Soil inorganic carbon: stocks, functions, losses and their consequences

Carbonate-containing minerals comprise an additional form of soil carbon known as soil inorganic carbon (SIC). Though SIC stocks are large, they have been disregarded in most studies to carbon sequestration. After reviewing the main forms of SIC (geogenic, biogenic and pedogenic carbonates) and the chemical processes leading to formation of pedogenic carbonates, we review the importance of SIC in the global C cycle and ecosystem functions. Besides pH regulation, SIC and dissolved Ca2+ from carbonates dissolution: i) increase plant growth due to better root growth, nutrient availability and acquisition, as well as provide protection against pathogens; ii) increase activities of soil microorganisms mineralizing nutrients; and iii) bind organic compounds which, consequently, stabilize organic matter, produce larger and stable aggregates, and control water permeability and balance. Consequently, the SIC is crucial not only for pH regulation, but also strongly contributes to many other soil functions and health. Finally, we assess future SIC losses under anticipated global change processes such as increased N deposition and fertilization, elevated CO2, invasive plant distribution and climate change. These SIC losses damage soil functionality and make it more vulnerable to a broad range of degradation factors, including erosion, topsoil and subsoil compaction, acidification and nutrient depletion. Crucial is that in contrast to organic carbon, the SIC losses are irrecoverable. We conclude that SIC is an important soil constituent responsible for a broad range of physical, chemical and biological soil properties and processes as well as ecosystem services such as cycles of C, N and other elements.