Comparing soil organic carbon stocks in contrasting Mediterranean pedosystems: How physical-chemical properties and land uses influence its behavior

Soils play a pivotal role in the global carbon cycle processes and behavior, with soil organic carbon stocks (SOCs) representing the largest terrestrial carbon (C) pool. Soil C sequestration enhanced soil quality and the overall ecosystem goods and services, together with socio-economic benefits. The Mediterranean areas are one of the world’s biodiversity hotspots for conservation priorities. The Sardinia Island (south Italy) hosts peculiar eco- and pedosystems thanks to its peculiar climatic, geological, geomorphological, and ancient land use history featured for one of the oldest civilizations (Nuragic, 6,000 BC). All factors are brought together in extremely peculiar soil conditions. This study aimed to investigate SOCs and related behavior in two contrasting Mediterranean pedosystems in Sardinia (Cambisols developed on granite vs Luvisols on limestone), featuring different land covers, by comparing: i) surface vs deep soil horizons (A vs B); ii) different land covers inside and between the two investigated pedosystems. Several properties were assessed, including vegetation (vascular plants), litter (C, N, C/N, litter carbon stocks, P, Ca, Mg, Na, K, Cu, Fe, Mn, and Zn), and soil physical- (sand, silt, and clay contents, bulk density) chemical features (pH, electrical conductivity, soil organic matter, total N, total P, available P, C/N, exchangeable H+Al, exchangeable cations, cation-exchange capacity, base saturation, Cu, Fe, Mn, and Zn), wherever feasible. Statistical analyses were performed to determine: i) significant differences (p < 0.05) between and among investigated horizons and land uses; ii) bivariate correlation between investigated parameters (Pearson’s Correlation Matrix (CM)); variability and complex multiple relationships among investigated parameters through factor analysis (FA), while a principal component analysis (PCA) was conducted to identify the soil physical-chemical parameters having greater importance in terms of observed variability within (a) the investigated pedosystem, as well as (b) among the different land covers. Results revealed that areas with natural or close-to-natural features exhibited significantly higher SOCs amounts compared to more intense and human-influenced land covers. Furthermore, the differences in substrate and following soil formation processes between the two investigated pedosystems led to different SOCs behaviors, demonstrating a strong relation between SOCs and intrinsic soil features. This emphasizes the greater importance of soil nature in influencing SOCs, greater compared to that shown by land cover variations. Consequently, this study blatantly demonstrates that SOCs investigations, if not carried out through an in-depth soil investigation, may lead to inaccurate, or even wrong, outcomes and following considerations.