Chapitre 5. Stocks de carbone dans les sols des zones agro-écologiques du Bénin

Introduction Situé en Afrique de l’Ouest, le Bénin s’allonge perpendiculairement au littoral du Golfe du Bénin entre les méridiens 1° et 4° et les parallèles 6° et 12° Nord. Les climats y sont chauds et humides. Les conditions pédologiques et la végétation du Bénin sont caractéristiques de la zone intertropicale. La surface totale cultivable représente 62,5 % de la superficie du pays et 20 % seulement des surfaces cultivables sont effectivement exploitées, soit 12,2 % du territoire national (..

Carbone des sols en Afrique

Les sols sont une ressource essentielle à préserver pour la production d’aliments, de fibres, de biomasse, pour la filtration de l’eau, la préservation de la biodiversité et le stockage du carbone. En tant que réservoirs de carbone, les sols sont par ailleurs appelés à jouer un rôle primordial dans la lutte contre l’augmentation de la concentration de gaz à effet de serre. Ils sont ainsi au centre des objectifs de développement durable (ODD) des Nations unies, notamment les ODD 2 « Faim zéro », 13 « Lutte contre le changement climatique », 15 « Vie terrestre », 12 « Consommation et production responsables » ou encore 1 « Pas de pauvreté ». Cet ouvrage présente un état des lieux des sols africains dans toute leur diversité, mais au-delà, il documente les capacités de stockage de carbone selon les types de sols et leurs usages en Afrique. Il propose également des recommandations autour de l’acquisition et de l’interprétation des données, ainsi que des options pour préserver, voire augmenter les stocks de carbone dans les sols. Tous les chercheurs et acteurs du développement impliqués dans les recherches sur le rôle du carbone des sols sont concernés par cette synthèse collective. Fruit d’une collaboration entre chercheurs africains et européens, ce livre insiste sur la nécessité de prendre en compte la grande variété des contextes agricoles et forestiers africains pour améliorer nos connaissances sur les capacités de stockage de carbone des sols et lutter contre le changement climatique

Dynamique de Fer ferreux (FeO) du sol de bas-fond en fonction de la répartition de l’eau selon la toposéquence au sud du Bénin: Phénomène de la toxicité ferreuse et production du riz

Objectif: La présente étude évalue la dynamique de la teneur du sol en (FeO) dans le bas-fond, mesure les conditions de développement du riz sur des sols immergés et établit les relations qui existent entre les paramètres de production et l’évolution de l’accumulation du (FeO) en fonction des répartitions de l’eau selon la toposéquence dans les casiers rizicoles.Méthodologie et résultats: Trois traitements ont été testés sur les parcelles : Écoulement naturelle de l’eau, inondation suivie de drainage, inondation permanente. Le prélèvement du sol est fait aux profondeurs 0-10 cm, 10-20 cm et 20-30 cm. Des échantillons composites ont été constitués par profondeur et un total de 504 échantillons de sol ont été analysés pour l'étude Les mesures du potentiel redox (Eh) et du pH sont réalisées sur le site par le pH letter pc228 mais l’évaluation de la teneur du sol en (FeO) et son dosage, sont réalisés au laboratoire au spectromètre à absorption atomique. Le rendement du riz est calculé en rapportant la production de riz paddy à l’ha. Une analyse de la variance à trois facteurs est effectuée, au moyen du logiciel SAS version 9.2. Le potentiel redox a varié entre 32,25±13,69 et 199,78±13,45 mv avec un pH allant de 4,56±0,16 à 7,79±0,32. La quantité de FeO a varié de 3132±534 à 498±96 ppm et les rendements de 2103±180 à 2854 ±174kg/ha au poids sec.Conclusions et application des résultats: Le bas-fond présente deux types d’hydromorphie : l’hydromorphie moyennement réductrice et l’hydromorphie très réductrice. L’effet conjugué des conditions de réduction et de la dynamique de la teneur du sol en fer ferreux a permis d’apprécier et de mesurer leurs impacts sur les paramètres de production du riz. Il existe une relation statistiquement significative (p<.0, 05) et inversement proportionnelle entre les paramètres Eh, Fer ferreux et pH. On constate que plus la valeur de Eh ne diminue, plus les valeurs du pH et du Fer ferreux ne sont élevées. Le traitement d’inondation suivie de drainage présente un fort tallage avec un taux de fertilité de 86,5% des talles et un meilleur rendement en grain de riz (2854kg/ha) est le plus recommandable pour la valorisation du bas-fond par la production rizicoleMots clés: dynamique, toxicité ferreuse, bas-fond, BéninEnglish AbstractObjectives: This study assesses the dynamics of soil content in FeO of the shallows, measure the rice growth conditions in submerged soils and establishes the relationship between production parameters and the FeO accumulation‘s evolution.Methodology and Results: Three treatments were tested on the plots. Composite samples were made by depth and a total of 504 soil samples were analyzed for the study. The redox potential (Eh) and pH measurements are carried out at the site by pH letter pc228; The evaluation of the FeO content and its determination are carried out in the laboratory using an atomic absorption spectrometer. The statistical analysis is performed using the software SAS 9.2. The redox potential varies between 32, 25±13, 69 and 199, 78±13, 45 mv with a pH from 4, 56±0, 16 to 7, 79±0, 32. FeO’s amount varies from 3132±534 to 498±96 ppm and the yields from 210±18 à 285±17 g/m2Conclusions and application of the results: The shallow presents two types of water logging : One slightly reductive and the second very reductive. The reduction conditions and the dynamics of the soil’s content in ferrous iron were used to measure and evaluate the effect of irrigation scheme on rice’s growth parameters. There is a statistically significant (p <0, 05), inversely proportional relationship between the parameters Eh, ferrous iron and the pH. It is found that more the value of Eh decreases, more the pH values and ferrous iron increase. This is confirmed by the Pearson’s correlation coefficient R, -0.8757 and -0.8830 indicating a strong relation between the three variables. The flooding treatment followed by drainage showed strong tillering with a fertility rate of 86.5% of tillers and performance (2854kg/ha) is the more recommended scheme in the shallow.Keywords: dynamic, iron toxicity, shallow, Beni

Municipal household waste used as complement material for composting chicken manure and crop residues

There are few organic materials available as agricultural soil amendment because their low chemical content means that large quantities are required. In order to improve the availability of raw materials for composting, as well as the quality of the compost produced, municipal solid waste (MW) was added to cotton-seed residue (CSR) and to the association of CSR with chicken manure (M) in different weight/weight (MW/added materials) ratios of 5:1 and 2:1. Aerobic composting was processed and compost yield was determined, as well as compost particle size and pH. Also, the compost bulk density and its water holding capacity were determined as well as contents of total nitrogen, carbon, phosphorus, calcium (Ca), magnesium and heavy metals. According to its pH and carbon/nitrogen ratio values, the municipal waste of Cotonou was judged to be a good raw material for composting in order to improve availability of the organic source of nutrients. The composts produced with MW+M+CSR had the highest potential for amending Ferralsols, especially with a mixture of 2:1 (200 kg MW+100 kg M+100 kg CSR) that could be applied at 10 t ha–1. However, further improvement in composting methods was suggested to increase Ca++ and reduce mercury contents, respectively. Moreover, potassium balance should be improved in the produced compost

Assessment of Nutritional Status of Rainfed Rice in Benin Using Diagnosis and Recommendation Integrated System (DRIS)

The fertilizer used in Benin by rainfed rice farmers, doesn’t meet the required expectation because of lack of many essential agronomic information to formulate the appropriate nutrient compositions. Despite all the advances in improvement of rice production, its yields in traditional cropping systems are very low and the only inputs of Nitrogen, Phosphorous and Potassium do not effectively increase rice yields. The purpose of this study was to assess nutritional status of rainfed rice crops in Benin. The study covered the rainfed rice production areas of Benin Center where 72 leaves samples were taken on 3600 plants from the farmer fields. The concentrations of nitrogen, phosphorus, potassium, calcium, magnesium, iron and zinc were determined in leaves samples. The preliminary DRIS (Diagnosis and Recommendation Integrated System) norms for the rainfed rice growing in the Benin center were selected for various nutrient ratios obtained from the high yield population of the rainfed rice crop. The yields of the two subpopulations were significantly different (p < .0001). The nutrient requirement for the rainfed rice production was ranked as N> Fe> Zn>K>Mg>P>Ca. The DRIS-derived sufficiency ranges for N, P, K, Ca and Mg from the nutrient indexing survey of the rainfed rice plants grown in Benin center were 1.91-3.66, 0.30-0.64, 2.00-3.89, 0.37-1.05 and 0.18-0.38g kg-1 respectively. The limits for Fe and Zn were 89.27-206.3 and 8.21-24.91 mg kg-1 respectively. On the basis of sufficiency ranges, 4.22, 57.75, 66.20, 56.34, 45.07, 46.50 and 29.57% of samples were low in N, P, K, Ca, Mg, Fe and Zn respectively. The DRIS norms put emphasis on nutrient balance and help to differentiate between healthy and unhealthy rice plants from the nutrition status. However, it needed further researches to determine the amount of the fertilizers to supply in order to maintain nutrient balance

Quantification of soil organic carbon in particle size fractions using a near-infrared spectral library in West Africa

International audienceParticle size fractionation enables a better understanding of soil organic carbon (C) dynamics since it separates fractions that differ in composition, residence time and function. However, this method is time-consuming and tedious; thus, its use has been greatly limited. Our objective was to evaluate the ability of an existing soil spectral library (SSL) from different regions of West Africa to predict the C amount in the fractions (gC kg-1 soil) of the samples in a new target set from Benin. The SSL included 181 samples from five countries, and the target set included 94 samples (depth ≤ 40 cm), most of which were coarse-textured; near-infrared reflectance (NIR) spectra were collected for 2 mm sieved samples (non-fractionated samples). The predicted variables were the C amounts in the non-fractionated soil and in the 50 μm fractions (F50, respectively). Different methods were tested to optimize the predictions: (i) SSL enrichment with 10 or 15 samples selected from the target set (spiking) and replicated six times (i.e. extra-weighted); (ii) locally weighted (local) partial least squares regression (PLSR), which is calibration by the spectral neighbours with the highest weights attributed to closest neighbours, and was compared to “global” (i.e., common) PLSR, where all calibration samples equally contribute; and (iii) spectrum pretreatments (e.g., smoothing, centring, derivatization). In addition, the intermediate precision of the conventional data (standard error of laboratory; SELint) was estimated through triplicate fractionation of three samples carried out by three operators (one per replicate). When the SSL alone was used for calibration, the predictions were inaccurate for the C amounts in the nonfractionated soil and in F50, with minimal benefit from the local PLSR over the global PLSR in general. For the non-fractionated soil, F50, the ratios of performance to the interquartile range in the validation set, RPIQVAL, were 1.6–1.8, 1.6–1.7, 1.9 and 1.9–2.1, respectively. Calibration with SSL spiked (i.e., completed with spiking samples) yielded an increase in RPIQVAL from 33 to 56% for the C amount in the non-fractionated soil and F50 (RPIQVAL reached 2.4–2.5, 2.2–2.3, 1.9–2.0 and 2.1–2.3, respectively), and the benefit of local PLSR was still limited. The SELint was based on a few samples and thus only provided a rough estimation; this estimate represented at least 65% of the prediction error for the C amounts in the fractions. Therefore, the SELint needs to be determined more extensively to both improve the model accuracy and refine the interpretation of the predictions based on NIR spectra. This library should be enriched with samples from other sites to represent other soil types