Among agroecological practices, the diversification of cropping systems with the introduction of trees is an adaptation solution to mitigate climate change (Bradshaw et al., 2004). Those combining trees and crops present functionally varied root systems that explore different soil depths. Moreover, while the production of aboveground biomass is recognized as one of the main levers to foster C storage in soils, the fate of root biomass has been less studied, especially in deep horizons. Deep roots from annual and perennial plants play a central role in the sustainable C sequestration through their turnover (Pradier et al., 2017), their exudation, and could become a determining criterion for plant selection. Indeed, root decomposition slows down with soil depth (Pries et al. 2018), and thus promote soil C storage. Our study aims to assess the contribution of roots to soil C sequestration, at different soil depth, under annual crops and trees in a sub-sahelian agroforestry ecosystem. The experiment was carried out in Sob village (Niakhar district, Senegal) in order to measure the root decomposition of two annual crops (pearl millet and cowpea) under the influence or not of a Legume tree (Faidherbia albida) and at different soil depth, down to 2m. Fine root decomposition of Faidherbia albida was also measured. Two pits of 2*2*2m were dug, one located at 1.5m from a Faidherbia albida tree of more than 80 year old (under the crown) and the second pit was located at 30m from the same tree, supposed with no tree influence. On three faces out of four of each pit, soil samples were collected at several depths and were analyzed (texture, C, N and P content, pH, bulk density). Soil humidity was monitored through TDR sensors from topsoil to 1.6m soil depth. The above- and belowground biomass of the annual crop (pear millet the year of sampling) was measured over the surface of each pit (4m²) including 4 plants. Pearl millet, Faidherbia and cowpea roots (from a juxtaposed field) were collected, washed and separated as a function of their diameter. Fine roots (Ø<2mm) were selected and 2g were put in litterbags of 2*2mm mesh size. Faidherbia medium root (2mm<Ø<10mm) decomposition was also monitored with 2.5g put in litterbags with the same mesh size. Litterbags were buried at 50cm from each pit face and at 20, 40, 90, 180cm soil depths. A total of 290 litterbags were buried with 3 replicates. At several sampling date (1.5, 3, 6, 9, 15 months) 50 to 70 litterbags were collected and roots litters were weighed to estimate the remaining mass. Results showed a tremendous effect of Faidherbia tree on soil fertility characteristics with higher mineral N and Olsen P content in topsoil under the tree. This trend was reversed under 30cm where the deep soil fertility seemed to be lower under the tree than far from it. The measurement of the crop aerial biomass at harvest showed an important increase (2 to 3 fold) under the tree compare to far from it. The belowground biomass was also increased but only in topsoil. Root to shoot ratio was higher far from the tree than under it. Fine and medium roots of Faidherbia albida were spread mainly in 40-100cm and 150-200cm soil layers. The root decomposition kinetics is currently under analyzes and would be further discussed in relation with C and N balance.