Appendix 3: The Bucket Theory*

The Bucket Theory of Soil Carbon Sequestration holds that Carbon levels can ONLY be increased by the addition of Organic Matter. The amount of organic carbon in soil is a balance between the build-up which comes from inputs of dead plant and animal material and the constant losses where the carbon is decomposed and the constituents separate to mineral nutrients and gases, or are washed or leached away. This theory limits the amount of carbon a soil can sequester to a theoretical ceiling of biomass introduced into the soil.
This theory overlooks critical aspects of microbiology of soils. Increased Organic Matter does nurture micro-organisms which manufacture soil carbon. But there are at least two other ways to increase carbon in soils:
1. Microbial Community Optimisation; and
2. Phototrophism (or in-soil photosynthesis).
Microbial Community Optimisation: Microbial communities are at their most effective when they are balanced. When one or several links in the chain are missing, the processes of decomposition and photosynthesis can never be fully effective. Just as a football team with several positions unmanned cannot hope to score. Inoculating soil with the missing members of the community is like putting players into empty positions. The effectiveness of the team is increased by an order of magnitude.
Photosynthetic Microbes: There is a class of microbial life called ‘autotrophic’ or ‘phototrophic’ that do not rely on Organic Matter for their sustenance. They use solar energy to grow via the process of photosynthesis. Cyanobacteria and Algae are examples. These add Carbon independently of other processes.
Autotrophic bacteria obtain their energy from sunlight (by photosynthesis) or the oxidation of ammonium, sulfur, and iron. They get their carbon from carbon dioxide.
• phototrophic cyanobacteria
• green sulfur-bacteria
• some purple bacteria
• many chemolithotrophic species, such as nitrifying or sulfur-oxidising bacteria*. Many species of algae live in soils and photosynthesise their carbon as plants do.

*Dr Jeff Baldock, CSIRO, presentation to Carbon Farming Conference, Orange, November 2009
** Hellingwerf K, Crielaard W, Hoff W, Matthijs H, Mur L, van Rotterdam B (1994)."Photobiology of bacteria". Antonie Van Leeuwenhoek 65 (4): 33147. doi:10.1007/BF00872217. PMID
7832590.