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

Organic matter (OM) is a multi-purpose tool in West African smallholder mixed-farming systems, but its supply has been decreasing for several decades. To assess the viability of a mixed-farming system of south Senegal, carbon (C), nitrogen (N) and phosphorus (P; available in soil and noted POD) budgets (stocks and flows) were thus quantified.The village territory of the study showed a ring-like organisation with growing intensification of fertilization and cropping practices from the periphery (bush ring) to the compounds (compound fields).Stocks in plant and soil averaged 54.7 tC, 2.63 tN and 43.5 kgP ha 1 in old fallows. They were 97, 29 and 251 % higher than in the bush cropped fields, plant biomass accounting for nearly all of the rise. C, N and P amounts recorded in the soil of compound fields were higher than those of the bush field, but the increase was restricted mainly to the 0 10 cm layer. However, the rather weak response of local sandy soils to management can be interpreted only by reassessing the bio-thermodynamical signification of soil organic carbon cycling in the maintenance of the integrity of local agroecosystems.Manageable stocks of the whole village territory were estimated to 29.7 tC, 1.52 tN and 28.6 kgP ha 1 in 1997. Carbon was stored mainly in soil. Livestock, crop harvest and wood collecting were responsible for respectively 59, 27 and 14 % of the C uptake on the village territory. As a result, large C flows were set towards 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, C depletion may reach 0.38 tC ha 1 y 1 and C demand may double during the next three decades. Without any intensification of farming practices, the viability of the system might soon be called into question.