The process where carbon dioxide is converted to soil humus has been occurring for millions of years, sequestering atmospheric carbon in soil as humified organic carbon restores natural fertility cycles, increase water-use efficiency, improves farm productivity, and provides resilience to climatic variation. Biological carbon capture and storage begins with photosynthesis, a natural process where green leaves transform sunlight energy, carbon dioxide and water into biochemical energy.
Carbon fixed during photosynthesis can be stored in a more a permanent form, such as wood (in trees or shrubs), or as humus (in soil). Humification is a process where carbon compounds are joined together into more complex and stable molecules in the soil from the exuded plant sugars. The formation of humus requires a vast array of soil microbes, including mycorrhizal fungi, nitrogen fixing bacteria and phosphorus solubilising bacteria, all of which obtain their energy from plant sugars (liquid carbon). Under appropriate conditions, 30-40% of the carbon fixed in green leaves can be transferred to soil and rapidly humified, resulting in rates of soil carbon sequestration in the order of 5-20 tonnes of CO2 per hectare per year. In some instances, high soil carbon sequestration rates have been recorded where there were virtually no ‘biomass inputs’, suggesting that the liquid carbon pathway was the primary mechanism for soil building. Understanding the soil building process is therefore of fundamental importance to the future viability of agriculture.