Root distribution and traits assessment according to depth in a subsahelian agroforestry system and potential impact on soil C stocks

International audienceIn agroforestry systems, the association of perennial and annual plant species can induce a spatial heterogeneity of the root biomass and traits according to soil depth and to the distance from the tree. Precise information is lacking on the role of belowground plant parts i.e. roots in soil C stocks, especially in agroforestry systems.This study aims to analyse the root strategies according to the soil properties and to quantify the root C inputs to the soils from both perennial (Faidherbia albida) and annual (pearl millet) species according to depth and to the presence of tree.Shortly, the root distribution, the root characteristics and some soil properties were assessed in the experimental agroforestry parkland “Faidherbia-Flux” (Niakhar, Senegal) at 5 soil depths and 2 locations (under and far (+30m) from the tree) in three independent replicates.The crop root biomass was equivalent and concentrated at 0-10 cm of depth despite higher aerial biomass under than far from the tree. On the contrary, Faidherbia roots were mainly concentrated below 100 cm of depth. Root traits such as length and diameter followed exponential decay with increasing depth. The C and N contents were higher in Faidherbia than in the crop root litter. In the topsoils only, the soil C and N contents were higher under than far from the tree.No difference in the root traits was noteworthy according to the location. Differences in soil quality according to the location occurred only in topsoil. The higher soil C and N contents under the trees might thus mainly be due to the tree aerial litter inputs rather than to the roots. Tree roots occupied deeper soil layers than the crop, probably as a response to the long period of cohabitation in this site. The presence of Faidherbia roots far from the tree could testify the extension of its exploration area to compensate the lack of nutrients in deep soil layers. The complementarity of both root systems ensured an efficient use of soil resources and significant carbon inputs below 100 cm of depth thanks to the presence of trees