Global Soil Organic Carbon Stocks in Natural and Urban Ecosystems

In recent years, sequestering elevated atmospheric carbon has become a central strategy to mitigate the impacts of climate change. While it still remains challenging for humans to reduce carbon emissions and meet carbon neutrality under the pressure of increasing global population and urbanization, a deeper understanding of global carbon sinks will inform future environmental policy.

In terrestrial environments, soil is the largest organic carbon sink and offers the greatest opportunity to mitigate the global carbon imbalance. Soil organic carbon (SOC) stocks, or SOC storage, originate in detritus and organic compounds released from plant roots; these compounds effectively store the carbon that plants absorb from the atmosphere. However, the capacity for soil to store carbon varies across climatic conditions and among vegetated biomes. The extent of soil carbon stored in urban environments remains largely unknown.

In this study, doctoral student Shih-Chieh (Peter) Chien and his advisor Dr. Jennifer Adams Krumins conducted a meta-analysis by collecting SOC data from the published literature. They defined each study location as a natural, urban green space, or urban intensive habitat. Then, they compared those carbon stocks within defined climatic zones, dominant vegetation types, and with respect to the Human Footprint (a unitless value of 0–50, an indicator showing the extent of human disturbance) in each study location. Peter’s analysis revealed that SOC stocks are higher in natural than in urban habitats, and he found a negative relationship between SOC and the human footprint in natural but not urban habitats. The highest SOC and greatest variation in natural habitats reflect the varied environmental conditions and vegetation types which characterize the way natural soils store carbon. Critically, these factors are less influential in urban environments where anthropogenic influences dominate. Urban ecosystems tend to be uniform as they are defined by human living constraints. This decreases variability in SOC stocks across increasing values of the Human Footprint.

Their research not only provides a useful reference for future natural resource management and urban design, but it also highlights the importance of urban SOC storage to managing global soil carbon budgets.

This research is also being written into a blog in the website of Global Soil Biodiversity Initiative (GSBI), and the work will be published in the beginning of 2022.