Agroforestry is an increasingly popular farming system enabling agricultural and providing several ecosystem services. In agroforestry, soil organic carbon (SOC) stocks are generally increased, but it is to disentangle the different factors responsible for this storage. carbon (OC) inputs to the soil may be larger, but SOC decomposition may be modified owing to microclimate, physical protection, or priming from roots, especially at depth. We used an 18-year-old silvoarable associating hybrid walnut trees (Juglans regia × ) and durum wheat (Triticum turgidum L. subsp.) and an adjacent agricultural control plot to quantify all OC to the soil – leaf litter, tree fine root senescence, crop residues, tree row herbaceous vegetation – and measured SOC stocks down to2 m of depth at varying distances from the trees. We then proposed model that simulates SOC dynamics in agroforestry accounting for both the soil profile and the lateral spatial heterogeneity. The model was to the control plot only. OC inputs to soil were increased by about 40 % (+1.11 t C ha⁻¹ yr⁻¹) down to 2 m of depth in the plot compared to the control, resulting in an additional SOC of 6.3 t C ha⁻¹ down to 1 m of depth. However, most the SOC storage occurred in the first 30 cm of soil and in the rows. The model was strongly validated, properly describing the measured stocks and distribution with depth in agroforestry tree rows and alleys. showed that the increased inputs of fresh biomass to soil explained the additional SOC storage in the agroforestry plot. Moreover, only priming effect variant of the model was able to capture the depth of SOC stocks, suggesting the priming effect as a possible mechanism driving deep SOC dynamics. This result questions the potential of soils to store large amounts of carbon, especially at depth. Deep-rooted trees modify OC inputs to soil, a process that deserves further study given its effects on SOC dynamics.
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Cardinael et al. (2018) studied this question.