Soil organic carbon stabilization mechanisms along two climo-cronosequences

The aim of the work is to study the mechanisms of soil organic carbon (SOC) sequestration as a function of time and climate. Two chronosequences located along a climate gradient (i.e., climo-chronosequence) were compared. The first (ADI) consists of river terraces of the Adige river (Veneto region) whereas the second (LED) of river-glacial terraces of Ledro lake (Trentino Alto-Adige region). All sites were grassland. Soil samples were collected (1 profile and 2 cores) for horizon and sub-sampled by depth (5 cm). Sub-samples were characterized by pH, EC, organic C (Corg), total nitrogen, texture, major and trace elements, and horizons were characterized by XRD. Basal soil respiration and enzymatic assays were determined for all sub-samples. Particulate organic matter (POM) and mineral-associated organic matter (MAOM) were also isolated and characterized by elemental (CHNS) and thermal (TGA-DSC) analyses. In the ADI chronosequence, the oldest site (125 ky) had the highest SOC stock, whereas, in LED chronosequence, the highest SOC storage was found in the youngest site (13-11 ky). In LED sites, the contribution of POM on SOC storage was predominant in the first 10 cm. The ratio between Corg in MAOM/POM ranged between 0.7-1.6, and increased with depth. On the opposite, in ADI, the MAOM played a more important role on SOC storage. Obtained data indicate that i) MAOM and POM may play a different role on SOC accumulation in chronosequences, ii) SOC stability generally increases with depth, and iii) the climate plays a major role, compared to time, on SOC sequestration.