Variability of soil carbon stocks under and outside the tree crown in the sylvopastoral zone of Senegal

Abstract

The pastoral livestock system is criticized for its environmen¬tal impact caused by high greenhouse gas emissions per unit of product, and the adverse effects on soil and vegetation. In Senegal, the carbon stock in vegetation has been monitored by the Center for Ecological Monitoring at 24 sites in the Sahel since 1987. However, soil C stock, an essential parameter for the humification process of tree litter and rhizodeposition of grasses, is not taken into account. This study aimed to determine the influence of tree and environmental factors on soil C content. Thus, 15 of the sites monitored in the Center were selected, and 480 soil samples were collected at 0-10 cm and 10-30 cm depth under and outside the tree crown. Total C and N contents were predicted by near-infrared spectroscopy (NIRS), and C and N stocks were calculated from 60 soil samples taken from under and outside the tree crown. A principal component analysis was carried out to identify the factors involved in C stock variation. Thus, in the sylvopastoral area of Senegal, these stocks varied along a north-south gradient; they were positively correlated with rainfall, tree cover, and biomass. They varied between 9.3 and 29.7 Mg C/ha at 10-30 cm. At a finer scale, the tree induced an increase in C and N contents under the crown in the 0-10 cm layer. This study showed that soil C stocks can be used to deter-mine the need for C compensation in Sahel through tree planting.