Content: 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 180 cm. Fine root decomposition of Faidherbia albida was also measured. Two pits of 2 × 2 × 2 m were dug, one located at 1.5 m from a Faidherbia albida tree of more than 80 year old (under the crown) and the second pit was located at 30 m 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 at different depths. The above- and belowground biomass of the annual crop (pear millet the year of sampling) was measured over the surface of each pit (4 m²) 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 2 g were put in litterbags of 2 × 2 mm mesh size. Litterbags were buried at 50 cm from each pit face and at 20, 40, 90, 180cm soil depths. 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. Soil C :N content indicated a different microbial strategy with higher values under the tree than far from the tree in topsoils and lower values under the tree than far from the tree in depth. However, it was not related to root litter decomposition. Despite the higher tree root biomass under the legume tree, C and N stocks did not vary according to the location far or under the tree. Under the tree, C inputs in the soil were increased with higher pearl millet plant production and yield. It affected mainly aerial plant parts and shallow roots but not deep roots in accordance with the greater soil fertility in the 0 – 40 cm under the tree compared to deeper soil layers. Root litter decomposition was mainly impacted by soil moisture and thus soil depth. Root litter quality through plant species was another important driver of root decomposition. The main differences between root litter decomposition were mainly observed during the first 1.5 months of decomposition. Low root decomposition in depth suggested greater soil carbon storage than in topsoils. Tree root litters were more recalcitrant than annual crop root litters and thus participate in soil carbon storage. The presence of a tree did not seem to impact crop or tree root litter decomposition but further research should focus to this aspect with replicated trees and different distances to the tree according to a gradient to confirm the influence of the tree presence on root decomposition kinetics.