Soil organic carbon sequestration and dynamics in two fluvial terraces along a chronosequence

Mechanisms of soil organic carbon (SOC) sequestration as a function of time and depth are investigated. Two fluvial terraces, showing the same land use, have been selected along a chronosequence (T1, 125 m a.s.l., ca. 125,000 years BP; T2, 15 m a.s.l., early Holocene). From each site, soil samples have been collected (1 profile and 2 cores) by horizon, and each horizon sub-sampled by depth. Five-cm thick sub-samples have been characterized for pH, EC, total organic C, total N, major/trace elements, SOC stability and texture, and particulate organic matter (POM) and mineral-associated organic matter (MAOM) have been isolated. The average organic C content in topsoil (20 cm) is quite constant in both sites (27.4 mg/g), whereas the average total N concentration ranges between 2.7-2.9 mg/g. SOC stock in topsoil is 50% higher in T1 (72±3 MgC/ha) than in T2 (49±5 MgC/ha). Although SOC accumulation decreases with depth, the two sites recorded a similar average C stock at 35 cm (89±9 and 76±8 MgC/ha, respectively). The average content of the MAOM pool is constant along the T2 profile (52%), while increases with depth in T1. Thermal analysis (TGA-DSC) suggests a general increase of the stability of MAOM and POM with depth in both sites, with T1 showing a largest increase of MAOM recalcitrance in deep soil respect to T2. While most of the studies on SOC sequestration and stabilization focuses on topsoils (0-20 cm), our preliminary data show that a significant stock of more recalcitrant organic C in the deeper layers.