Root distribution and properties of a young alley-cropping system: effects on soil carbon storage and microbial activity

Purpose In agroforestry systems, the root distributions and properties of the annual and perennial vegetation are poorly known, although they are recognized for fostering soil carbon (C) stocks through annual root-derived C inputs and by altering microbial activity. This study aimed to evaluate the potential contribution of roots from the understory vegetation strip (UVS) and the crop to top- and subsoil C stocks (0–100 cm) to a 3-year-old agroforestry system of the alley-cropping type. Methods Root biomass, chemical composition, functional traits and anatomical structure were assessed in parallel to a characterization of soil physicochemical and microbiological properties, including enzymatic activity, at different depths and locations perpendicular to the tree line. Results The root biomass and properties were heterogeneous in the young alley-cropping system due to the presence of different plant communities and the heterogeneity of the soil mineral N content according to the location perpendicular to the tree line. The soil microbiological properties and organic C stocks did not vary horizontally at this stage of agroforestry but should be monitored through multiple-time samples to confirm a differentiation in subsequent years suggested by the tight link between root stoichiometry and microbial extracellular enzymatic activities that we found. Conclusions Altogether, our results suggested that increasing the root biomass in topsoil in agroforestry systems positively contributes to increasing soil organic C stocks, but in deeper soil layers, an increase in litter inputs with a high C:N ratio might accentuate microbial N limitations and limit soil C storage.