Organic carbon decomposition rates with depth and contribution of inorganic carbon to CO 2 emissions under a Mediterranean agroforestry system

International audienceAgroforestry systems have been much studied for their potential to store soil organic carbon (SOC). However, few data are available on their specific impact on potential SOC mineralization, especially at depth in subsoils. Moreover, many soils of the world, especially in arid and semi‐arid environments, also contain large stocks of soil inorganic carbon (SIC) as carbonates. Consequently, the organic carbon dynamics has been poorly investigated in these soils due to the complexity of measurements and of the processes involved. To assess mineralization rates of SOC with depth, we incubated soil samples coming from an 18‐year‐old agroforestry system (both tree row and alley) and an adjacent agricultural plot established on a calcareous soil in France. Soil samples were taken at four different depths: 0–10, 10–30, 70–100 and 160–180 cm. Total CO2 emissions, the isotopic composition (δ13C, ‰) of the CO2 and microbial biomass were measured. The SIC concentrations ranged from 48 to 63 g C kg−1 soil and the SOC concentrations ranged from 4 to 17 g C kg−1 soil. The contribution of SIC‐derived CO2 represented about 20% in the topsoil and 60% in the subsoil of the total soil CO2 emissions. The microbial biomass and the SOC‐derived CO2 emissions were larger in the topsoil, but the decomposition rates (day−1) remained stable with depth, suggesting that only the size of the labile carbon pool was modified with depth. Subsoil organic carbon seems to be as prone to decomposition as surface organic carbon. No difference in CO2 emissions was found between the agroforestry and the control plot, except in the tree row at 0–10 cm. Our results suggest that the measurement of soil respiration in calcareous soils could be overestimated if the isotopic signature of the CO2 is not taken into account. It also advocates more in‐depth studies on carbonate dissolution‐precipitation processes and their impact on CO2 emissions