Building soil organic carbon (SOC) is a cornerstone of regenerative agriculture, directly impacting soil health, crop productivity, and climate resilience. But how do we effectively increase SOC? There are only three primary pathways for building soil carbon:
- Decomposition
- Humification
- Biochar Production
This lesson will explore each pathway, their efficiency, and how they fit into a regenerative farming system. But before we start, make sure to watch this video from our YouTube channel – Agriculture Explained (and make sure to subscribe!)
1. Decomposition
Decomposition involves breaking down plant and organic material into smaller components through microbial activity. For example, when you terminate a cover crop, the biomass—both above-ground leaves and stems, and below-ground roots—decomposes into labile carbon (a readily available organic matter). Microbes consume this labile carbon, releasing nutrients and eventually converting some of it into resistant organic matter, a stable form of SOC.
Efficiency of the Decomposition Pathway
- Carbon Conversion Efficiency (CCE): 8.3%
This means only 8.3% of the original carbon in the biomass becomes resistant organic matter. The rest is tied up in microbial biomass or released back into the atmosphere as CO₂.
Key Benefits
- Nutrient Cycling: Decomposition releases nutrients like nitrogen, phosphorus, and potassium, making them available for subsequent crops.
- Microbial Stimulation: Supports soil microbes, including decomposer bacteria and fungi.
Limitations
Decomposition has low efficiency for carbon sequestration, making it more valuable for nutrient cycling than for building SOC.
2. Humification
Humification is the process of converting root exudates—carbon-rich compounds released by plant roots—into humus, a stable and highly beneficial form of organic carbon. Microbes, particularly mycorrhizal fungi, play a central role in this process by forming organo-mineral complexes that stabilize carbon in the soil.
Efficiency of the Humification Pathway
- CCE: 46%
Nearly half of the carbon entering this pathway is converted into humus, making it the most efficient method for building SOC.
Key Benefits
- Soil Health: Humus improves soil structure, water retention, and nutrient-holding capacity.
- Long-Term Stability: Carbon stored in humus is highly resistant to decomposition.
How to Optimize Humification
- Plant cover crops and perennial pastures to maximize root exudates.
- Support beneficial microbes, including mycorrhizal fungi, by minimizing tillage and reducing synthetic inputs.
Humification is the gold standard for building soil carbon, offering both efficiency and long-term stability.
3. Biochar Production
Biochar is produced by burning organic material, such as wood or crop residues, in a low-oxygen environment (a process called pyrolysis). This converts the biomass into a stable form of carbon. However, raw charcoal must be activated with water, minerals, and microbes to become effective biochar for agricultural use.
Efficiency of the Biochar Pathway
- CCE: 41.4%
This pathway retains a significant amount of carbon, though less than humification.
Key Benefits
- Soil Structure: Biochar improves aggregation and water retention due to its porous nature.
- Nutrient Retention: Biochar can hold nutrients and reduce leaching, especially in sandy soils.
Challenges
- High Costs: Producing biochar at scale is expensive. For example, increasing SOC by 1% in the top 30 cm of soil would require 52 tons of biochar per hectare, costing approximately $65,000.
- Sustainability Concerns: Large-scale biochar production often requires significant biomass, raising questions about its environmental impact.
Biochar is best used as a complementary tool for delivering nutrients and microbes, rather than as a primary method for building SOC.
Choosing the Right Pathway for Your Farm
Each pathway has its strengths and limitations. Here’s how to decide which is right for your system:
- Decomposition: Use for nutrient cycling and microbial stimulation, especially with cover crops and green manures.
- Humification: Focus on humification for efficient and stable carbon storage. Practices like planting diverse cover crops and minimizing tillage will enhance this pathway.
- Biochar: Consider biochar as a medium when applying microbes and nutrition but be mindful of costs.
Building Soil Carbon Sustainably
The most effective strategy for building SOC combines these pathways, integrating decomposition, humification, and biochar into a holistic system. For example:
- Use cover crops to enhance decomposition and humification.
- Add small amounts of biochar as a microbial and nutrient carrier.
- Support microbial communities by reducing synthetic inputs and maintaining year-round plant cover.
Building soil carbon isn’t just about sequestering carbon—it’s about creating healthier, more resilient soils that support sustainable farming.
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