The Liquid Carbon Pathway
One of the exciting things about the world of soil microbiology is that we still know so little and we are learning all the time. For many years we thought that carbon built up in soils from the breaking down of decomposing surface matter. The problem with this form of soil carbon sequestration is that it is not stable: the carbon is quickly consumed by the soil microbes and respirated back into the air as CO2.
Today we know stable carbon is exuded from plant roots into the soil where it forms humus and where it can remain for decades. This concept, where carbon is transformed from a gas (CO2) to a liquid (plant sugars) to a solid (humus), was only recently described by Dr Christine Jones and she named it the Liquid Carbon Pathway.
During photosynthesis carbon is taken from the air in the form of CO2 and then turned into leaves, wood, roots and plant root fluids, known as 'exudates'. The importance of these exudates is revealed in how much energy plants expend on them. A crop plant puts roughly 30% of its energy into above ground growth, 30% into growing its root system and an astonishing 40% into the soil in the form of exudates.
The reason why? The plant uses those exudates to create symbiotic relationships with mycorrhizal fungi. These fungal networks exchange those exudates for the minerals and water which the plant needs to grow. Mycorrhizal fungi in turn use the exudates to create their own sticky carbon exudate called Glomalin. Glomalin is critical in the formation of soil aggregates which are essential in the creation of soil structure with pores for air and water storage. With the increased water holding capacity bought about by increased soil carbon the plant's photosynthetic capacity increases. This leads to more carbon being pumped down into the soil - a most virtuous feedback loop, all fuelled for free by the sun.
Farming practices like ploughing, leaving fields fallow, and the use of synthetic fertilisers all disrupt this feedback loop and hence the Liquid Carbon Pathway. However with the introduction of management practices like no-till, multi-species cover crops and short duration, high density grazing, photosynthesis can be enhanced and farmlands can produce nutritious food while once again becoming carbon sinks.