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High organic inputs explain shallow and deep SOC storage in a long-term agroforestry system ¿ combining experimental and modeling approaches

Cardinael R., Guenet B., Chevallier T., Dupraz C., Cozzi T., Chenu C.. 2018. Biogeosciences, 15 (1) : p. 297-317.

DOI: 10.5194/bg-15-297-2018

Agroforestry is an increasingly popular farming system enabling agricultural diversification and providing several ecosystem services. In agroforestry systems, soil organic carbon (SOC) stocks are generally increased, but it is difficult to disentangle the different factors responsible for this storage. Organic carbon (OC) inputs to the soil may be larger, but SOC decomposition rates may be modified owing to microclimate, physical protection, or priming effect from roots, especially at depth. We used an 18-year-old silvoarable system associating hybrid walnut trees (Juglans regia¿×¿nigra) and durum wheat (Triticum turgidum L. subsp. durum), and an adjacent agricultural control plot to quantify all OC inputs to the soil ¿ leaf litter, tree fine root senescence, crop residues, and tree row herbaceous vegetation ¿, and measure SOC stocks down 2¿m depth at varying distances from the trees. We then proposed a model that simulates SOC dynamics in agroforestry accounting for both the whole soil profile and the lateral spatial heterogeneity. OC inputs to soil were increased by about 40¿% (+1.11¿t C¿ha-1¿yr-1) down to 2¿m depth in the agroforestry plot compared to the control, resulting in an additional SOC stock of 6.3¿t C¿ha-1 down to 1¿m depth. The model described properly the measured SOC stocks and distribution with depth. It showed that the increased inputs of fresh biomass to soil explained the observed additional SOC storage in the agroforestry plot. Moreover, modeling revealed a strong priming effect that would reduce the potential SOC storage due to higher organic inputs in the agroforestry system by 75 to 90¿%. 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 studies given its potential effects on SOC dynamics.

Mots-clés : agroforesterie; séquestration du carbone; matière organique du sol; système racinaire; racine; enracinement; profondeur; structure du sol; cycle du carbone; juglans regia; juglans nigra; triticum turgidum; hybride; france

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