheric (CO2)-C-14 records. 3. Stable N isotope ratios increased along the humification gradient of diets, and values for earthworms and termites varied from 1.8 parts per thousand to 9.9 parts per thousand and from -1.5 parts per thousand to 15.9 parts per thousand, respectively. Epigeic (litter-feeding) earthworms had younger diet ages (2-4 years), whereas endogeic (soil-feeding) earthworms generally exhibited older diet ages (5-9 years). Grass-feeding termites had young diet ages (2 years), and wood/soil-feeding termites had the oldest diet ages (c. 50 years). Soil-feeding termites were similar in diet age (7-12 years) to wood feeders (8-11 years), with the exception of one species (18-21 years) that consumes large-diameter wood. 4. A significant positive relationship was found between diet ages and stable N isotope ratios of the two groups in the savanna. This relationship held in the gallery forest when termites feeding on woody tissues were not considered. These results show that the stable N isotope ratios of organisms can increase with diet age, unless C in the diet has been stored as organic matter, such as woody tissue, that is able to age without being subject to humification processes. 5. Given that above-ground food webs are often sustained directly by material and energy flow from below-ground food webs, in addition to trophic interactions, gradual enrichment of N-15 with the humification of below-ground diets should be considered when interpreting stable N isotope ratios of terrestrial food webs

Comparative study of earthworm communities, microbial biomass, and plant nutrient availability under 1-year Cajanus cajan (L.) Millsp and Lablab purpureus (L.) Sweet cultivations versus natural regrowths in a guinea savanna zone

In tropical savannas where soils are generally sandy and nutrient poor, organic farming associated with enhanced soil biological activity may result in increased nutrient availability. Therefore, legumes have been introduced in the humid savanna zone of Cte d'Ivoire, owing to their ability to fix atmospheric N and to continually supply soil with great quantity of organic materials in relatively short time. The main objective of this study was to assess the influence of two legume (Cajanus cajan and Lablab purpureus) cultivations on earthworm communities and P and N availability. Trials were carried out under farmers' field conditions; C. cajan was planted on savanna soils (trial 1) while L. purpureus was established on new Chromolaena odorata-dominated fallow soils (trial 2). Native vegetations were considered as controls. Changes in soil properties (earthworm abundance and diversity, microbial biomass carbon (MBC), and plant available P and N) were assessed using the biosequential sampling. After 1 year, both the legume stands showed a significantly higher density of earthworms, compared with the respective controls. This trend was linked to an increase in the abundance of the detritivores Dichogaster baeri Sciacchitano 1952 and Dichogaster saliens Beddard 1893, and the polyhumic Stuhlmannia zielae Omodeo 1963. Equally, legume had beneficial impacts on the average number of earthworm species, the Shannon-Weaver index of diversity and MBC in savanna (trial 1). Available P and ammonium significantly increased under both legume cultivations and were significantly and concurrently linked to litter quality and earthworm activities as shown by multiple regressions. As a result, legumes could improve nutrient availability in the sandy soils of central Cte d'Ivoire by positively affecting soil biological activity and this could bring farmers to cultivate crops on savanna lands

Conventional versus agro-ecological intensification: assessing the effect of conservation agriculture in maize cropping systems with the DSSAT model in Côte d’Ivoire (West Africa)

Integrated soil fertility management options are being promoted as ways of adapting agricultural systems to sustain yields on highly degraded and poor soils encountered throughout West Africa. The efficiency of these practices may be affected by high variability and uncertainty associated with seasonal rainfall, especially for areas such as Côte d’Ivoire, where intra-seasonal rainfall has been observed to change from 1 year to another. The DSSAT crop simulation model was used in this study as a tool to evaluate the impacts of soil improvement options including inorganic fertilizer and conservation agriculture generating higher carbon sequestration and crop yield in maize agro-ecosystems. The model was calibrated using agronomic data for three cropping seasons from 2009 to 2010 in Goulikao (Center-West Côte d’Ivoire) and Ahérémou 2 (Central Côte d’Ivoire), respectively and validated against independent datasets of yield of 2003–2004 seasons in the buffer zone of the Lamto Natural Reserve, Central Côte d’Ivoire. The model predicted average maize yields of 1454 kg ha−1 across the sites versus an observed average value of 1736 kg ha−1, R2 of 0.72, and RMSE of 597 kg ha−1 after the default values for stable soil organic matter fraction used in the model were substituted by the estimated one. For the validation, the predicted higher maize yield was consistently related to fallow biomass inputs and different rates of fertilizer, thus generating a RMSE of total aboveground biomass and grain yield of 606 kg ha−1 and 350 kg ha−1, respectively. The impact of fallow residues and cropping sequence on subsequent maize yield was simulated and compared with conventional fertilizer and control data using 12 years historic climate time series. We conclude that soil fertility improvements through conservation agriculture can sustain grain yield at the same level as conventional inputs of urea against larger climate variability. However, this system may be substituted by conventional agriculture when climate forecast reveals a dry cropping season year