Articles | Volume 15, issue 1
Biogeosciences, 15, 297–317, 2018
Biogeosciences, 15, 297–317, 2018

Research article 15 Jan 2018

Research article | 15 Jan 2018

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

Rémi Cardinael1,2,3,4, Bertrand Guenet5, Tiphaine Chevallier1, Christian Dupraz6, Thomas Cozzi2, and Claire Chenu2 Rémi Cardinael et al.
  • 1Eco&Sols, IRD, CIRAD, INRA, Montpellier SupAgro, Univ. Montpellier, Montpellier, France
  • 2AgroParisTech, UMR Ecosys, Avenue Lucien Brétignières, 78850 Thiverval-Grignon, France
  • 3CIRAD, UPR AIDA, 34398 Montpellier, France
  • 4AIDA, Univ Montpellier, CIRAD, Montpellier, France
  • 5Laboratoire des Sciences du Climat et de l'Environnement, UMR CEA-CNRS-UVSQ, CE L'Orme des Merisiers, 91191 Gif-Sur-Yvette, France
  • 6System, INRA, CIRAD, Montpellier SupAgro, Univ. Montpellier, Montpellier, France

Abstract. 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 measured SOC stocks down to 2 m of 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. The model was calibrated to the control plot only.

Measured 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 agroforestry plot compared to the control, resulting in an additional SOC stock of 6.3 t C ha−1 down to 1 m of depth. However, most of the SOC storage occurred in the first 30 cm of soil and in the tree rows. The model was strongly validated, properly describing the measured SOC stocks and distribution with depth in agroforestry tree rows and alleys. It showed that the increased inputs of fresh biomass to soil explained the observed additional SOC storage in the agroforestry plot. Moreover, only a priming effect variant of the model was able to capture the depth distribution 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 potential effects on SOC dynamics.

Short summary
The introduction of trees in an agricultural field modifies organic matter (OM) inputs to the soil (litterfall, root litter), the microclimate, and the stabilization and decomposition processes of OM. These changes could affect soil organic carbon (SOC) storage, but the importance of each process is not well known. In a long-term agroforestry trial, we showed that SOC storage could be explained by high OM inputs to the soil but that enhanced decomposition could also have reduced this potential.
Final-revised paper