Article

# Carbon sinks in small Sahelian lakes as an unexpected effect of land use changes since the 1960s (Saga Gorou and Dallol Bosso, SW Niger)

Authors:
• Institut National Supérieur d'Agronomie et de Biotechnologie (USTM)
• Université de Rouen-Normandie
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... The Rock-Eval pyrolysis has been used to study OM-rich sediments from tropical environments (e.g., Sifeddine et al. 1995;Biscara et al., 2010;Fujisaki et al., 2015). In Africa, a variety of Holocene lacustrine and palustrine environments from Niger, Cameroon, and Gabon have been investigated using Rock-Eval pyrolysis Debret et al., 2014;Mabicka Obame et al., 2014). Although this method provides valuable information regarding the sedimentary dynamics and the sediment sources in the studied lake basins, the common presence of siderite, particularly in tropical lake sediments, can alter the primary signal of the SOM Sifeddine et al., 2001;Jacob et al., 2004;Debret et al., 2014). ...
... Samples representing the deep lake sedimentation were obtained in continuous 5-cm increments, whereas event-deposits (turbidites) were not sampled. The Rock-Eval dataset of the Barombi Mbo sequence was further compared with published datasets (Sanei et al., 2005;Carrie et al., 2012;Sebag et al., 2013;Mabicka Obame et al., 2014). Note that Sanei et al. (2005) and Carrie et al. (2012) started their Rock-Eval pyrolysis phase at 300°C (not at 200°C, see below), and as a result, their S2 peak and their HI calculation can be underestimated. ...
... In summary, the sector with HI:OI > 1.25 indicates aquatic OM and biological tissues rich in lipids (Algae in Fig. 5), the sector with 1.25 > HI:OI > 0.5 indicates biogenic OM from terrestrial organic sources, such as fresh plant tissues and soil litters (Litter in Fig. 5), and the sector with HI:OI <0.5 indicates detrital soil OM from organomineral and mineral horizons (Soil in Fig. 5). These observations are in line with other published dataset (e.g., Disnar et al., 2003;Boussafir et al., 2012;Saenger et al., 2013;Sebag et al., 2013;Mabicka Obame et al., 2014;Debret et al., 2014;Sebag et al., 2016) and support the use of the Van Krevelen pseudo-diagram for the interpretation of the siderite-effect-free results from Lake Barombi sediments. ...
Article
Originally developed for use in the petroleum industry, Rock-Eval pyrolysis is a technique commonly applied to lake sediments to infer paleoenvironmental reconstructions. The standard Rock-Eval parameters provide information on the amount of total organic and inorganic carbon (TOC and MinC, respectively), and are usually interpreted as proxies for the source (aquatic or terrestrial) of the primary production of organic matter (Hydrogen Index vs Oxygen Index). Although this method usually provides valuable evidence, the common presence of siderite in tropical lake sediments can alter the primary signal of the sedimentary organic matter (SOM). Indeed, the CO2 and CO released by the pyrolysis of siderite are integral to the calculation of the SOM-related standard Rock-Eval parameters. In this study, we analyze sediments from a core collected in the Lake Barombi (southwest Cameroon) and describe the impact of siderite on standard Rock-Eval parameters. We propose a workflow that allows standard Rock-Eval parameters to be corrected, based on the analysis of thermograms. The proposed corrections provide siderite-effect-free parameters, accurately reflecting the changes in sedimentary organic matter composition.
... Rapid population growth and economic development have led to the emission of a massive amount of carbon-containing substances into the environment in recent decades, resulting in global warming. Aquatic environments, such as coastal zones, wetlands, offshore waters, and deep oceans are the Earth's main carbon sinks [1,2]. Blue carbon is stored as biological biomass and sediment [3,4]. ...
... The biomass of the algal cells was calculated using the relationship between the algal cell density and the absorbance obtained from Section 2.3.1. The growth inhibition rate (GIR) was calculated using Equation (1), and the concentration for 50% effect (EC 50 ) was calculated by fitting the Logistic model [30]. ...
Article
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Aquaculture facilities are a potential source of antibiotics in aquatic environments, having adverse effects on the algae species. In this study, the toxicity induced by enrofloxacin (ENR) on the algae Scenedesmus obliquus was evaluated. The uptake of ENR and the change in the growth and photosynthesis of algae were analyzed. At the exposure doses of 10–300 μg/L, the accumulated levels of ENR in algae were 10.61–18.22 μg/g and 12.09–18.34 μg/g after 48 h and 96 h of treatment, respectively. ENR inhibited the growth of algae, with a concentration for 50% effect of 119.74 μg/L, 53.09 μg/L, 64.37 μg/L, and 52.64 μg/L after 24 h, 48 h, 72 h and 96 h of treatment, respectively, indicating the self-protection and repair ability of algae in a short period of time. Furthermore, the chlorophyll contents decreased in all treatment groups, and the photosynthetic system Ⅱ parameters decreased in a dose-dependent manner under ENR stress, suggesting that ENR caused a disorder in the electron transport of the photosynthesis of algae, and the carbon fixation and assimilation processes were thus damaged. These results indicate that ENR poses a considerable risk to aquatic environments, affects the carbon sinks, and even has an adverse effect on human health.
... On Les Gleysols du Sénégal présentent des valeurs de stock de COS inférieures par rapport aux polders, ici les données de Cheverry (1974) et Cheverry et Fromaget (1970) Un autre facteur possible est la gestion particulière dans chaque polder. Obame et al. (2014) ont démontré le potentiel de petits lacs dans la région sahélienne à stocker le C organique, de sorte que si l'on considère que l'irrigation utilise de l'eau de surface naturelle du lac, qui n'est sans doute pas exempt de COD, nous devons aussi considérer l'importance de l'entrée détritique du rivage ou du bassin versant, qui sont également utilisés à des fins pastorales. ...
... Utilisée pour étudier la MO sédimentaire (i.e. Giovanni et al., 1999 ;Macaire et al., 2005 ;Copard et al., 2006) elle a montré son intérêt pour analyser la composition de la MOS et des litières (Baudin et al., 2015 ;Carrie et al., 2012 ;Delarue et al., 2013 ;Disnar et al., 2003 ;Obame et al., 2014 ;Saenger et al., 2013 ;Sebag et al. 2006Sebag et al. , 2013aSebag et al. , 2013bVolland-Tuduri, 2005). ...
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En régions arides et semi-arides, la gestion du carbone (C) du sol est inséparable des contraintes thermo-hydriques. Le concept de la séquestration du C pour ces régions est encore trop peu connu ou compris, en raison du manque des données expérimentales.Avant de pouvoir effectuer un bilan C à l’échelle des profils de sols et du paysage interdunaire, l’évaluation du stock de C du sol est nécessaire ainsi que la caractérisation de sa stabilité. L’étude a été conduite sur les sites des polders de Bol, nord-est du lac Tchad. Une chronoséquence par « approche synchronique » a été choisie selon les différents âges de poldérisation et de mise en culture ; les sites sélectionnés t0, t10, t60, t62 et à t65 correspondent à un historique de 0 à 65 ans. Le t0 représente le site de référence de sédiment récent. Les sols étudiés sont chimiquement fertiles ; ils se différencient sur les formations quaternaires anciennes de la cuvette tchadienne. Ce sont des sols d’apports alluviaux, hydromorphes, localement salés, cultivés de manière intensive. Dans ce dispositif, 36 profils pédologiques ont été creusés et décrits, y compris 3 profils dans la zone alluviale t0. Les sols ont été échantillonnés de la surface jusqu’à 1 m de profondeur, par incréments de 0,1 m.Les stocks de C organique (COS) (0-1 m) se différencient selon les traitements: à t0 200 ± 0,8 ; t10 241 ± 0,9 ; t60 183 ± 34 ; t62 174 ± 0,3 et à t65 189 ± 1,1 MgC ha-1 (P=0,004) ; les stocks de C inorganiques (CIS) pour la même profondeur sont négligeables et varient de façon significative entre traitements (P = 0,03) de 1 pour t0 à 6 MgC ha-1 pour t65 représentant environ 5 % de stocks de COS. Pour la couche de sol 0-0,3 m, les stocks de COS exprimés en masse de sol varient en moyenne de 53 à 64 MgC ha-1 entre t65 à t0 avec un taux de perte moyenne annuelle de -0,17 MgC ha-1 an-1. L’analyse par pyrolyse Rock-Eval confirme le caractère hérité de la matière organique de l’écosystème lacustre. Dans la couche de surface des sols (0-0,4 m), le caractère résistant de cette matière organique a également été révélé par des valeurs élevées observées des stocks de carbone résistant (ResC), entre 62,6 ± 13,8 et 79,9 ± 5,8 Mg C ha-1 de t0 à t65 par rapport à celles de carbone labile (LabC), plus faibles. Le compartiment résistant représente de 17 à 23 % du stock total de C. La relation entre l’acquisition des propriétés des sols suite à la poldérisation a été discutée au regard de la caractérisation physique des sols et de la nature de la matière organique. Après plus d’un demi-siècle de mise en poldérisation, les stocks de COS ont présenté un caractère relativement stable et résistant à la biodégradation dans ce contexte semi-aride.
... The NEP was positively correlated with the RF as the second most dominant influencing factor (R 2 = 0.13, P < 0.001) of the NEP while the TEM was insignificant to the NEP. In view of the interactions among the influencing factors, temperature might indirectly influence the spatial variation of the NEP through vegetation growth (Hazarika et al. 2005;Nayak and Patel 2010;Mabicka et al. 2014;Liu et al. 2015;Zhou et al. 2017). The temperature in the study area was stable. ...
... (2000)andDragoni et al. (2011) that land coverage changes are typically accompanied by abundant carbon exchange. Moreover,Mabicka et al. (2014) andZhang et al. (2014) reported that increasing the vegetation coverage and changing the land use types are effective measures for protecting soil carbon pool. China has launched various national forestry ecological construction projects in the BThree-North Regions^of China since the TNSFP was implemented in 1978. ...
Article
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The variations of vegetation carbon sequestration have become a gauge for evaluating the ecological effect of vegetation restoration. In this study, the spatiotemporal patterns of the net ecosystem production (NEP) were simulated using an improved CASA model and GSMSR model. It showed that the NEP markedly increased in the tableland of Loess Plateau during 2003–2012, with an annual average growth of 3.65 g C·m⁻² a⁻¹. The mixed broadleaf-conifer forest ranked first (127.23 g C·m⁻² a⁻¹) while the bare land and sparse vegetation presented the lowest carbon sequestration (14.64 g C·m⁻² a⁻¹). The NEP manifested a significantly uneven overall spatial distribution: high in the southwest and low in the northeast. The spatial variations of NEP resulted from the combined effects of geographic position, terrain, meteorology, and soil and vegetation, respectively. Quantitative isolation revealed that the most dominant factor of vegetation carbon sequestration was soil and vegetation, while terrain exerted insignificant impacts on the NEP.
... These studies have similar and coherent OI values, suggesting a specific type of OM even though their settings are different. Conversely, very high OI RE6 values (up to 1500 mg O 2 g À1 TOC) have been reported (Sebag et al., 2013;Mabicka Obame et al., 2014) in tropical environments and thus possibly indicate highly degraded OM. OI RE6 values higher than 1000 mg O 2 g À1 TOC have as well been reported in C horizons from cambisols in mountain areas where OM is scarce and strongly degraded (Giguet-Covex et al., 2011;Bajard et al., 2017). ...
... EMA-OI RE6 in this study (110-1872 mg O 2 g À1 TOC) reach anomalously high values, most of them far beyond the usual range to characterize OM sources (Peters, 1986) in aquatic sediments (Meyers, 1997;Meyers and Lallier-Vergès, 1999;Ariztegui et al., 2001;Steinmann et al., 2003;Baudin et al., 2015) and soils (Disnar et al., 2003). Some studies have reported high OI RE6 values from lignins (800 mg O 2 g À1 TOC; Carrie et al., 2012), C horizons in cambisols (>1000 mg O 2 g À1 TOC; Giguet-Covex et al., 2011;Bajard et al., 2017) and predominantly clastic lake sediments with abundant terrestrial OM in lateritic environments (>850 mg O 2 g À1 TOC; Mabicka Obame et al., 2014;Sebag et al., 2018). However, high EMA-OI RE6 values could be likely the result of other matrixeffects, which in our case subsist the carbonates matrix-effect correction and affect the EMA-OI RE6 index. ...
Article
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Qualitative and quantitative changes of organic and carbonate carbon in sedimentary records are frequently used to reconstruct past environments, paleoproductivity and sediment provenance. Amongst the most commonly used proxies are Total organic carbon (TOC), Mineral carbon (MinC), as well as Hydrogen (HI) and Oxygen Indices (OI) of organic matter (OM). Rock Eval pyrolysis enables the assessment of these quantitative and qualitative parameters with a single analysis. This is achieved through transient pyrolysis of the samples up to 650°C followed by combustion up to 850°C, with hydrocarbons, CO and CO2 measured during the thermal decomposition of both OM and carbonate minerals. Carbonate minerals with low thermal cracking temperatures such as siderite (<400°C) can induce significant matrix effects, which bias the TOC, MinC and OI Rock-Eval parameters. Here we assess the applicability of End-Member Analysis (EMA) as a means to correct Rock-Eval thermograms for siderite matrix effects. For this, we performed Rock-Eval pyrolysis on sideritic sediments of Lake Towuti (Indonesia). New thermal boundaries were constrained in Rock-Eval thermograms using EMA to limit siderite matrix effects and improve TOC, MinC, and OI calculations. Our approach allowed us to: 1) evaluate the influence of siderite matrix effects on Rock-Eval thermograms; 2) properly exploit a Rock-Eval dataset to characterize the type and sources of OM in siderite-rich sediments; and 3) identify the OM behind degradation and mineralization processes. The Rock-Eval dataset revealed sediments with a substantial amount of refractory OM, especially in those where TOC is high and HI characteristic of autochthonous biomass. These results, associated to alternative indices used to assess OM preservation, suggest that refractory OM is residually enriched following strong degradation of labile compounds. Finally, relatively labile and refractory organic fractions may be consumed in the formation of siderite during this sequential process of OM mineralization.
... Ainsi, suite à la remontée de la nappe phréatique, sont apparus des lacs pérennes qui constituent des pièges à sédiments. Les sédiments carottées sous les lames d'eau de ces lacs ont montré un taux de comblement de 2 à 3 cm par an [18,19]. C'est pour mieux comprendre ce processus de comblement que cette étude a été réalisée en vue de quantifier et caractériser la dynamique des particules en suspension (MES) participant au comblement de ces lacs. ...
... Une lecture de la hauteur des eaux est réalisée chaque matin à 6 heures. Ces lectures indiquent l'évolution de la hauteur du lac mais aussi celle du niveau piézométrique du fait qu'à Bangou Bi, c'est essentiellement la nappe phréatique qui affleure sous forme de lac [19]. Parallèlement à cette mesure, les hauteurs de pluie, en 2008 et 2009, ont été déterminées après chaque événement pluvieux au moyen d'un pluviomètre totalisateur installé à 70 m de Bangou Bi. ...
Article
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L’envasement des cours d’eau est une des graves conséquences des changements climatiques et des fortes pressions anthropiques au Sahel. Des taux de comblement de l’ordre de 2 à 3 cm/an ont été mis en évidence dans ceux de la région de Niamey (Sud-Ouest Niger). Dans ce contexte, ce travail est fait pour déterminer les impacts des variations piézométrique et pluviométrique sur la dynamique des matières en suspension (MES) participant au comblement des lacs (Bangou Kirey et Bangou Bi). Les résultats obtenus montrent que Bangou Kirey, alimenté par des koris drainant un plus vaste bassin versant, contient au moins 30 fois plus de MES que Bangou Bi. La concentration en MES dans Bangou Kirey a, cependant, été impactée par l’effet de la remonté de la nappe phréatique et les apports en sédiments fins apportés par les eaux de ruissellement. Elle a, en effet, été diluée par la remonté en surface de ladite nappe. Par ailleurs, si l’influence des pluies, plus importantes et plus fréquentes, de la deuxième partie de la saison des pluies (après mi-juillet) est moins marquée, les premières pluies ont été celles qui ont causé plus d’érosion ayant abouti à un accroissement continu de la concentration en MES.
... Rock-Eval provides information on quantity and quality of organic matter without sample preparation. It also gives information on stoichiometric of organic carbon [58,68]. Disnar et al. [58] provided essential information on the amount and composition of tropical SOM. ...
Chapter
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In strongly weathered tropical soils, humus and humic substances (HSs) appear to play an important role in soil fertility because they represent the dominant reservoir and source of plant nutrients. As the refractory organic carbon form of soil, HSs play a vital role in the atmospheric CO 2 sequestration. Detailed classification of humus forms in tropical ecosystems and the dynamics and function of humus are still poorly understood. Nevertheless, in tropical environment many studies indicated that it is very difficult to differentiate between tropical humus, at least in normally drained soil. Moders, mulls, and Amphimull are the dominant humus forms in the topsoil of tropical environment. Knowing the mechanisms of formation, the dynamics and the methods of characterization of humus in tropical zones are a scientific challenge. This chapter aims to share recent findings from a broad humus in tropical soil and research related to this theme.
... This is broadly consistent with the percentage of deep sandy soils in the watersheds (approximately 30% for the Mauritania site, 40% for Agoufou and about 50% for the Niger site), as these soils contribute less to runoff. WIR cannot be attributed solely to runoff for all lakes, since water inflow may contain contributions from water table, as reported elsewhere in Southern Niger (see for instance Obame et al., 2014). Concerning the decadal evolution, an increase in WIR is observable since the 50 or 70s in all the studied watersheds, even if this is not statistically significant for the Mauritanian site. ...
... However, this fraction (20% of the total transported according to some estimates; Lal, 2003) cannot be technically termed "sequestration" (Olson et al., 2014;Bernoux et al., 2006). Because it is taken out of the C cycle, it is termed by some as a "C sink" (Table 5; Obame et al., 2014;Quine and Van Oost, 2007;Van Oost et al., 2005). Yet, even the "sink hypothesis" is questionable because of the assumption based on "dynamic replacement of C" at the eroding site. ...
Article
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Accelerated soil erosion, driven by anthropogenic activities such as conversion of natural ecosystems to agroecosystems and mechanical tillage, has numerous adverse impacts on ecosystem services. In addition to degrading soil quality and reducing agronomic/biomass productivity on-site through a decrease in use-efficiency of inputs, off-site impacts of accelerated erosion include eutrophication and contamination, sedimentation of reservoirs and waterways, and emissions of greenhouse gases (e.g., CO2, CH4 and N2O). While advancing food and nutritional security, adoption of restorative land use and recommended management practices are important to strengthening numerous ecosystem services such as improving water quality and renewability, increasing below and above-ground biodiversity, enhancing soil resilience to climate change and extreme events, and mitigating climate change by sequestering C in soil and reducing the emission of CO2, CH4 and N2O. An effective control of accelerated erosion is essential to sustainable development and improving the environment.
... This is broadly consistent with the percentage of deep sandy soils in the watersheds (approximately 30% for the Mauritania site, 40% for Agoufou and about 50% for the Niger site), as these soils contribute less to runoff. WIR cannot be attributed solely to runoff for all lakes, since water inflow may contain contributions from water table, as reported elsewhere in Southern Niger (see for instance Obame et al., 2014). Concerning the decadal evolution, an increase in WIR is observable since the 50 or 70s in all the studied watersheds, even if this is not statistically significant for the Mauritanian site. ...
Article
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Changes in the flooded area of ponds in the Gourma region from 1950 to present are studied by remote sensing, in the general context of the current multi-decennial Sahel drought. The seasonal and interannual variations of the areas covered by surface water are assessed using multi-date and multi-sensor satellite images (SPOT, FORMOSAT, LANDSAT-MSS, –TM, and-ETM, CORONA, and MODIS) and aerial photographs (IGN). Water body classification is adapted to each type of spectral resolution, with or without a middle-infrared band, and each spatial resolution, using linear unmixing for mixed pixels of MODIS data. The high-frequency MODIS data document the seasonal cycle of flooded areas, with an abrupt rise early in wet season and a progressive decrease in the dry season.
... La longueur de l'ensemble du réseau ravinaire a connu un accroissement entre 1975 et 2015. Cet accroissement a particulièrement été important entre cette longueur dans les koris dans les années 2000 relativement à 1975 (période sèche) intervient dans une période relativement plus pluvieuse [15] et dans un contexte d'encroûtement des sols [10]. En effet, l'augmentation de l'encroûtement des sols et le retour de la pluviométrie accroissent les ruissellements et renforce ainsi le ravinement [16]. ...
Article
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Au Sahel, l'érosion hydrique cause des dommages majeurs aux terres exploitées dans les systèmes agraires. L'objectif de ce travail a été de quantifier la dynamique spatio-temporelle d'un processus de cette érosion, le ravinement, à l'échelle du bassin versant du lac Kongou situé au Nord-est de Niamey. L'étude a été fondée sur une cartographie diachronique du réseau hydrographique aux moyennes d'une photographie aérienne de 1975 et des images Google Earth de 2004, 2008, 2015. Il est ressorti qu'entre 1975 et 2015, les réseaux hydrographiques de tous les sous-bassins versants ont connu une complexification de leurs connectivités. L'ordre de Shreve, déterminant la connectivité du réseau ravinaire, a été multiplié par 2,5. Aussi, la longueur totale des drains secondaires a été multipliée par 2,5 en moyenne entre 1975 et 2015. Par ailleurs, les reculs des têtes des ravines et des berges ont été très importants pouvant atteindre jusqu'à 8,55 m / an voire 11,35 m / an. Cette forte érosion par ravinement contribuerait sans nul doute à l'ensablement du lac kongou en particulier, et des lacs de la région de Niamey qui ont des taux de comblement dépassant 2 cm / an. ABSTRACT Spatio-temporal dynamics of gullying in watershed of Kongou Lake, southwest of Niger Hydric erosion processes have major damages to soil of agrarian systems of the Sahel. This work aimed to characterize the gullying spatio-temporal
... Such research has encouraged the consideration of carbon emissions in land planning systems and the exploration of land management strategies based on a low-carbon economy to achieve both carbon emissions reduction and sustainable land-use planning [4][5][6][7]. Many scholars have investigated areas in Africa, Germany, Ireland, Denmark, Spain, using techniques such as contrast analysis, correlation analysis, and remote sensing-dynamic monitoring to study soil and vegetation carbon reserves of different land use types, estimate and analyze carbon emission characteristics of different land use patterns, and help inform land use policies to achieve reductions in carbon emissions [8][9][10][11][12][13]. Additionally, many scholars have estimated the adjusted carbon budget of forest due to land ownership and cover changes [14][15][16]. ...
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Changes in land use affect the terrestrial carbon stock through changes in the land cover. Research on land use and analysis of variations in carbon stock have practical applications in the optimization of land use and the mitigation of climate change effects. This study was conducted in Baixiang and Julu counties in the Taihang Piedmont by employing the trend analysis method to characterize the variation in county land use and carbon stock. The findings show that in both counties, agricultural and unused land areas decreased while built-up land area increased, and the reduction in cropland was the main reason behind the agricultural land reduction. An inflection point appeared on the cropland curves of Julu, because the cropland area decreased by 1576.97 hm$$^{2}$$ from 2004 to 2006. Cropland area in Baixiang decreased from 1996 to 1998 by a total of 129.89 hm$$^{2}$$ and then remained relatively stable after 1998. The total carbon storage and variation in land use in the two counties displayed similar trends. Total carbon reserves in Julu increased by 2.76 $$\times$$ 10$$^{4}$$ tC (carbon equivalent), while those in Baixiang decreased by 0.63 $$\times$$ 10$$^{4}$$ tC. Carbon stock of built-up land in Julu and Baixiang increased by 2.44 $$\times$$ 10$$^{4}$$ and 1.22 $$\times$$ 10$$^{4}$$ tC, respectively.
... The physical land resource degradation in the IHR is mainly caused by landslides, mudslides, collapse of manmade terraces, soil loss from steep slopes and decline of forest and pastures areas (ICIMOD, 1994). Several best as well as recommended management practice have been suggested for restoration of degraded lands (Raizada and Juyal, 2012;Glendell and Brazier, 2014;Hansen and Nestlerode, 2014;Obame et al., 2014). The integrated use of organic-inorganic nutrient sources is considered a useful management strategy for improving soil quality, soil fertility and crop productivity especially under low input agricultural systems susceptible to degradation (Lal, 2014;Olson et al., 2014;Ghosh et al., 2016). ...
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In semiarid Sahelian region, the dynamics of soil organic carbon (SOC) and water are key to sustainable land management. This work focuses on the behaviour of carbon. A total of 33 soil profiles in four polders, ranging from 10 to 65 years in age, were sampled, analysed (0–1 m), and matched with marsh soil profiles in recent sediments considered as reference (t0) for carbon stocks determination. SOC and soil inorganic carbon (SIC) stocks show a spatial variability between polders. SOC stocks were t0 200 ± 0.8; t60 183 ± 34; and t65 189 ± 1.1 MgC·ha⁻¹, whereas the SIC stocks were negligible. These results show the highest stocks of soil carbon observed for this climatic region. The SOC stocks were also calculated for the equivalent soil mass at a defined depth (0–0.3 m); the corrected calculation of SOC stocks (Scorr) for 2450 Mg·ha⁻¹ of equivalent soil mass is t0 64 ± 1.9, t60 59 ± 9.8, and t65 53 ± 2.2 MgC·ha⁻¹; the stocks decrease by −7.8% and −17.2% from t0 to t60 and t65. Carbon was inherited from the pre-existing·marsh and the polders have conserved high stock values.
Article
The Northern coastline, from Kayar to Saint Louis (Senegal) is characterized by a succession of dunes and inter-dunes. Coastal wetlands located in the inter-dunes which bear Organic Matter (OM) rich soil where market garden agriculture is the main activity. These zones locally called “Niayes” are characterized by shallow groundwater table threatened by anthropogenic pressure, climate and environmental impacts. The scientific interest of this research work is to characterize the composition of soil OM to discuss its origin, composition, and diagenetic evolution in this depositional environment of Mboro research site with regard to natural and anthropogenic pressure. Three selected core samples with depth interval of 5 cm aligned in along a slope transect in the Mboro wetland were carried out. Soil OM were analyzed for Particulate OM, C/N, Rock-Eval HI and OI, δ¹³C, δ¹⁵N whereas sediments and water were respectively analyzed for grain size distribution, water contents and redox potential down to 50 cm depth. We have demonstrated through the results a vertical variability in the content, the nature and preservation quality of sedimentary OM with the occurrence of a peat storage zone. The isotopic markers depict a strong contribution of C3 plants, macrophytes and to a lesser extent of algae and microbial organic materials. High concentrations of dissolved nutrients occurring mainly on the surface slope may derive from the use of fertilizers to improve agricultural yields. These results constitute an added value in the scientific knowledge of this sector of the Niaye of Senegal.
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Ferruginous conditions were a prominent feature of the oceans throughout the Precambrian Eons and thus throughout much of Earth’s history. Organic matter mineralization and diagenesis within the ferruginous sediments that deposited from Earth’s early oceans likely played a key role in global biogeochemical cycling. Knowledge of organic matter mineralization in ferruginous sediments, however, remains almost entirely conceptual, as modern analogue environments are extremely rare and largely unstudied, to date. Lake Towuti on the island of Sulawesi, Indonesia is such an analogue environment and the purpose of this PhD project was to investigate the rates and pathways of organic matter mineralization in its ferruginous sediments. Lake Towuti is the largest tectonic lake in Southeast Asia and is hosted in the mafic and ultramafic rocks of the East Sulawesi Ophiolite. It has a maximum water depth of 203 m and is weakly thermally stratified. A well-oygenated surface layer extends to 70 m depth, while waters below 130 m are persistently anoxic. Intensive weathering of the ultramafic catchment feeds the lake with large amounts of iron(oxy)hydroxides while the runoff contains only little sulfate, leading to sulfate-poor (< 20 µM) lake water and anoxic ferruginous conditions below 130 m. Such conditions are analogous to the ferruginous water columns that persisted throughout much of the Archean and Proterozoic eons. Short (< 35 cm) sediment cores were collected from different water depths corresponding to different bottom water redox conditions. Also, a drilling campaign of the International Continental Scientific Drilling Program (ICDP) retrieved a 114 m long sediment core dedicated for geomicrobiological investigations from a water depth of 153 m, well below the depth of oxygen penetration at the time of sampling. Samples collected from these sediment cores form the fundament of this thesis and were used to perform a suite of biogeochemical and microbiological analyses. Geomirobiological investigations depend on uncontaminated samples. However, exploration of subsurface environments relies on drilling, which requires the use of a drilling fluid. Drilling fluid infiltration during drilling can not be avoided. Thus, in order to trace contamination of the sediment core and to identify uncontaminated samples for further analyses a simple and inexpensive technique for assessing contamination during drilling operations was developed and applied during the ICDP drilling campaign. This approach uses an aqeous fluorescent pigment dispersion commonly used in the paint industry as a particulate tracer. It has the same physical properties as conventionally used particulate tracers. However, the price is nearly four orders of magnitude lower solving the main problem of particulate tracer approaches. The approach requires only a minimum of equipment and allows for a rapid contamination assessment potentially even directly on site, while the senstitivity is in the range of already established approaches. Contaminated samples in the drill core were identified and not included for further geomicrobiological investigations. Biogeochemical analyses of short sediment cores showed that Lake Towutis sediments are strongly depleted in electron acceptors commonly used in microbial organic matter mineralization (i.e. oxygen, nitrate, sulfate). Still, the sediments harbor high microbial cell densities, which are a function of redox conditions of Lake Towuti’s bottom water. In shallow water depths bottom water oxygenation leads to a higher input of labile organic matter and electron acceptors like sulfate and iron, which promotes a higher microbial abundance. Microbial analyses showed that a versatile microbial community with a potential to perform metabolisms related to iron and sulfate reduction, fermentation as well as methanogenesis inhabits Lake Towuti’s surface sediments. Biogeochemical investigations of the upper 12 m of the 114 m sediment core showed that Lake Towuti’s sediment is extremely rich in iron with total concentrations up to 2500 µmol cm-3 (20 wt. %), which makes it the natural sedimentary environment with the highest total iron concentrations studied to date. In the complete or near absence of oxygen, nitrate and sulfate, organic matter mineralization in ferruginous sediments would be expected to proceed anaerobically via the energetically most favorable terminal electron acceptors available - in this case ferric iron. Astonishingly, however, methanogenesis is the dominant (>85 %) organic matter mineralization process in Lake Towuti’s sediment. Reactive ferric iron known to be available for microbial iron reduction is highly abundant throughout the upper 12 m and thus remained stable for at least 60.000 years. The produced methane is not oxidized anaerobically and diffuses out of the sediment into the water column. The proclivity towards methanogenesis, in these very iron-rich modern sediments, implies that methanogenesis may have played a more important role in organic matter mineralization thoughout the Precambrian than previously thought and thus could have been a key contributor to Earth’s early climate dynamics. Over the whole sequence of the 114 m long sediment core siderites were identified and characterized using high-resolution microscopic and spectroscopic imaging together with microchemical and geochemical analyses. The data show early diagenetic growth of siderite crystals as a response to sedimentary organic matter mineralization. Microchemical zoning was identified in all siderite crystals. 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The study of the organic matter in sediments and sedimentary rocks focuses on the interaction between the biosphere and geosphere. A proper appreciation of the subject requires an understanding of the environmental controls which govern the production of organic matter in the biosphere, the ecological and sedimentological processes which control its deposition and distribution, the biogeochemical, and geomicrobiological factors which influence its preservation, and the geochemical and physical processes which determine its modification during its incorporation in the geosphere. This makes the study of sedimentary organic matter one of the most multidisciplinary pursuits within the whole field of earth sciences.
Article
Management of soil organic matter (SOM) is esential to sustaining the quality and productivity of soils around the globe. This appears to be particularly true in the tropics where there is a greater proportion of nutrient poor, highly weathered soils that are more susceptible to losses of SOM. Developing management practices that promote the maintenance and storage of SOM in the tropics depends on understanding the factors that control SOM dynamics. This paper describes the role that soil physical properties (mineralogy, texture, and structure) play in regulating the accumulation and loss of SOM in tropical soils. Two different approaches are presented here. The first approach explores relationships between total SOM and soil physical properties in the tropics. These include effects of climate and mineralogy on latitudinal gradients in SOM, interactions between texture and mineralogy as determinants of SOM storage and relationships between SOM and the structural stability of soils. The second approach describes characteristics of SOM associated with different physical constituents of the soil, with particular attention to particle-size fractions and aggregated particles of different sizes. In each case we summarise findings on the distribution of SOM among fractions and characterise its biochemical composition, bioavailability and turnover. Evidence for and against the physical protection of organic matter from microbial attack in tropical soils is also given. Wherever possible, we compare and contrast the findings for tropical soils with those of temperate soils. The influence of landuse management on physical control of SOM dynamics is discussed as an overriding factor with each aproach.
Article
Long-term fertility experiments (LTFEs) are a tool to investigate the sustainability of cropping systems. The present study analyzed two LTFEs for intensive rice-based irrigated systems in the Senegal River valley at Ndiaye and Fanaye (Sahel savanna). The trials were established in 1991, contain six different fertilizer treatments and rice is grown two times per year. Soil types are a typical Orthithionic Gleysol and an Eutric Vertisol at Ndiaye and Fanaye, respectively. The objectives of the presented study were to analyze the effect of intensive irrigated rice cropping on the soil resource base by studying the changes of soil characteristics over time and by comparing soil N, P and K pools in different fertilizer treatments. In the LTFE at Ndiaye, topsoil pH values increased significantly from 5.5 to about 6.5 and electrical conductivity was high but remained stable. Soil organic carbon (SOC) and total soil nitrogen (TSN) dropped slightly after 16 consecutive seasons but the difference was statistically not significant. At both sites, exchangeable N ranged between 1.6 and 2.8% of TSN and fixed N accounted for 5.5–8.2% of TSN, with slightly higher values in Fanaye. Treatment differences in N dose had no significant effect on these parameters. Results of δ13C analysis showed a decrease due to rice cropping at both sites, and the measurements indicate high turnover rates of soil organic matter. Soil analyses of total soil P and K and of different pools indicated only small changes when these elements were applied at medium quantities. In contrast, treatments with N application only showed considerable soil P and K depletion, and rice cultivation without P and/or K application cannot maintain soil fertility. The soil mining process is relatively quick for P due to the naturally low soil P status, whereas the high soil K reserves buffer even important negative K balances for decades. It is concluded that irrigated rice cultivation in the region can maintain soil fertility if at least medium P doses are applied together with nitrogen.
Article
Rock–Eval 6 analysis, a well established screening tool for petroleum geochemistry, is being increasingly used to characterise the varying species of organic matter (OM) in the bulk samples of recent aquatic sediments. This is particularly important due to recent scientific attention on the role of OM in biogeochemical distribution of environmentally hazardous compounds (e.g., trace metals) in recent sediment archives. Rock–Eval’s automated use, low sample volume requirements and its high analytical accuracy and precision makes it an ideal tool for relatively rapid screening of OM in sediment cores. However, to date, there has been no broad scale standardisation to determine what may be contributing to each signal (e.g., S1, S2, S3, RC). We have selected a wide variety of representative, pure biochemicals (proteins, lipids, carbohydrates and lignins) and biological standards (phytoplankton, copepods, tree bark and conifer needles) to better understand the Rock–Eval 6’s measured organic matter parameters in the unconventional environmental samples. These data have been corroborated with organic petrographical and elemental (CHNS/O) data. Our results show that small organic molecules (<500 Da) are largely responsible for the S1 hydrocarbon peak while lipids and aquatic biological standards are contributing most in the S2 signal, and in particular the more labile “S2a” signal. Furthermore, carbohydrates, lignins and terrigenous plant standards are most responsible for the S3 signal. We also note that the S3 signals (CO/CO2 ratios: OICO, OICO2 and OIRE6) are the best discriminants for the source of OM. Finally, step wise pyrolysis of biological standards coupled with elemental analysis (CHNS/O) suggests that S2 and, to a lesser extent, S3 (S3CO and/or S3CO2), would be most responsible for metal-binding elements such as S and N, with implications for element biogeochemical cycles.
Article
Sediment samples collected along a 6-m core, drilled in the deepest part of the Lagoa do Caçó (NE Brazil), have been investigated in order to determine source(s) and degradation conditions of the organic matter (OM) with special emphasis on paleoenvironmental implications. Bulk organic geochemistry (Rock-Eval pyrolysis, C/N determination, δ13C and δ15N measurement) and petrography combined with sedimentological evidence and radiocarbon dates allowed to identify four major intervals documenting major environmental changes that occurred during the last 20,000 years. The first interval, dating back to the end of the Last Glacial Maximum (LGM), contains well-preserved OM derived from higher plants. This material was most probably produced in an ephemeral palustrine system and rapidly buried by sands. This level is thought to have been deposited under relatively arid climate conditions associated with strong but episodic rainfalls. Between 19,240 and 17,250 Cal years BP, the climate appears to have been more humid and seasonality more pronounced as suggested by the presence of a permanent lake. After a drastic environmental change dating back to 17,250 Cal years BP, the sediment became truly lacustrine with restricted mineral input and highly degraded higher plant-derived organic matter. After that, a stepwise improvement in the preservation of OM occurred, as revealed by several pronounced shifts in the Rock-Eval TpS2 signal. These changes could document abrupt climatically driven changes during the Late Glacial. Finally, around 5610 Cal years BP, environmental conditions, approaching those prevailing today were established. Minor climatic changes during the Holocene were probably buffered by a high water table which might explain the lack of paleoenvironmental fluctuations.
Article
The historical sediment record of Lake Sonachi (Kenya) was used to study the influence of lake depth and mixing regime on patterns of sedimentation in a small, fluctuating tropical soda lake. Lake Sonachi last desiccated com- pletely in the early nineteenth century and has fluctuated between 3- and 18-m lake depth over the past 115 yr. A freeze-core of offshore sediments describes recent lake history as a succession of meromictic episodes, represented by varved or subannually laminated muds, and holomictic episodes, represented by more coarsely layered muds. Two interbedded horizons of colloidal amorphous silica dated to a period of rising lake level after a prolonged lowstand were deposited by abiogenic, pH-driven precipitation from the water column and represent the instanta- neous sequestering of an estimated 63 mg liter 21 or possibly .50% of the lake's dissolved-silica reservoir. Changes in offshore sedimentation and inferred bottom dynamics over time indicate that sediment resuspension and focusing in Lake Sonachi occur mostly during infrequent events of deep circulation between the average mud deposition boundary depth at ;2 m and the chemocline depth at 4-5 m; wind-driven sediment redistribution across the lake floor is important only at lake depths of #3 m. Dry sediment accumulation has varied between 86 and 620 g m 22 yr 21 over the past 175 yr, with no relationship to lake depth but, on average, lower rates during meromixis (199 6 90 g m 22 yr 21 ) than during holomixis (349 6 152 gm 22 yr 21 ). Net organic carbon accumulation offshore varied between 0 and 92 g m 22 yr 21 , with no significant relationship to either lake depth or mixing regime at the time of deposition. Sedimentary organic carbon content (5.7-26.9%) is negatively correlated with bulk sediment accumu- lation; 73% of this variation is accounted for by the clastic dilution of sedimented planktonic algal production with low-organic littoral sediments redeposited offshore.
Article
Globally, lakes are currently accumulating organic carbon (OC) at an estimated annual rate of about 42 Tg · yr-1. Most of the OC in all but the most oligotrophic of these lakes is autochthonous, produced by primary production in the lakes. The sediments of reservoirs accumulate an additional 160 Tg annually, and peatlands contribute 96 Tg annually. These three carbon pools collectively cover less than 2% of the Earth's surface and constitute a carbon sink of about 300 Tg · yr-1. Although the oceans cover 71% of the Earth's surface, they accumulate OC at a rate of only about 100 Tg · yr-1.
Article
This study presents a synthetic approach based on the combined use of sediment and C org accumulation rates of well studied lake systems and oceanic margins. Importance of latitude is expressed by the advected flux of terrestrial sediment and, especially, of particulate organic carbon. This most important factor varies throughout the Quaternary, particularly with the intensification of fluxes during Holocene. These changes are linked to the strengthening of monsoon circulation.In various lake systems from Cameroon (Barombi Mbo, Ossa, Assom and Bambili), Gabon (Kamalete) and Congo (Kitina and Sinnda), the global sedimentation and the C org accumulation were slow during dry period and increased during wet period. This relationship is verified to the scale of the Gabon and Congo oceanic margins where the accumulation rates increase during extent of ombrophilous forest. However, the greatest fluxes of organic carbon during wet periods would be balanced by higher concentrations values during the dry period resulting in a nearly homogenous carbon accumulation. These carbon concentrations are generally explained by the input of coarse debris by abrupt floods and by a less degraded organic matter as a result of the cooling of the climate. But, according to specific morphology features or vegetation cover, some lake systems exhibit distinct trends of the sedimentary and C org accumulation rates: (1) Highest accumulation rates coincided with a forest retreat when the slope is too steep (Kamalete in Gabon and Bosumtwi in Ghana) or when river flood overrun (Nguene in Gabon) (2) Increase of the accumulation rate are registered without change of the vegetal cover and express only rainfall growth (Bambili and Assom).
Article
A comparative study was made of organic carbon accumulation in five lacustrine environments in Brazil (large open water lake, ponds with floating meadows and marshes). The most representative cores were used to calculate the accumulation of total organic carbon (TOC) in the sediment over the past 12,000 cal years BP. Carbon accumulation rates were determined using the TOC concentration, sediment «in situ» density, ¹⁴C calibrated ages and the interpolated sedimentation rate (cm year⁻¹) or sediment accumulation rate (g m⁻² year⁻¹). The C/N ratio and microscopic observations were used to characterise the sedimentary organic matter.
Article
Accurate and reliable estimates of carbon (C) storage in landscapes are critical to the development of effective policies and strategies to mitigate atmospheric and climate change. Carbon stocks of two native woody shrub (Guiera senegalensis J.F. Gmel and Piliostigma reticulatum (DC.) Hochst) communities and associated soils within Senegal's Peanut Basin were determined and the spatial structure of soil C quantified. These shrubs are of interest because they dominate semiarid sub-Sahalien Africa and commonly coexist with row crops but have been largely overlooked as a key vegetative component of this landscape. Peak-season shrub biomass C was measured in forty-five 0.81 ha plots at 8 locations using allometric relationships along with soil sampling (0 to 40 cm depth) and analysis for organic C and bulk density. Soil samples to a depth of 20 cm were taken every 2 m in 24×20 m grids and every 0.5 m in four nested 3 m×3 m grids containing at least one shrub or tree canopy, and geostatistical techniques were then used to quantify scale and degree of soil C spatial dependence. Estimates of peak-season biomass C ranged from 0.9 Mg C ha−1 to 1.4 Mg C ha−1 with an overall mean of 1.12 Mg C ha−1 (SEM=0.079) in the G. senegalensis sites and from 1.3 to 2.0 Mg C ha−1 (mean=1.57 Mg C ha−1; SEM=0.18) in the P. reticulatum communities. The overall mean of SOC to 40 cm was 17 and 17.2 Mg C ha−1 respectively, at the G. senegalensis and P. reticulatum sites with 57% of that C residing in the top 20 cm. Semivariograms of soil C showed moderate spatial dependence and spatial autocorrelation at distances of less than 0.56 and 1.34 m at the G. senegalensis and P. reticulatum sites, respectively. Comparison across the different grids showed that the presence of shrub canopies at either site had much closer relationship to soil C levels than trees.
Article
The hydrology of the Sahel is characterized by the degradation of the drainage network, resulting in the lack of large watersheds over which the spatial integration of the hydrological processes could be studied. The main hydrological units are small endoreic areas, measuring a few hectares to a few square kilometres and the surface runoff is collected into pools. A detailed investigation of the role of these pools in the hydrology of the HAPEX-Sahel Central Super-Site was carried out from 1991 to 1993. The first results of this investigation are presented. A typology in three classes of the endoreic systems (valley bottoms; sinks; plateaux) is proposed. The behaviour of one representative pool in each class is analysed, showing that the partition between evaporation and deep infiltration depends on the level of filling of the pools. The bottom of the pool is clogged by clay deposits, which prevent infiltration. Above a threshold varying between 1 and 2 m most of the water stored in the pool after runoff infiltrates, contributing to the recharge of the aquifers. On a seasonal basis, deep infiltration accounts for less than 50% of the water collected by the plateau pool, and more than 80% for the valley bottom pools. Almost all the water running off to the sink pools infiltrates rapidly and deeply into the ground. The valley pools (both valley bottoms and sinks) appear to be the major contributors to the recharge of the upper aquifer. The proportion of the HAPEX-Sahel Central Super-Site water balance that is taken by the deep infiltration from the pools varies greatly depending on the temporal distribution of rainfall. Whereas similar seasonal rainfalls were recorded in 1991 and 1992, it is estimated that 5% of the water precipitated over the valley pool watershed infiltrated towards the aquifer in 1991 and 20% in 1992. This difference is explained by a very irregular time distribution of precipitation in 1992, most of the major rainfall events being observed over a short period during the intensive observation period. In conclusion some preliminary figures are given regarding the importance of recharge from the pools as compared with in situ recharge.
Article
Lagoa do Caçò is an equatorial lake in northeast Brazil, where the Inter-Tropical Convergence Zone (ITCZ) enters the South American continent, and so at a key position for deciphering Quaternary climatic variation. Preliminary results have shown that the sedimentary organic matter (OM) in the lake records the shifts in the ITCZ since the Last Glacial Maximum. The nature and fate of the OM have been determined by studying surface sediments and potential OM sources. We present here the results of organic petrographic and Rock-Eval pyrolysis assessments of the biological origin and preservation state of the OM in modern sediments from two deep transects of the lake. The results led us to understand how an oligotrophic lake has accumulated 6 m of OM-rich sediment during the last 20 kyr. Palaeoenvironmental studies have shown that the lake level fluctuated during this time. Our results explain the influence of lake bathymetric variation on early diagenesis and enable interpretation of the corresponding sedimentary OM record, which is dependent on the bathymetric variation.
Article
Geochemical analysis of sedimentary organic matter in recent lacustrine sediments appears to be a useful tool in providing information concerning past environmental conditions. However, such analysis is often made without knowing the geochemical characteristics of the organic matter derived from the watershed and, more explicitly, its soils. The present work deals with (i) a geochemical investigation (Rock-Eval pyrolysis) of soil organic matter sampled in a lake watershed, and (ii) the study of the sedimentary organic matter trapped in the lake deposits. The research was conducted on Chaillexon Lake which was created by a rock collapse that dammed the palaeovalley of the Doubs River about 12 000 years ago. Since this event, the sediment trap provides a continuous palaeoclimatic record for the Postglacial period.Results obtained lead to two main conclusions. First, the variability of Rock-Eval pyrolysis values observed in soils modifies the common interpretation given to these parameters in the characterization of sedimentary organic matter. Indeed, variations in these parameters point not only to varying proportions of terrestrial and lacustrine organic matter in a lacustrine infilling but also to variations of the terrestrial supply linked with the evolution of vegetal cover in the catchment. The second conclusion is that the story of the Chaillexon lacustrine system is marked by a rather sudden soil and forest development at the Preboreal–Boreal transition (9000 BP). Copyright © 1998 John Wiley & Sons, Ltd.
Article
The lack of appropriate data has long been a major obstacle to the study of the Sahelian rainfall at the event scale. In this paper, use is made of the EPSAT-Niger recording rain-gauge data to characterize the convective rain events of the central Sahel. Although some considerations lead to the identification of two main types of mesoscale convective systems, it is shown here that the most relevant stratification in terms of statistical analysis of the event rainfall distribution is between the events of the margins and those of the core of the rainy season. In fact, the average storm rain-depth appears to be non-stationary in time, with storm rain-depths slightly higher in the core of the rainy season than on the margins. Separation between the core and the margins thus allows the fitting of an exponential model to the observed storm rain-depth distributions of each period (core and margins), although a better fit would certainly be obtained if a proper modelling of the time non-stationarity was carried out. It is then shown that there is little, if any, correlation between the mean storm rain-depth of a given year and the overall abundance of the corresponding rainy season. This is a validation of previous works, which reached the same conclusion using daily rainfall data only. One major result of this work is that the statistics characterizing the rain events in the Sahel display little fluctuations, either in space or from year-to-year, as compared with those observed for the total seasonal rainfall. Each year and at each station the average storm rainfall remains close to 12 mm during the margins of the rainy season and close to 15 mm in the core. During the same period, the average seasonal rainfall over the study area ranged from 400 to 660 mm and for any given year the ratio between the maximum and the minimum point seasonal rainfall was of the order of 2. It is therefore concluded that the main source of rainfall variability in the Sahel is linked to the variability in the number of events rather than in the magnitude of these events. © 1998 Royal Meteorological Society.
Article
We estimate that recent increases in organic carbon accumulation in the sediments of lakes and reservoirs amount to about 0.02 ṁ 1015 gC yr−1 and 0.2 ṁ 1015 gC yr−1, respectively. The reservoir accumulation represents a small but significant fraction of the carbon missing in current global budgets.
Chapter
Two metres of sediments were recovered from the Lac du Bouchet (Massif Central, France) to assess the influence of climatic changes on organic lacustrine sedimentation. Palaeoenvironmental reconstruction was realised on the basis of a quantitative evaluation of the different mineral and organic fluxes. The late glacial period is marked by high mineral and low organic fluxes. The transition to the Holocene is mainly characterised by an increased flux of organic components (terrestrial biomass and phytoplankton) and a relative decrease in the mineral fluxes. The respective evolution of phytoplanktonic and terrestrial higher-plant fluxes reflects the local palaeoenvironmental variations concerning both the sedimentation and surrounding basin. The end of the Atlantic period shows a climatic cooling and the transition from Sub-Boreal to Sub-Atlantic underlines the installation of present climatic conditions. Anthropogenic influence is visible from the middle of the Sub-Atlantic. The regional palaeoenvironmental variations are also recorded in other European sites (SchalkenMehrener Maar, Lago di Monticchio), although slight differences occur due to local effects related to the geomorphology of the basins. This palaeoclimatic evolution parallels global climatic changes as recorded by the recognition of similar major trends in tropical sites (Carajas, Amazonia).
Article
Organic matter constitutes a minor fraction of marine and freshwater sediments, yet its important contribution to the sedimentary record can be used to reconstruct marine and continental paleoenvironments. The organic matter content of sediments is the residue of past biota. The amounts and types of organic matter present in sediments consequently reflect environmental conditions that impacted ecosystems at different past times. General sources of the organic matter are inferred from bulk properties such as elemental compositions, carbon and nitrogen stable isotope ratios, Rock-Eval pyrolysis data, and organic petrography. Details of organic matter origins are refined by analyses of biomarker molecular compositions. Source changes are proxies for fluctuations in sea-level, oceanic surface currents, and continental climates. Algal paleoproductivity rates are indicated by organic-carbon mass accumulation rates and carbon and nitrogen stable isotopic compositions. These parameters record past availability of nutrients and, therefore, are proxies of surface mixing in the oceans and amount of land runoff to lakes. Sea-surface paleotemperatures are recorded by the number of carbon–carbon double bonds in lipid biomarkers produced by marine algae. Larger proportions of the double bonds are proxies for the cooler surface waters that accompanied periods of global glaciation and intervals of enhanced upwelling. The δ13C and δD values of plant organic matter record past concentrations of carbon dioxide in the atmosphere and changes in delivery of atmospheric moisture, respectively. Diagenesis, which causes the concentration and composition of organic matter in sedimentary settings to differ from those of the original biologically synthesized materials, can bias organic geochemical paleoenvironmental records. The magnitude of this potential source of misinformation must always be considered and evaluated. Comparison of multiple organic geochemical proxies of past conditions helps to compensate for the effects of diagenetic alterations and thereby to improve interpretations of paleoenvironmental change.
Article
Throughout Earth history, almost all preserved organic matter has been incorporated in marine sediments deposited under oxygenated waters along continental margins. Given modern oceanic productivity and sediment burial rates of 50 × 1015 and 0.16 × 1015 gC yr−1, respectively, organic preservation in the marine environment is < 0.5% efficient. Although correlative information is often used to suggest that productivity, sediment accumulation rate, bottom water oxicity, and organic matter source are key variables, the mechanisms governing sedimentary organic matter preservation have remained unclear.The factors which directly determine preservation vary with depositional regime, but have in common a critical interaction between organic and inorganic materials over locally variable time scales. More than 90% of total sedimentary organic matter from a wide variety of marine depositional environments cannot be physically separated from its mineral matrix. This strongly associated organic component varies directly in concentration with sediment surface area and thus appears to be sorbed to mineral grains. Sediments accumulating outside deltas along continental shelves and upper slopes characteristically exhibit mineral surface area loadings approximately equivalent to a single molecular covering. These monolayer-equivalent coatings include a fraction of reversibly bound organic molecules that are intrinsically labile, but resist appreciable mineralization as they pass rapidly through oxygenated surface sediments and are preserved within underlying anoxic deposits. The delivery of mineral surface area is the primary control on organic matter preservation within these expansive coastal margin regions where roughly 45% of all organic carbon accumulates.Deltaic sediments account for roughly another 45% of global carbon burial, but often exhibit much less than monolayer-equivalent organic coatings. This pattern is seen in periodically oxygenated sediments off the mouth of the Amazon River, even though the component clastic minerals are discharged by the river with monolayer coatings. Comparably extensive losses of organic matter, including distinct particles such as pollen grains, occur in the surfaces of deep-sea turbidites in which long term reaction with O2 is clearly the causative factor. Sub-monolayer organic coatings also are observed in continental rise and abyssal plain sediments where slower accumulation rates and deeper O2 penetration depths result in increased oxygen exposure times and little (~ 5% of the global total) organic matter preservation. A transition zone between monolayer and sub-monolayer organic coatings apparently occurs on lower continental slopes, and is marked along the Washington coast by parallel offshore decreases in total organic matter and pollen between 2000–3000 m water depth.Sediments underlying highly productive, low-oxygen coastal waters such as off Peru and western Mexico are characteristically rich in organic matter, but account for only ~ 5% of total organic carbon burial. These sediments show a direct relationship between organic matter content and mineral surface area, but at organic loadings 2–5 times a monolayer equivalent. Organic materials sorbed in excess of a monolayer thus also may be partially protected. Such high sedimentary organic contents may result from equilibration with DOM-rich porewaters, or very brief O2 exposure times which allow preservation of extremely oxygen-sensitive organic materials such as pigments and unsaturated lipids. Thus organic matter preservation throughout much of the ocean may be controlled largely by competition between sorption at different protective thresholds and oxic degradation.Future research strategies should be specifically directed at delineating the mechanisms for organic matter preservation in marine sediments. In particular, special effort is needed to determine the amounts and types of sorbed organic materials and the nature of their bonding to mineral surfaces. The extent and dynamics with which organic molecules are partitioned between porewaters and solid phases also should be determined, as well as the effects of these phase associations on their reactivities toward chemical and biological agents. In addition, processes for slow oxic (and suboxic) degradation of organic materials bear investigation in deep-sea sediments, as well as in other extreme environments such as oxidizing turbidites, weathering shales, and soils. Such studies should include characterizations of hydrolysis-resistant organic materials and emphasize the complementary use of biochemical compositions with readily separable particles such as pollen to calibrate and typify the mechanisms and stages of sedimentary organic degradation.
Article
Among the three sites distributed along the West African latitudinal gradient in the AMMA-CATCH observation system, the experimental setup in the Niamey area of south-west Niger samples the cultivated Sahel environment, for hydrological, vegetation and land surface processes. The objective is to investigate relationships between climate, land cover, and the water cycle, in a rapidly changing semiarid environment. This paper first presents the main characteristics of the area, where previous research, including the EPSAT and HAPEX-Sahel experiments, had evidenced a widespread decadal increase in water resources, concurrently with severe drought conditions. The specifics of AMMA-CATCH research and data acquisition at this site, over the long-term (∼2001–2010) and enhanced (∼2005–2008) observation periods, are introduced. Objectives and observation strategy are explained, and the main characteristics of instrument deployment are detailed. A very large number of parameters – covering rainfall, vegetation ecophysiology, phenology and production, surface fluxes of energy, water vapour and CO2, runoff and sediment, pond water, soil moisture, and groundwater – were monitored at local to meso scales in a nested structure of sites. The current state of knowledge is summarized, connecting processes and patterns of variation for rainfall, vegetation/land cover, and the terrestrial hydrologic cycle. The central role of land use and of its spectacular change in recent decades is highlighted. This paper provides substantial background information that sets the context for papers relating to the south-west Niger site in this AMMA-CATCH special issue.
Article
Lake basins (∼2.7×106 km2, about 0.8% of the ocean surface or 2% of the land surface) bury a surprisingly high amount of atmospheric carbon (∼70×106 t/a) which reaches more than one fourth of the annual atmospheric carbon burial in the modern oceans. This is mainly accomplished by the rapid accumulation of lacustrine sediments and a very high preservation factor (on average 50 times higher than that in the oceans). Lakes with relatively large drainage areas commonly display the highest carbon accumulation rates. In most cases, burial of organic matter is more important than that of carbonate carbon produced by silicate weathering, in contrast to the oceans where the burial of atmospheric carbonate carbon almost reaches the same amount as that of organic carbon. Exceptions to this rule are closed lake basins in arid to semiarid climate which precipitate a major part of their atmosphere-derived dissolved inorganic carbon (DIC) as carbonate. These results are demonstrated in some detail for L. Qinghai, China, (low contribution of atmospheric carbonate carbon) and L. Turkana, East Africa, (high contribution from silicate rocks). Further data are gained by estimates for a number of closed and open lakes. The drainage areas of the lakes withdraw atmospheric carbon at rates of mostly 1–4 g/m2/a, calculated from the lacustrine carbon burial. Carbon burial rates in lakes commonly increase with change to wetter and warmer climate (partially larger lake surfaces, higher rates of seasonal carbonate precipitation, trend to stratified lake waters with oxygen-deficient bottom water). Anthropogenic influence mostly enhances the production and preservation of organic carbon in lake basins (often by a factor of 3–4). After the last glacial maximum, the joint action of the globally spreading vegetation, peat growth, and carbon burial in lakes would have been able to reduce the atmospheric carbon pool to one third to one half of its present amount within a time period of 1 ka. However, CO2 exchange between the atmosphere and the ocean has brought about an overall increase in the atmospheric CO2 during the Holocene. The contribution of lakes and artificial reservoirs in counteracting man-made CO2 emissions should not be neglected.
Article
A land surface map of the region of Niamey (2–3°E, 13–14°N), Niger, derived from a classification of SPOT multispectral sattelite data with 20 × 20-m resolution, is described. Sixteen distinct classes were defined based on landform, vegetation soil crust type and land use. The classification system was designed to emphasize the role of these factors in hydrological and ecological processes. The primary purpose was to provide information for integrating local-scale measurements of land-atmosphere interactions in HAPEX-Sahel (Hydrological and Atmospheric Pilot Experiment) up to a region 1° × 1° square, and to assist hydrologists in interpreting the spatial distribution of soil moisture. A large number of ground descriptions were integrated with high spatial resolution multispectral satellite reflectance data. Since most of the hydrological and atmospheric observations in HAPEX-Sahel were carried out in only three land cover types (tiger bush on the plateaux, fallow savanna and millet fields in the sand valley or plains), the 16 classes were merged successively degree of nine then six so that three final maps were produced, each one reflecting a different degree of aggregation.Some difficulties could not be resolved. One stemmed from the continuum linking land left fallow and cultivation. This uncertainty can be a problem since bush-fallow and cropped fields are hydrologically and ecologically quite distinct. Another major difficulty was the detection of sparse vegetation cover using the satellite data.Each surface condition class was assigned a range of runoff capability parameters derived from numerous rainfall simulation tests conducted in the Sahelian zone. Consequently, runoff production could be assessed for each 20 × 20-m pixel. However, due to the endoreic nature of the watersheds in the region, the use of the map to predict runoff production must be restricted to small areas ranging from the pixel size to catchments of 1–10 km2.
Article
The Continental Terminal water-table near Niamey (S-W Niger) has been the subject of a dense and detailed survey conducted over a period of almost 15 yr. The continuous rise in the groundwater level was unexpected but manifest and varied between 0.01 and 0.45 m yr−1. As shown by corroborating measurements made throughout the twentieth century, this rise has been taking place for much longer and present levels are the highest ever recorded. Since the beginning of the 1960s, groundwater resources have increased by up to 150% (+15% on median), in spite of the severe droughts of the 1970s and 1980s. Based on isotopic data, infiltration is estimated at around 5 mm yr−1 for median over the long-term. Hydrodynamic observations show that in recent years it has exceeded 20 mm yr−1. The rise acceleration during the past decade is apparent from groundwater level chronicles. Even if fluctuations in rainfall may interfere, this phenomenon is mainly explained by a change in land-use. In this semi-arid area, intense land clearing has modified the hydraulic properties of the top cm of the soil and has consequently increased surface runoff. As runoff concentrates in temporary endoreic ponds and then infiltrates to the water-table, higher runoff implies higher groundwater recharge and a subsequent rise in the water-table. This is one of the best documented examples of a long-term rise of an African water-table in such a semi-arid context.
Article
Carbon sequestration in soil organic matter is increasingly advocated as a possible win–win strategy in the rehabilitation of degrading dryland agro-ecosystems because it simultaneously contributes to the reduction of global atmospheric greenhouse gas concentrations while enhancing local land productivity. A study was conducted in Senegal's Old Peanut Basin to assess current carbon stocks and to examine management options for their increase. Average soil and woody biomass carbon contents were 11.3 and 6.3 t carbon (C) ha−1, respectively. CENTURY, a biogeochemical model, was used to simulate soil and biomass carbon over a period of 25 and 50 years under a series of land use and management options. These simulated practices resulted in C dynamics ranging from −0.13 t C ha−1 yr−1 from a worst-case millet–sorghum rotation to +0.43 t C ha−1 yr−1 on intensively managed agricultural fields. Agroforestry simulations involving Faidherbia albida (Del.) Chev. and Leucaena leucocephala (Lam.) deWit. also resulted in promising carbon gain (+0.22 and +0.12 t C ha−1 yr−1, respectively), suggesting that improving agricultural practices is key to enhancing food production and mitigating climate change. Results from a sensitivity analysis suggest that woody biomass carbon is more sensitive to long-term changes in precipitation and temperature than soil carbon. Other management strategies likely to result in lower rates of soil carbon sequestration, including short-term improved fallows, should also be considered viable opportunities because promoting too narrow a set of ‘best management practices’ risks weakening local adaptability and opportunistic management regimes, both of which are crucial elements in small-scale farming systems in drylands.
Article
The relationship between organic carbon (OC) and grain size found in most continental shelf sediments is here reinterpreted in terms of the surface area of the sediments. Cores from many North American shelf environments show downcore decreases in OC to similar refractory background concentrations if expressed relative to the surface area of the sediments. This consistent concentration is 0.86 mg-OC m−2, which is equivalent in concentration to a monolayer of organic matter coating all mineral surfaces. A more global collection of sediment-water interface samples show that this relationship is even more extensive, with exceptions occurring in areas of very high riverine sediment input, organic pollution, or low-oxygen water columns. Density separations indicate that organic matter is largely adsorbed to mineral grains. The microtopography of surfaces was examined with N2 sorption and most surface area was found to be inside pores of <10 nm width. These data lead to a hypothesis that organic matter is protected by its location inside pores too small to allow functioning of the hydrolytic enzymes necessary for organic matter decay. Such protection would likely work in concert with other protection mechanisms such as humification. This consistent surface area correlation with OC concentration may explain control of spatial and temporal variations in OC burial rates by sedimentation rates; the pore protection hypothesis provides a causal mechanism for this observed control.
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In the West African semiarid belt of the Sahel, for the second half of the XXth century, lasting droughts (1970s–1980s) and one of the World's highest population growths have resulted in major land cover and hydrological changes that can be quantified using aerial photographs. This paper aims to provide one of the longest combined observations of land cover and hydrological changes for semiarid areas using a time series of normalised mosaics of aerial photographs dating back from 1950, field inquiries, and updated groundwater data. The 500 km2 study area in southwest Niger was chosen (i) for its rural environment representative of the rain-fed agriculture belt of the Sahel and (ii) to encompass the main hydrological study sites investigated in this region over the past two decades (Hapex-Sahel and AMMA experiments, 1990–2000s). Results have significant implications for future freshwater availability and food security in the Sahel.
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Soils are a major location of carbon (C) storage in most terrestrial ecosystems and changes in soil C pools therefore play an important role in the C cycle. Human conversion of native ecosystems to agro-ecosystems, overgrazing and lack of agricultural management influence the amount, quality and turnover of soil organic carbon (SOC). This paper focuses on changes in reactivity, distribution and total stocks of SOC in the upper 1 m of soils representing savanna as well as groundnut (Arachis hypogaea L.) and millet (Pennisetum glaucum L.) fields cultivated for up to 40 years within the semi-arid part of Senegal. The soil C stock varied from 4.4 kg C m−2 in open woodland savanna to 2.7 kg C m−2 in grass savanna. Clearing and cultivation of savanna land has resulted in a loss of SOC up to 24% in the upper 1 m equal to 1.1 kg C m−2 over 40 years, or more than six times the short-term C loss associated with clearing of above-ground biomass in nowadays semi-arid savanna in Senegal. Observed SOC losses the first 4–8 years following cultivation were consistent for all sites, whereas observed long-term losses suggest important variations related to a north–south (N–S) climatic gradient across the region. Laboratory respiration measurements reveal similarity with respect to SOC reactivity, which is consistent with the observations of significant higher soil CO2 effluxes from savanna soils than cultivated soils. The results call for further attention on region-specific changes in SOC stocks (time scales) due to cultivation and improved understanding of the long-term controls of SOC cycling after clearing of tropical savanna.
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Groundwater level measurements taken over a 4-year period from an extensive network of wells and boreholes within the HAPEX-Sahel (Hydrologic Atmospheric Pilot Experiment in the Sahel) degree square (south Niger), together with existing data, have provided an insight into infiltration and recharge processes taking place in the porous phreatic aquifer of the Continental Terminal formation. Despite high spatial and temporal variability of aquifer response to rainfall (rises of between 0 and 9 m are recorded), a pattern of recharge can be recognised. Aquifer responses vary from site to site, but the type of response at any single point tends to be consistent from year to year. Recharge is dominated by infiltration from temporary drainage networks (pools and streams) and aquifer response depends to a large extent on aquifer hydraulic characteristics and distance from the nearest infiltrating zone. In many wells, for which data extending back to 1987 is available, water levels show a consistent year by year rise. This is interpreted as a process of aquifer recovery following the severe drought of the 1970s and early 1980s, though part may also be attributable to changing patterns of land management (e.g. woodland clearance). Initial estimates of regional recharge are from 50–60 mm year−1, or in other words about 10% of annual rainfall. The figure is supported by other methods of investigation (hydrochemical analyses; water budgets of pools).