A review of global data indicates that over half of the world's soil carbon is stored more than 30 cm (12 inches) below the surface, a depth typically excluded from standard carbon accounting practices.
Key Findings
The review, led by researchers from the University of Western Australia, Amity University, and Tsinghua University, synthesizes global data on deep soil carbon, defined as carbon stored below 30 cm depth. According to the analysis, subsoil layers contain an estimated 850 billion tons (petagrams) of organic carbon, representing 50–60% of the total carbon found in the top meter of soil.
Standard carbon accounting, as defined by the Intergovernmental Panel on Climate Change (IPCC) and used by most national carbon markets, measures only the top 30 cm.
Behavior and Stability of Deep Soil Carbon
The review reports that deep soil carbon cycles slowly, with ages ranging from 2,000 to 10,000 years. Its stability is attributed to organo-mineral bonding (including clay and iron oxides), low oxygen levels, and reduced microbial activity at depth.
The review identifies several factors that can destabilize this carbon. According to the authors, warmer temperatures, heavier rainfall, and deep tillage increase its vulnerability. A process called the "priming effect" may occur when fresh carbon from deep roots stimulates microbial activity, causing microbes to break down ancient stored carbon and potentially converting the deep soil from a carbon sink to a source.
Management and Implications
The review outlines several management strategies being studied to enhance deep carbon storage:
- Planting deep-rooted perennial grasses and trees, which deposit carbon directly at depth.
- Mechanical soil inversion (deep ploughing) to bury carbon-rich topsoil.
- Adding clay, biochar, or iron minerals to enhance organo-mineral protection at depth.
"The authors call for deeper soil sampling to improve carbon accounting and climate models."
The authors of the review also recommend developing composite materials for sequestration and combining plant breeding with microbiome engineering to optimize deep carbon storage.
The review is published in the journal Carbon Research.