Investigating the influence of climate vs. time on mechanisms of soil organic carbon sequestration

The soil organic carbon (SOC) pool represents the largest stock in terrestrial ecosystems. Given the massive role of soil as a carbon (C) sink and its related ecosystem functions, understanding soil organic matter (SOM) dynamics is a crucial scientific challenge. Two chronosequences characterized by different climates were investigated to determine the main effects and interactions of time and climate on SOC sequestration and stability. The drier chronosequence (ADI) consists of four sites (ADI125, ADI10, ADI8 and ADI3) ranges from 125,000 (ADI125) to 3,000 years BP (ADI3) while the wetter one (LED) is constituted by three sites (LED16, LED14, and LED10) have an age spanning from 16,000 (LED16) to 10,000 years BP (LED10). All sites are grasslands and do not show evident impacts of human disturbance. Soil samples (1 profile and 2 cores per site) were collected by horizon, and each horizon was sub-sampled by depth (each 5 cm). The sub-samples were characterized for pH, EC, density, total organic C (OC), total N, texture, mineralogy, total and extractable elements, thermal and biological stability. Particulate and mineral-associated organic matter (POM and MAOM, respectively) were isolated and characterized by elemental and thermal analyses. Considering a common depth, the SOC stock increases with soil age in the drier (ADI) chronosequence. The older (ADI125) and the younger (ADI3) soils stock 60 and 27 MgC ha-1, respectively, in the topsoil (0-15 cm), and 88 and 49 MgC ha-1, respectively, considering the depth 0-30 cm. In wetter climatic conditions (LED), the youngest soil (LED10) shows the highest SOC stock both at 15 and at 30 cm of depth (86 and 138 MgC ha-1, respectively). Between sites having the same age but different climate, LED10 (the wetter and colder site) stocks ~2 times more OC than ADI10 (the drier and warmer site) in the first 30 cm of depth. Significant stocks of SOC are accumulated below 30 cm, e.g., 38% and 46% of total SOC in ADI10 and LED16, respectively. According to the thermal and biological data, the SOM accumulated in LED is more labile, and shows no increase of stability with soil depth and age; conversely, in the dryer chronosequence (ADI), the SOM stability increase with soil age and depth. Comparing the biological data from soils having the same age, the wetter LED10 emits 3× more CO2 than the drier ADI10 in the topsoil. The contribution of POM and MAOM to C storage differs between the two chronosequences. In topsoil, the POM represents the main pool, especially in LED, while the MAOM plays an increasing role with depth independently of the chronosequence. The concentration of OC in mineral-associated (MAOC) and particulate (POC) pools along the profile is 2× and 3× higher in wetter than in drier soils, respectively. Thermal indices suggest that, in drier chronosequence, MAOM stability increases with depth and age, while in wetter conditions such a relationship is not observed. Instead, POM is characterized by lower thermal stability, regardless of chronosequences. In conclusion, the results of this research show that age and climate play an essential role in SOC dynamics in both chronosequences. In detail, a wetter climate determines a higher OC accumulation in both pools, although these greater OC stocks are negatively correlated with their thermal stability. Thus, although more OC is potentially stored in wet and cool climate, it is more prone to global warming. Conversely, age plays a more pronounced effect on SOM accrual and MAOM formation in the drier system, where OC stabilization is less covered by the climatically accumulated labile SOM. Thus, soils with medium-high pedogenetic development and characterized by a drier climate, may represent a more stable C sink. Thus, this research contributes to fill the knowledge gap on the dynamics of SOM and related pools (i.e. POM and MAOM) as a function of both time and climate.