Sarah Robertson
BSc Honours, 2011
Deep soil organic carbon and its significance to global carbon budgets and carbon sequestration
Summary
Within the carbon cycle (C cycle), C flows from sources to sinks, such as the atmosphere, the ocean, vegetation, and soil. Soils constitute the largest sink. Soil organic C (SOC) is an important component of the C cycle that occurs within the soil. Ongoing research is being conducted upon the role of SOC as a sink of atmospheric CO2, and the possibility of offsetting C-emissions from the burning of fossil fuel and from land-use changes.
Though the upper 0.2m of the soil profile usually contains 40% of the SOC, large amounts of SOC can also be stored deeper in the profile. This deep SOC (>0.3m in depth) is often not included in global C budgets, but may significantly impact the global C cycle. The objective of my review was to investigate what is currently known about mechanisms of soil organic matter (SOM) stabilization, SOC distribution in deep soil layers, deep SOC composition, and why deep SOC is important to consider in global C budgets.
Stabilized SOM is either chemically or physically protected from decomposition. In deep soils, mineral interactions with SOM are the primary, but not the only, mechanism of stabilization. SOC stability increases with soil depth, so larger fractions of deep SOC are stable, compared with SOC in the upper soil profile. The distribution of SOC is affected by bioturbation or vertical mixing by soil organisms, soil texture (there is more SOC in finer-textured soils, due to slower turnover rates), precipitation and temperature.
Vegetation, however, exerts the strongest influence on deep SOC distribution, especially the belowground biomass, which can contribute SOM directly to the deep soil. As for deep SOC composition, root litter, root exudates, dissolved OM (DOM), and bioturbation are the four main sources of OC to deep soils. The largest single source of deep SOC is DOM transport. These different sources of SOC vary with depth, and result in differing chemical and physical qualities and fractions of SOC.
Recent estimates of global SOC stocks have found a significant amount of C stored in soils between 1-3m. These estimates represent a significant addition to what is typically cited to be found in soils globally. Researching deep SOC dynamics will help to better understand the importance of soil processes as a whole and the potential role that soils can play in different C-sequestration techniques.
