This series of lessons introduces soil carbon in Mediterranean regions, where soils are highly diverse but often contain carbon in both organic and inorganic forms. The course explores the origin, forms, distribution, and importance of soil carbon, as well as why and how it is measured. It also highlights the role of soil carbon in agriculture, ecosystem functioning, and global carbon cycles.

This lesson describes the general composition of soils and the two main forms of soil carbon: soil organic carbon (SOC) and soil inorganic carbon (SIC). SOC derives mainly from decomposed organic matter, while SIC consists mostly of carbonates such as calcite and dolomite. The lesson also presents their spatial distribution, both globally and within soil profiles.

This lesson explains the origins and diversity of soil organic and inorganic carbon forms. SOC originates from plant photosynthesis and enters soils through plant residues, where it is transformed by soil organisms and partly stabilized in mineral-associated forms. SIC originates from rock weathering, soil processes, and biological activity, forming primary, secondary, and biogenic carbonates. The lesson includes illustrations and highlights the interactions between organic and inorganic carbon pools in soils.

This lesson focuses on the functions of soil carbon, especially SOC. The SOC supports soil biodiversity by serving as a food source for microorganisms, which in turn recycle essential nutrients. It also improves soil structure, water retention, and resistance to erosion, while contributing to carbon storage and pollutant retention. The functions of soil inorganic carbon (SIC) in calcareous soils are also briefly discussed.

This lesson describes the dynamics and measurement of soil carbon. SOC is a dynamic pool influenced by inputs (plant residues, organic amendments) and outputs (decomposition and CO₂ emissions). It also explains interactions between organic and inorganic carbon, including carbonate formation and dissolution driven by biological activity and soil acidification. Finally, it presents methods for measuring SOC and SIC, including chemical, thermal, isotopic, and spectroscopic approaches.

In Mediterranean and dryland ecosystems, soil carbon consists of both organic and inorganic forms that are closely interconnected. SOC is essential for soil fertility, structure, and climate regulation, while SIC represents a major and often overlooked carbon pool. Understanding and quantifying both forms is crucial to improve knowledge of soil functioning and to support carbon accounting at regional and global scales.