The carbon cycle: between climate and soil
An article by Michaela Dippold, Georg-August-Universität Göttingen
In the context of climate change, there is a lot of talk about the ability of soils to store the greenhouse gas CO2. While changes to land usage often result in the carbon stocks in the soil being reduced and CO2 being released, sustainable agriculture aims to achieve humus balance. The idea is to ensure that our carbon stocks are not lost under any circumstances – not only because of the climate, but also because they improve soil fertility. One key role in this is played by soil microorganisms.
Microorganisms as the engine of carbon and nutrient cycles in the soil
Microorganisms in the soil live on carbon and release nutrients from the humus; these can then – either directly or after their cell death – be absorbed by our crops. On average, 100 billion cells of these tiny organisms can be found in one gram (i.e. half a teaspoon) of soil. These cells die every three to 100 days, with new ones growing in their place, meaning that they are replaced several times throughout the year. The healthier the soil is, the more microorganisms we will find, though this also means that their annual renewal will be all the more intensive.
In other words, plants can often access the valuable nutrients that can be found in the constant cycle of living microorganisms and dead organic soil substance. Because of this cycle, the nutrients are never all available at the same time, and can therefore never be washed completely out of the soil and into the groundwater by a single occurrence of heavy rainfall.
How do we manage the microorganisms in the soil?
To promote growth of the microorganisms, we need to supply soil with its nutrients, i.e. the carbon, while at the same time protecting the nutrient stocks. However, because we engage in agriculture to produce our own food from our soils, we are constantly removing carbon and nutrients that are not easily replaced in our globalised economy.
To offset the nutrient losses, we use precisely dosed quantities of fertilisers. After all, over-fertilisation means that nutrients will be lost into the groundwater, rivers, seas and the atmosphere, where they are almost always harmful. To keep lots of nutrients in the soil, in a form that plants can access, we need active soil life with many microorganisms, though this requires large amounts of carbon in the soil.
Besides systematically returning crop residues to the soil – that is to say everything that we cannot use directly as food – it is also important to take advantage of those periods during the year that are not used for food production to replenish the carbon stocks in the soil. Winter cover crops have proven to be an extremely promising strategy in this context. Rather than being harvested, they are worked completely into the soil in the spring in order to increase the carbon stock.
The area of soil in which microorganisms keep nutrient stocks in flux through their constant renewal can even be extended by deeper layers by using particularly deep-rooting cover crops. This allows us to keep even more nutrients available, while storing more carbon in our soils at the same time.