Soil organic carbon sequestration and dynamics along a chronosequence on fluvial terraces
Authors: Galluzzi Giorgio , Plaza César , Priori Simone , Giannetta Beatrice , Zaccone Claudio 
Published: March 2022

&lt;p&gt;The aim of this work is to investigate the mechanisms of soil organic carbon (SOC) sequestration as a function of time and depth. A chronosequence, consisting of two orders (T2 and T1) of the Adige river terraces (Veneto region, North of Italy) and 3 sites (Q2, Q3, and Q4), has been investigated. The highest and oldest terrace (T1) is located in Montalto di Gaium, 125 m above the current Adige riverbed level. This terrace was probably formed during the last interglacial (&lt;em&gt;ca.&lt;/em&gt; 125,000 years BP) and was characterized by Paleudalf soils. Conversely, T2 represents the youngest order of terraces (probably formed during the early Holocene) and is situated 15 m above the current riverbed level. The Q2 site was located in T1 whereas Q3 and Q4 in T2; all sites have a common vegetation. From each site, soil samples have been 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 have been characterized for pH, electrical conductivity, total organic C (C&lt;sub&gt;org&lt;/sub&gt;), total N (N&lt;sub&gt;TOT&lt;/sub&gt;), texture, and micro and macro nutrients. Particulate organic matter (POM) and mineral-associated organic matter (MAOM) have been isolated using a physical fractionation method and characterized by elemental (CHNS) and thermal analysis (TGA-DSC).&lt;/p&gt;&lt;p&gt;The average C&lt;sub&gt;org&lt;/sub&gt; content in the topsoil (20 cm) is quite constant in the three sites (27.4 mg/g), whereas the average N&lt;sub&gt;TOT&lt;/sub&gt; concentration ranges between 2.7 and 3.1 mg/g. In all sites, the C&lt;sub&gt;org&lt;/sub&gt; concentration along the profile is positively correlated with N&lt;sub&gt;TOT&lt;/sub&gt; (p&lt;0.001); moreover, a positive and significative correlation between C&lt;sub&gt;org&lt;/sub&gt; and clay (p&lt;0.001) was observed exclusively in Q2, while in all sites Ca, instead of Al or Fe, seems to play a major role in C&lt;sub&gt;org&lt;/sub&gt; sequestration. SOC stock in topsoil is 47% higher in Q2 (T1) (72&amp;#177;3 MgC/ha) than in Q4 (T2) (49&amp;#177;5 MgC/ha), but such a difference decreases at 35 cm (96&amp;#177;2 and 76&amp;#177;7 MgC/ha, respectively). Furthermore, in the site showing the deepest soil profile (Q3), the SOC accumulated between 35 and 80 cm (42 MgC/ha) represents the 33% of the total. The average content of the MAOM pool is constant along the T2 (Q3 and Q4) profiles (52%), while increases with depth in T1 (up to 62% in deeper layers).&lt;/p&gt;&lt;p&gt;Thermal indices (e.g., WL&lt;sub&gt;400-550/200-300&lt;/sub&gt;, TG-T&lt;sub&gt;50&lt;/sub&gt;, DSC-T&lt;sub&gt;50&lt;/sub&gt;) suggest that the stability of bulk SOM generally increases with depth in the three sites. Moreover, a general increase in the thermal stability of both MAOM and POM is observed with depth in all sites, with Q2 (i.e., the site in the oldest terrace) showing a larger increase of MAOM thermal stability in deeper soil compared to Q3 and Q4 (located on the youngest terrace).&lt;/p&gt;&lt;p&gt;While most of the studies on SOC sequestration and stabilization focuses on topsoils, our preliminary data show that a significant stock of more recalcitrant organic C accumulates in deeper soils. Future data will help to better understand the effect of time on SOC distribution among different pools and as a function of depth.&lt;/p&gt;