Organic carbon sequestration along a soil chronosequence on fluvial terraces (Adige river, Italy)

The aim of this work is to investigate the mechanisms of soil organic carbon (SOC) sequestration as a function of two factors, namely climate and time. Two-to-three chronosequences, located along a climate gradient and consisting of 3 sites each, will be investigated. The first chronosequence soils were studied in two orders (T1 and T2) of fluvial terraces of the Adige river (Veneto region, North of Italy). The highest and oldest terrace (T1) is situated at Montalto di Gaium, at an altitude of 125 m above the Adige riverbed level. This terrace was presumably formed during the last interglacial (ca. 125,000 years BP) and was characterized by paleudalf soils. On the opposite, T2, situated at 15 m above the actual riverbed level, represents the youngest order of terrace in this area, and probably formed during early Holocene. These fluvial terraces have different age but a common land use. From each site along this chronosequence, soil samples have been collected (1 profile and 2 cores per site) by soil horizon, and each horizon then sub-sampled by depth (each 5 cm). Five-cm thick sub-samples have been characterized for pH, electrical conductivity (EC), total organic C, total N and texture. Particulate organic matter (POM) and mineral-associated organic matter (MAOM) have been isolated and will be characterized by elemental analysis (CHNS), thermogravimetric analysis (TGA-DSC), X-ray diffraction (XRD) and other spectroscopic techniques (i.e., ICP-MS, FTIR, NMR). The average organic C content in the topsoil (20 cm) is quite constant in the three sites (27.4 mg/g), whereas the average total N concentration ranges between 2.7 and 3.1 mg/g. SOC stock in the topsoil increases with soil age, being 30% higher in soils from T1 than from T2. Although SOC accumulation decreases with depth, soils along the chronosequence recorded approximately the same average C stock around 35 cm (76-85 Mg/ha). At the same time, in the site showing the deepest soil profile, the SOC accumulated between 35 and 80 cm represents ca. 30% of the total. TGA-DSC data suggest that the thermal stability/recalcitrance of SOM generally increases with depth. While most of the studies on SOC sequestration and stabilization focused on topsoils (10-20 cm), our preliminary data highlight the importance of investigating also deeper layers. Future data will help to better understand the effects of climate and time on SOM distribution among different pools and as a function of depth.