Carbon storage and soil organic matter dynamics in agroforestry systems in a temperate Mediterranean climate
Thesis defended on 27 November 2015 at the ABIES Doctoral School
Soil organic carbon (SOC) plays an important role in maintaining soil properties, it is a major carbon sink at terrestrial level and very sensitive to land management methods. Agroforestry systems associate trees and crops (agrosylviculture) and/or livestock (sylvopastoralism) on the same plot. Agroforestry systems may play a role in the mitigation of, as well as adaptation to, climate change. This thesis set out (1) to quantify the storage and forms of SOC in soils in agroforestry systems, (2) to explain the SOC storage found and (3) to propose a model of SOC dynamics over several years.
The SOC stocks were measured in five agrosylvicultural plots and in one sylvopastoral plot in France. A synchronic approach was used, i.e. a comparison between an agroforestry plot and the adjoining agricultural plot. The SOC stocks were quantified from 20 cm down to a depth of 2 m depending on the soils studied, allowing for distance from trees. On the experimental site at Restinclières (INRA, 18 year old plantation) that was studied in particular, there was greater SOC storage down to a depth of 1 m. The SOC storage rates were estimated at 0.25 ± 0.03 t C ha-1 yr-1 in the 0-30 cm horizon and 0.35 ± 0.04 t C ha-1 yr-1 in the 0-100 cm horizon. In the set of plots studied, the average storage rate was 0.24 ± 0.07 t C ha-1 yr-1 in the 0-30 cm horizon. The distance from the trees did not appear to affect the SOC stocks in the interrows. Carbon stocks were high in the grass growing in the lines of trees.
We also set out to explain the SOC storage processes in agroforestry systems by quantifying all the soil organic matter inputs. The fine root biomass was estimated using deep pits, up to 4m deep. The mortality of the fine roots of the trees was studied using minirhizotrons at various depths and distances from the trees. We estimated that the soil in the agroforestry plot received about 40% more organic matter input than the agricultural plot (3.80 t C ha-1 yr-1 compared to 2.69 t C ha-1 yr-1) at a depth of 2 m. The fine tree roots and the roots of the crops each accounted for about 30% of the OM input in the agroforestry plots.
The organic matter was fractionated into particles to determine the additional forms of SOC. Most of the additional carbon was in particulate organic matter (POM) the size of sand (50-200 µm and 200-2000 µm), and was located mainly in the surface horizons. The SOC mineralization potential of the two plots was measured by incubating soil from different depths and by incubating soil that had been subject to different levels of destructuration. The SOC mineralization rates decreased exponentially with the soil depth in the same way in the agricultural plot and in the interrows in the agroforestry plot. Destruction of the macro-aggregates in the incubated soils did not lead to an increase in mineralisation, indicating that these POMs are not protected within these aggregates.
To gain a better understanding of the SOC dynamics in agroforestry systems, a two dimensional carbon dynamics model was built, discretised depending on the depth and taking account of the distance from trees. The modelling results suggested that carbon storage in agroforestry systems results from increased input of OM in the soil rather than a decrease in SOC mineralisation.
This study showed the importance and potential of agroforestry systems for increasing SOC stocks in agricultural soils. However, it raises the question of the sustainability of this storage compartment.