The contribution of agroforestry systems to climate change mitigation - Assessment of C storage in soils in a Mediterreanean context

National audienceAgroforestry is a land use type where crops and tree are grown together in the same place and at the same time. a groforestry systems have the advantage of providing multiple products (e.g. wood, fruits) or services (e.g. biodiversity enhancement, erosion control) whilst maintaining agricultural production. i f they are known to store carbon into the biomass of the trees, they could also increase soil organic carbon (SOC) stocks. However their impact has rarely been studied under temperate or Mediterranean conditions and has mostly concerned superficial soil layers. Our objectives were (i) to quantify and spatialize s OC stocks in an agroforestry system and in an adjacent agricultural plot, (ii) to assess what s OC fractions are responsible for possible additional carbon storage, and (iii) to quantify all organic inputs entering the soil. t he trial was established in 1995 in southern France. Hybrid walnut trees are intercropped with durum wheat. s OC stocks were measured on 200 soil cores down to 2 m soil depth, and particle-size fractionation was performed on 64 soil samples. Carbon stocks of trees and of the herbaceous vegetation in the tree rows were also quantified. A trench was dug to 4 m soil depth to quantify tree fine root distribution and biomass. Minirhizotrons were installed at different depths to study tree fine root turnover. a nnual additional s OC storage rates were estimated at 259 ± 59 kg C ha-1 yr-1 (0-30 cm) and at 350 ± 88 kg C ha-1 yr-1 (0-100 cm). a dditional storage was mainly due to particulate organic matter fractions (> 50 μm) and 10 to 15% was associated to clay particles. When the aboveground biomass of the trees was taken into account, total organic carbon storage rate reached 1.11 ± 0.16 Mg C ha-1 yr-1. High tree root densities were observed at depth, but root turnover decreased with depth. a groforestry systems provide higher amounts of carbon at depth than other agricultural practices, such as no-till farming, and could therefore provide a more stable C storage in the long-term. t his study was funded by a D e M e within the a gripsol project as part of the r eacctif progra