There is a lot of talk about a global soil carbon debt, the loss of carbon to the atmosphere from 12,000 years of agricultural practices.
It exists. Humans owe the soil a vast volume of carbon from the clearing of natural vegetation for crops and rearing livestock causes soil carbon to burn and mineralise into gas.
The details of these losses and the potential to recover some of them are discussed and argued over by scientists. And whilst there is disagreement over the actual amount, as is usual among the white-coated ones, the consensus is the volume is large and significant to the total greenhouse gas emissions. At the same time, politicians grapple with policy options to encourage the sequestering of carbon back into the soil.
And it is a big deal. The collective impact of humans on soil is collapse-inducing, so anything that can be done to keep soils healthy must be.
We have several posts that dance around this problem.
- Soil matters—graphs that should scare the pants off you
- Farmers use soil and water to grow crops and raise animals
- More carbon in the soil than in the atmosphere
- Fixing climate with soil carbon has a big bonus
Here we want to make one crucial point that appears lost in all the scientific debate and political rhetoric. We will use a quote from a recent scientific paper to make it
Carbon accumulation in soil will occur only when the biomass-C input is greater than the C losses by erosion, decomposition and leachingde Moraes Sá, J. C., Lal, R., Briedis, C., de Oliveira Ferreira, A., Tivet, F., Inagaki, T. M., … & Romaniw, J. (2022). Can C-budget of natural capital be restored through conservation agriculture in a tropical and subtropical environment?. Environmental Pollution, 298, 118817.
Only when the carbon input is greater than the output.
Returning carbon to the soil is not achieved by magic. It is simple physics and chemistry where more has to go in than out.
Natural systems do this through plants. Photosynthesis draws carbon dioxide out of the air and converts it into biomass. Some of that biomass is eaten, but most of it dies and returns to the soil after a short or long period of storage in the living plant.
When this plant litter, along with the excreta and dead bodies of the animals, reaches the soil, it decomposes and is lost to the atmosphere.
Occasionally this transfer is accelerated if the plants catch fire.
In agricultural systems, something similar happens. Plants grow, and the most nutritious parts are harvested for food, animals eat other parts while the remainder returns to the soil. Sometimes the plants are grown exclusively for livestock which also becomes food, and overall, there is an export of carbon from the fields.
Farmers know this, of course. They can add carbon into the system directly as a combination of manure, cover crops, mulches or soil amendments such as biochar. Farmers can also prime the plants to be more productive by adding fertiliser and helping them out by controlling pests and diseases.
These extra input sources can make the net carbon input greater than the output.
However, these actions cost energy, resources, time and effort, so there is a temptation to minimise their use to help balance the books.
Artificial inputs have to come from somewhere else. The manure from the dairy down the road, the biochar from the pyrolisis plant that burns sugar cane bagasse. These organic inputs are borrowing carbon from somewhere else.
Fertiliser and pesticide applications are borrowing energy from the ancient past, and using this energy creates unwanted waste we now call greenhouse gases.
But we digress to reinforce the point.
Soil can only record a net gain in carbon if the carbon we take away and the carbon lost through decomposition is less than the carbon left behind in the soil.
And that carbon needs to come from the atmosphere, or else it is just borrowed, stuffing up the carbon balance of another parcel of land.
So when we see talk of organic agriculture, sustainable agriculture, sustainable intensification or any of the back-to-nature options, for them to work, they must be applied in a way that leaves more carbon in situ than is taken away.
Carbon accumulation should be the first and perhaps the only thing that matters.
de Moraes Sá, J. C., Lal, R., Briedis, C., de Oliveira Ferreira, A., Tivet, F., Inagaki, T. M., … & Romaniw, J. (2022). Can C-budget of natural capital be restored through conservation agriculture in a tropical and subtropical environment?. Environmental Pollution, 298, 118817.