Soil carbon

Soil carbon is the solid carbon stored in global soils. This includes both soil organic matter and inorganic carbon as carbonate minerals. It is vital to the soil capacity in our ecosystem. Soil carbon is a carbon sink in regard to the global carbon cycle, playing a role in biogeochemistry, climate change mitigation, and constructing global climate models. Microorganisms play an important role in breaking down carbon in the soil. Changes in their activity due to rising temperatures could possibly influence and even contribute to climate change. Human activities have caused a massive loss of soil organic carbon; however, there is potential for human activity to intentionally divert carbon back to the soil.

Soil carbon is present in two forms: inorganic and organic. Soil inorganic carbon consists of mineral forms of carbon, either from weathering of parent material, or from reaction of soil minerals with atmospheric CO₂. Carbonate minerals are the dominant form of soil carbon in desert climates. Soil organic carbon (SOC) is present as soil organic matter. It includes relatively available carbon as fresh plant remains and relatively inert carbon in materials derived from plant remains: humus and charcoal. Soil carbon is critical for terrestrial organisms and is one of the most important carbon pools, with the majority of carbon stored in forests. Biotic factors include photosynthetic assimilation of fixed carbon, decomposition of biomass, and the activities of diverse communities of soil organisms. Climate, landscape dynamics, fires, and mineralogy are some of the important abiotic factors. Anthropogenic factors have increasingly changed soil carbon distributions. For example, anthropogenic fires destroy the top layer of the soil, exposing soil to excessive oxidation. Industrial nitrogen fixation, agricultural practices, and land use and other management practices are some anthropogenic activities that have altered soil carbon.

Soil carbon distribution and accumulation arises from complex and dynamic processes, which are influenced by biotic, abiotic, and anthropogenic factors. While many environmental factors affect the total stored carbon in terrestrial ecosystems, photosynthesis, respiration, and decomposition are the main drivers in balancing the total amount of stored carbon on land. Atmospheric CO₂ is taken up by photosynthetic organisms and stored as organic matter in terrestrial ecosystems. Microbes, fungi, plant roots, and other biota of the soil are consistently releasing some CO2 back into the atmosphere through respiration; additionally, microbes, and saprophytic fungi work to break down organic matter in the soil- some of which is released as carbon into the atmosphere, and some is retained as humus in the soil from microbial excrement. These natural processes form the basis of the carbon cycle.

Although exact quantities are difficult to measure, human activities have caused substantial losses of soil organic carbon through land use changes, such as deforestation and other agricultural practices. For example, the destruction of rainforests has resulted in a significant release of stored carbon from terrestrial ecosystems into the atmosphere as carbon dioxide (CO2).

Of the 2,700 Gt of carbon stored in soils worldwide, 1550 GtC is organic and 950 GtC is inorganic carbon, which is approximately three times greater than the current atmospheric carbon and 240 times higher compared with the current annual fossil fuel emission. The balance of soil carbon is held in peat and wetlands (150 GtC), and in plant litter at the soil surface (50 GtC). This compares to 780 GtC in the atmosphere, and 600 GtC in all living organisms. The oceanic pool of carbon accounts for 38,200 GtC.

About 60 GtC/yr accumulates in the soil. This 60 GtC/yr is the balance of 120 GtC/yr contracted from the atmosphere by terrestrial plant photosynthesis reduced by 60 GtC/yr of plant respiration. An equivalent 60 GtC/yr is respired from soil, joining the 60 GtC/yr plant respiration to return to the atmosphere.

Climate change has significant impacts on soil formation, as temperature and moisture levels alter the development of chemical and physical properties in the soil. Therefore, changes in climate will impact the soil in many ways that are still are not fully understood, but changes in fertility, salinity, moisture,temperature, SOC, sequestration, aggregation etc. are predicted.

Soil also has carbon sequestration abilities where carbon dioxide is fixed in the soil by plant uptakes. This accounts for the majority of the soil organic matter (SOM) in the ground, and creates a large storage pool (around 1500 Pg) for carbon in just the first few meters of soil and 20-40% of that organic carbon has a residence life exceeding 100 years. Researchers are interested in discovering ways that human intervention can adequately restore carbon to the soils, through farming and other practices.