The Two Ways Soil Carbon Is Lost

1. Microbial Decay
   Microbes break down soil organic matter to access energy, releasing carbon dioxide (CO₂) back into the atmosphere in the process. While decomposition is a natural and essential part of the soil ecosystem, excessive microbial activity—without mechanisms to stabilise carbon—leads to significant carbon losses.

2. Oxidation
   Oxidation is a chemical process similar to burning, except it occurs without a flame. It happens when soil aggregates are broken apart, exposing organic matter to oxygen, which accelerates carbon loss. This process is triggered by practices such as tillage, overuse of synthetic fertilisers, and excessive pesticide applications.

How Conventional Cropping Systems Lose Carbon

Let’s look at a conventional wheat system as an example.

• After harvest in December, the soil is left bare or with only stubble retention—but no living roots—until May.
• For five months, no plants are photosynthesising, meaning no carbon is being added to the soil.
• During this fallow period, herbicides or tillage are often used to control weeds, further impacting microbial communities.
• When planting occurs in May, fertiliser, seed treatments, and fungicides are applied, reducing beneficial soil microbes like mycorrhizal fungi, which play a critical role in stabilising carbon.
• The wheat crop photosynthesises for only about four months, meaning carbon is only being added to the soil for a third of the year.

The result?

• Minimal carbon inputs and high carbon losses.
• Soil organic carbon levels have dropped from around 5% (pre-European settlement) to below 2%, with many soils now below 1% SOC.

Key Practices That Contribute to Soil Carbon Loss

1. Short Growing Seasons

   • Limited photosynthesis means less carbon is pumped into the soil via root exudates.
   • Without living plants for most of the year, carbon losses outpace inputs.

2. Overuse of Pesticides & Herbicides

   • Many pesticides and herbicides oxidise soil carbon, releasing it as CO₂.
   • These chemicals harm beneficial microbes, especially mycorrhizal fungi, which help convert carbon into stable humus.

3. High-Analysis Fertilisers (e.g., Synthetic Nitrogen)

   • Excessive use of synthetic nitrogen and phosphorus inhibits mycorrhizal fungi, reducing their ability to stabilise carbon.
   • Nitrogen fertilisers can also accelerate soil oxidation, leading to faster carbon depletion.

4. Tillage

   • Tillage breaks apart soil aggregates, exposing organic matter to oxygen and accelerating microbial decomposition.
   • It also disrupts fungal networks, which are essential for converting organic carbon into long-term stable humus.

5. Overgrazing in Pastures

   • Overgrazing reduces plant recovery time, leading to less photosynthesis and fewer root exudates feeding soil microbes.
   • Bare soil is exposed, increasing oxidation and microbial decomposition rates.

How to Prevent Soil Carbon Loss

1. Keep the Soil Covered Year-Round

• Cover crops and perennial plants help maintain continuous carbon inputs.
• Residue management reduces oxidation and protects microbial life.

2. Practice No-Till Farming

• No-till preserves soil structure and protects organic matter from excessive oxidation.
• It also supports fungal networks, which stabilise carbon into humus.

3. Reduce Pesticide & Herbicide Use

• Instead of frequent herbicide applications, integrate cover crops and crop rotations to manage weeds naturally.
• Reduce synthetic pesticides to preserve beneficial microbial communities.

4. Limit Synthetic Fertilisers

• Use humic substances like fulvic and humic acids to buffer fertilisers and prevent soil oxidation.
• Introduce biological sources of nitrogen, such as legumes, to reduce reliance on synthetic inputs.

5. Implement Regenerative Grazing

• Use holistic or adaptive multi-paddock (AMP) grazing to prevent overgrazing.
• Allow sufficient recovery time for pasture regrowth, ensuring continuous photosynthesis.

Conclusion

Preventing soil carbon loss is just as important as building it. By keeping the soil covered, reducing chemical inputs, and supporting microbial life, farmers can stop the decline of soil organic carbon and even begin sequestering more carbon over time.

Build Soil Carbon With Agresol

We specialise in using regenerative practices to build soil health and increase soil organic matter levels. Start regenerating your farm by joining our consulting service, or get started with a FREE 30 minute consultation.