Organic matter dynamics in mixed-farming systems of the West African savanna : a village case study from south Senegal

Dynamique de la matière organique à l'échelle d'un terroir agro-pastoral de savane ouest-africaine (sud-Sénégal)

Raphaël J. Manlay (1)

1 IRD, Laboratoire d'étude du comportement des sols cultivés (LCSC), Montpellier, France

Organic matter (OM) is a multi-purpose tool in smallholder mixed-farming systems of West African savannas (WAS). It provides people and animals with energy, nutrients and construction materials. Its cycling also ensures the biological maintenance of the system, mainly through allowing and manuring. In the WAS, population grows faster than yields of food crops. Since access to industrial inputs will remain low in the coming years, OM availability can be used as an indicator of sustainability of local farming systems. Carbon (C), nitrogen (N) and phosphorus (P; total in plant, available in soil and noted POD) budgets were thus quantified in a tropical mixed-farming system of southern Senegal to assess its viability from the plot to the village levels. The village territory of the study showed a ring-like, compound-centred organisation. Growing intensification of fertilization and cropping practices, from the periphery to the compounds, led to the distinction of (1) a plateau bush ring under semi-permanent cultivation (2) a compound ring under continuous cultivation receiving manure from night corralling and household wastes (3) lowland, seasonally flooded, rice fields. Soils were sandy Lixisols, except in the lowland (Gleysols). Human and livestock densities were respectively 33 inhabitants and 51 tropical livestock units km-2. According to the plot study set at the beginning of the rainy season, stocks in plant and soil (down to 40 cm deep) increased from 27.8 tC, 2.03 tN and 17.3 kgP ha-1 in groundnut crops of the bush ring to 54.7 tC, 2.63 tN and 43.5 kgP ha-1 in fallows aged more than 10 years. Plant biomass accounted for nearly all of the rise. In soil, fast increase within the very first year (+20 %) was followed by stagnation (C, N) or even decrease (POD) of stocks. In six more or less manured cereal fields of the compound ring, stocks were 29.9 tC, 2.30 tN and 83 kgP ha-1. Large increase of soil POD stocks in comparison to cropped bush fields was recorded at all depths and in all plots. Although significant, increases of soil C and N stocks were restricted to the 0-10 cm layer, and occurred mainly in fields adjoining farmyards, which received organic inputs with highest chemical inputs. Highest C, N and P amounts were found in lowland rice fields as a result of soil fine texture. To account for the weak response of local sandy soils to management, a plant-soil conceptual model, reassessing the thermodynamical signification of soil organic carbon cycling was proposed. In these tropical sandy soils, biota strongly controls soil physical properties and nutrient availability to plant. Perennial rooting systems, or manure exogenous inputs, are thus needed to sustain the ecosystem structural and functional integrity, since they fuel the soil biota with C-mediated energy and nutrients. Integrative estimates for manageable stocks of the whole village territory were 29.7 tC, 1.52 tN and 28.6 kgP ha-1 (soil stocks: 0-20 cm layer considered only) in 1997. Main C pools were soil (52 %), woody above-ground biomass (22 %) and stumps (8 %). Crop harvest, livestock, and wood and straw collecting were responsible for respectively 27, 59 and 14 % of the C outflows from the area exploited by the village. Livestock accounted for 79-85 % of the C, N and P returns to the soil. As result of these transfers large C inputs were brought to the compound ring (3.8 tC ha-1 y-1). N and P depletion of the system amounted to 4 kgN and 1 kgP ha-1 y-1, suggesting that the system was close to nutrient balance. Under current demographic growth rate, simulated C depletion of the village farming system may reach 0.38 tC ha-1 y-1 during the next three decades; meanwhile C flows related to livestock activity and crop and wood harvest should double. The viability of the system might thus be called into question within the next few years, if practices of cropping and animal husbandry are not intensified.

La matière organique (MO) remplit de multiples fonctions dans les systèmes agro-pastoraux ouest-africains. Or l'offre en ressource organique y diminue depuis plusieurs décennies. Afin d'estimer la viabilité d'un terroir agro-pastoral dans le sud du Sénégal, les bilans (stocks et flux) du carbone (C), de l'azote (N) et du phosphore (P ; assimilable seulement dans le sol et noté POD) y ont donc été établis.
Le terroir choisi était organisé en auréoles, avec une intensification croissante des pratiques de la périphérie (auréole de brousse) vers le village (champs de case).
Les stocks dans le système sol-plante étaient de 54,7 tC, 2,63 tN et 43,5 kgP ha-1 dans les jachères âgées, soit des croîts respectifs de 97, 29 et 251 % par rapport aux cultures de brousse (l'augmentation ayant lieu essentiellement dans la biomasse végétale). Les stocks mesurés dans le sol des champs de case étaient supérieurs à ceux des cultures de brousse, essentiellement dans l'horizon 0-10 cm. Cependant, la faible réponse globale des stocks de carbone des sols sableux à la jachère et à la fumure ne peut être interprétée qu'en réévaluant le rôle bio-thermodynamique joué par le carbone dans l'intégrité des agroécosystèmes locaux.
Les stocks moyens sur le terroir contrôlables par l'homme furent estimés à 29,7 tC, 1,52 tN and 28,6 kgP ha-1 en 1997. Le carbone était stocké surtout dans le sol. Elevage, récoltes et collecte de bois ont représenté respectivement 59, 27 et 14 % des prélèvements de carbone dans le terroir. Grâce à eux, des flux importants de carbone ont été établis vers les champs de case (3,8 tC ha-1 an-1), et les pertes minérales globales ont été estimées à 4 kgN et 1 kgP ha-1 an-1. Le terroir était donc proche de l'équilibre minéral.
Mais selon la dynamique démographique actuelle, la perte de carbone pourrait atteindre 0,38 tC ha-1 an-1, et la demande en carbone doubler durant les 30 prochaines années. Sans intensification des pratiques, ceci remettrait en cause la viabilité du système.

Keywords: Biomass; Carbon; Fallow; Fertility; Flux; Livestock; Manure; Mixed-farming system; Nitrogen; Organic matter; Phosphorus; Plant; Senegal; Soil; Stock

Keywords: Azote; Biomasse; Carbone; Elevage; Fertilité; Flux; Fumier; Jachère; Matière organique; Phosphore; Plante; Racine; Sénégal; Sol; Stock; Terroir agro-pastoral; Tropiques

Domains: Earth Sciences

Raphaël J. Manlay. Dynamique de la matière organique à l'échelle d'un terroir agro-pastoral de savane ouest-africaine (sud-Sénégal). Sciences de la Terre. ENGREF (AgroParisTech), 2000. Français. ⟨NNT : 2000ENGR0062⟩. ⟨tel-00005770v1⟩