Soil organic carbon sequestration and dynamics along two climo-chronosequences

The aim of this work was to investigate the mechanisms of soil organic carbon (SOC) sequestration as a function of depth, time, and climate. To achieve such a goal, two climo-chronosequences (i.e., chronosequences located along a climate gradient) were investigated. The first chronosequence (ADI) consisted of fluvial terraces of the Adige River (Veneto region, Italy), whereas the second one (LED) of fluvio-glacial terraces of the Ledro Lake (Trentino-Alto Adige region, Italy). In particular, the ADI chronosequence included three orders of terraces (T1, T2, and T3) and 4 sites (Q2, Q3, Q4, and Q5). The oldest terrace (T1) was located in Montalto di Gaium, 125 m above the current Adige riverbed level. This terrace was probably formed during the Last Interglacial (ca. 125,000 years BP) and was characterized by Paleudalf soils. Conversely, T3 represented the youngest order of terraces (probably formed during the early Holocene) and was situated few meters above the current riverbed level. The Q2 site was located in T1, Q3 and Q4 in T2, while Q5 in T3. The LED chronosequence, formed during the Last Glacial Period, consisted of three sites (Q1, Q2, and Q3) located in 3 terraces (T1, T2, and T3, respectively). The oldest terrace (T1), at 800 m a.s.l., was presumably formed during the first readvance of Alpine glaciers (ca. 16,000 years BP), whereas the youngest terrace (T3) emerged during the last retreat of glacial deposits (ca. 13-11,000 years BP). All sites were grasslands. From each site, soil samples were collected (1 profile and 2 cores per site) by soil horizon, and each horizon sub-sampled by depth (each 5 cm). Five-cm thick sub-samples were characterized for pH, EC, total organic C (Corg), total N (Ntot), texture, and major and trace elements. Particulate organic matter (POM) and mineral-associated organic matter (MAOM) were isolated using a physical fractionation method and characterized by elemental (CHNS) and thermal analysis (TGA-DSC). The average Corg content in the topsoil (15 cm) was quite constant in ADI Q2, Q3, and Q4 (ca. 30 mg/g), whereas the youngest site (ADI Q5) had the lowest Corg concentration (ca. 16 mg/g). In the first three ADI sites, the Corg concentration along the profile was positively correlated with Ntot (p< 0.001); moreover, a positive and significative correlation between Corg and clay (p< 0.001) was observed exclusively in ADI Q2, whereas calcium played a more important role in Corg sequestration in both ADI Q2 and ADI Q3. In LED, the Corg of the topsoil decreased with the age: the youngest site (LED Q3) had higher Corg content (ca. 84 mg/g) with respect to the oldest (LED Q1; ca. <50 mg/g). Among sites with the same age but different climate (i.e., ADI Q3 and LED Q3), the average Corg content was higher in the colder and rainier site (LED Q3). [...]