Soil carbon sequestration in a Mediterranean agroforestry system

The Earth's soils are a large reservoir of carbon (C), containing about 1500 Pg C, which represents two to three times the C contained in the atmosphere. This reservoir is extremely sensitive to land use and can act as a source or as a sink of atmospheric CO 2. Agroforestry systems are expected to sequester C into both above and belowground biomass. Such systems could also increase soil organic carbon (SOC) stocks due to higher organic inputs including leaf litter, pruning residues, tree fine root s' turnover and root exudates. However, although tropical agroforestry systems have been thoroughly investigated, there are very few estimates of C sequestration in soils from temperate conditions. The objectives of this study were (i) to quantify the SOC stocks down to 2m soil depth in an 18 - year-old agroforestry system and in an adjacent agricultural plot , and (ii) to assess which SOC fractions are responsible for this additional storage. The experimental field was established in 1995 in Restinclières, South of France, on an alluvial carbonated Fluvisol. In the agroforestry system, hybrid walnuts (13x4 - 8m spacing, 85 trees ha-1 ) are intercropped with durum wheat, wh ereas in the adjacent agricultural plot, only durum wheat is cultivated. Spontaneous vegetation also gr ows on the tree row. About 200 soil cores were sampled from ten soil layers from 0 to 2m into the two plots of 625 m2 each. Bulk densities , texture and SOC contents were determined for each soil samples at each soil depth, using field spectroscopy. Carbon stocks were spatialized at the field scale. To determine which SOC fractions were affected by the agroforestry system, soil particle size fractionation was performed on 64 soil samples from 0-10, 10-30, 70-100 and 160 - 180 cm soil depth. C stocks were characterized by a high, but organized spatial variability. Spatial analysis showed doubled SOC contents on the tree row compared to the inter-row in surface soil layers, probably due to high inputs from the natural vegetation. Thanks to our extensive sampling scheme, we are able to provide the first quantification of soil C sequestration in these very representative agroforestry systems. After 18 years, this agroforestry system stored 4.9 ± 1.2 MgC ha - 1 down to 30 cm, and 6.3 ± 1. 8 MgC ha-1 down to 1 m. Annual additional soil carbon storage rates were estimated to be 272 ± 68 kgC ha-1 year-1 (0 - 30 cm) and 352 ± 98 kgC ha - 1 year - 1 (0 - 100 cm). This additional SOC storage was mainly due to the particulate organic matter fraction (50 - 200 and >200 μm), whereas only 10 to 15% was associated to clay particles. The total annual carbon storage rate would be about 1 MgC ha -1 year-1 when trees' biomass is taken into account, which is a lot compared to other techniques used to improve carbon storage in agriculture, like no-till farming or conservation agriculture. (Texte integral)