Q91. When building soil carbon, is there any difference between charcoal/biochar and carbon present in the soil life such as roots, microbes and fungi? Which is more stable?

Yes, there is a difference in carbon from biochar vs roots/microbes.

Stability
Biochar is a type of biological carbon, made by pyrolising organic matter. It is very resistant to breakdown (>100 years). It captures the carbon in the organic material and turns it into a resistant form of carbon that won’t be released back into the atmosphere. It is more stable than roots, microbes and fungi.

Crop roots, microbes and fungi are more easily degradable than biochar. These organic matter sources are broken down and turned over, releasing nutrients and keeping soil biology running. Over time, improved crop growth and microbial activity can lead to increases in soil carbon. Note that if a root grows into a soil aggregate, the root can release exudates that are then bound to mineral surfaces, making them part of the stable carbon pool. In sands, there are few mineral surfaces so organic matter is readily decomposed.

Functionality
In terms of functionality, biochar and organic carbon sources do different things. Biochar itself does not capture carbon once mixed into the soil but helps improve soil properties that lead to better plant growth and further carbon sequestration. It improves soil fertility by increasing the cation exchange capacity and often has a liming effect. Biochar can also improve soil structure to a degree.

Carbon from soil life cycles and turns over in the soil as part of the various types of soil organic matter. This cycling provides multiple soil health benefits, such as nutrient cycling, maintaining soil structure, and supporting plant growth through interactions with roots and soil organisms.

Building soil carbon
When adding biochar, you are importing a source of carbon, which theoretically increases carbon levels faster than through sequestration from soil life. However, at economically feasible and practical rates of application, you won’t necessarily have a measurable impact on total soil carbon in the top 10 cm.

Plus, the goal of ‘the higher the better’ somewhat misses the point. Carbon can build up in strongly acidic or saline soils because conditions are poor for microbial turnover. This build up isn’t overly useful and crop productivity suffers.

Carbon cycling in the soil is essential for a productive and resilient soil, but won’t necessarily lead to carbon sequestration, especially in sandy soils and/or low rainfall areas. This might not be a bad thing when the goal is a resilient soil.