High organic inputs explain shallow and deep SOC storage in a long-term agroforestry system – Combining experimental and modeling approaches
Résumé
Question
In temperate agroforestry systems, soil organic carbon (SOC) stocks are generally increased [1], but is difficult
to disentangle the different factors responsible for this SOC storage. Organic carbon (OC) inputs to soil may be
larger, but SOC decomposition rates may also be modified owing to physical protection, microclimate, or
priming effect from tree roots. Our objective was therefore to assess if organic inputs could be the main driver
of SOC storage in agroforestry systems.
Methods
This study was performed in an 18-year-old silvoarable system associating hybrid walnut trees and durum
wheat, and in an adjacent agricultural control plot. SOC stocks and all OC inputs to the soil, i.e., leaf litter, tree
fine root senescence [2,3], crop residues, and tree row herbaceous vegetation were quantified down 2 m
depth [4]. We then adapted a model [5] to simulate SOC dynamics in agroforestry accounting for both the
whole soil profile and the lateral spatial heterogeneity.
Results
Measured OC inputs to soil were increased by about 40% (+ 1.11 t C ha-1 yr-1) down 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 (Fig. 1). The model described properly the measured SOC stocks and distribution with depth, especially
when priming effect was taken into account (Fig. 2).
Conclusions
Modeling showed that the observed SOC storage in an agroforestry system would result from increased OC
inputs, which were measured, and from an acceleration of SOC decomposition. This compensatory mechanism
could reduce a lot the potential of agroforestry soils to store SOC, especially at depth. Deep-rooted trees
modify OC inputs to soil, a process that deserves further studies given its potential effects on SOC dynamics.
Domaines
Sciences du Vivant [q-bio]
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