4Carbon dioxide + water + energy = carbohydrate + oxygen + water
or 6CO2 + 12H2O + energy (sunlight) = C6H12O6 + 6O2 + 6 H2O
It’s as simple as this. A plant takes carbon dioxide out of the atmosphere plus water out of the soil and converts them into a simple sugar (a carbohydrate) which is stored within the plant to be used either as a future energy source or the carbon is used to create the ‘backbone’ of the plant (starches, cellulose etc). Whilst locking up the carbon, the plant emits life-giving oxygen (plants are the source of all the oxygen in the atmosphere!)
Note also that the sun’s light energy is converted into chemical energy in the process. Hence, all our fossil fuels and all our biofuels are simply sunlight energy that has been captured by plants and turned into chemical energy which is then ‘stored’ in the plant.
Soil Organic Matter and the Carbon: Nitrogen Ratio
The first thing to know is that different plant materials have different Carbon to Nitrogen (C:N) ratios. A leafy legume (say clover) may have a C:N ratio of 15:1. Wheat straw, on the other hand, may have a C:N ratio of 90:1
Now, to keep things simple, let’s say the first bacterium that comes along to eat the organic material has a C:N ratio of 6:1 – its body has 6 atoms of carbon for every atom of nitrogen. The bacterium eats the clover plant which has a ratio of 15:1. 60% of the carbon consumed by the bacterium will be used as an energy source during the respiration process and will be lost as carbon dioxide. Conveniently, 60% is 9 of the 15 parts of carbon, leaving just 6 parts of carbon to 1 part of nitrogen (C:N = 6:1). This happens to be exactly the proportion of C:N needed by the bacterium to build its ‘body’, with the result that no N is absorbed or secreted.
This bacterium then dies and gets eaten by a larger organism. Higher up the food chain, the C:N ratio tends to increase, so this second organism may have a C:N ratio of 8:1. The second organism consumes 6 atoms of C for every one atom of N. However, this organism also needs to use 60% of this consumed carbon as an energy source as it respires. Hence, 3.6 of the 6 parts of C disappear into the atmosphere, leaving a C:N ratio of 2.4:1 to be ‘built’ into the second organism’s body. As it only needs a ratio of 8:1, there is surplus N in its diet and this N is secreted into the soil to be used by the plants.
This second organism then gets eaten by a third, larger one, 60% of the C is lost, surplus N is secreted into the soil and the process continues. The end result is that adding a plant material such as clover or another leafy plant with low C:N ratios will quickly result in N becoming available to the plant. This is one of the reasons why cover crops work so well.
However, in many arable situations, the majority of the organic matter laid onto the soil surface will be straw. As I mentioned earlier, straw could have a C:N ratio of 90:1. OK, let’s assume the same initial bacterium comes along and eats the straw. Its ‘body’ is made up of C & N in a ratio of 6:1. It consumes the straw and respires 60% of the carbon as an energy source. This uses up 54 atoms leaving 36 atoms for every atom of N – hence after respiration the ‘straw’ has a C:N ratio of 36:1.
However, to build its ‘body’, this organism needs the C & N to be balanced at 6:1 (or 36:6) and not 36:1, in other words it needs to find another 5 atoms of N to be able to use the straw. This N is taken from the soil reservoir, hence the extra straw actually uses up N and you have to add extra nitrogen to allow the straw to be broken down fully. Once that first bacterium has got the C:N ratio down to 6:1, then the chain occurs as before, a larger organism comes along and consumes the bacterium, resulting in surplus N becoming available for the plants etc
As the soil organic matter – plant and animal – increases, you reach a critical point: the N ‘secreted’ (as bacterium and higher life forms are eaten) is greater than the N required (to digest the straw or other high-carbon residues). This is the point at which you can start to reduce the N added, as the cycle starts to become self-sufficient.
Add into this mix nitrogen fixation from bacteria, both those on the roots of legumes and the free living bacteria, and you can see how nitrogen can build in the soil.
Just out of interest, one of the people I met in America – Gabe Brown – was finding that his soil was so active after 15+ years of direct drilling (and organic farming) with cattle included in the rotation, that even his high C:N ratio straw residues were breaking down within a few weeks. He was struggling to keep cover on his soil and was trying to grow ever more lignified crops to slow this process down!