Storing Carbon in No-Till Farming
On this page, we will answer the 2 questions everyone asks, namely:
- Can I store carbon?
- What are the methods to achieve carbon storage in my soil?
The 5 levers to store carbon in the soil
Several factors allow increasing the OM content of a soil, some related to climate and the mineral composition of the soil, and others related to the cropping system:
- Presence of roots
- A low C/N ratio of residues (favor nitrogen-rich residues)
- Presence of earthworms
- Humid and cold conditions
- A sufficiently high content of clay and calcium in the soil
- External OM inputs (livestock manure, compost, RCW, ...)
And secondly, reducing or even stopping tillage will reduce mineralization and thus minimize carbon losses. But the importance of this factor is less compared to those mentioned above. If the main source of soil humus comes from roots, we understand that our annual plants (cover crops and crops) are rather ineffective compared to grassland species. A small example below comparing an annual plant and a perennial plant.
It has also been observed in monitored trials that diversity grown simultaneously greatly increases the rate of sequestered carbon.
The humus cycle
K1: humification coefficient.
K2: secondary mineralization coefficient.
1-K1: primary mineralization coefficient.
How to store carbon?
What technical and scientific literature teaches us is that humus in the soil is mainly microbial carbon derived from the consumption of plant roots.
Thus, the more roots there are, the more microorganisms can consume them and the more humified carbon we will have. Indeed, the aerial plant material transforms only slightly into humus; it is mainly the root part that has this effect.
What does a soil microorganism eat?
For microorganisms to develop effectively, they need a complete ration. They require carbon, but also nitrogen, phosphorus, and sulfur.
Soil microorganisms are composed of matter having roughly the same carbon/nitrogen/phosphorus/sulfur ratios, that is, a C/N/P/S of 100/8/2/1.5. To develop effectively, microorganisms need food as close as possible to these ratios. For example, microorganisms feeding mainly on straw will mechanically be deficient in nitrogen, phosphorus, and sulfur. Thus, if there is too much carbon, microorganisms will degrade the carbon as CO2 to end up with a mineral ratio of their food roughly corresponding to their constitutive ratio.
Therefore, if we take very carbon-rich straw and add nitrogen, phosphorus, and sulfur, we will increase its ease of consumption by microorganisms and thus increase its humification rate. This is what the Australians below did on 4 sites (Hamilton, Harden, Buntine, and Leeton), by adding or not fertilizers to the straw[1].
Does ACS store carbon?
Reducing tillage limits mineralization and loss of organic matter. What is seen in trials is that no-till alone stores very little carbon but mainly concentrates it on the surface, as can be seen in these various examples in France and Switzerland.
Indeed, No-till systems with annual crops produce too few roots for effective carbon storage. However, grassland systems store a lot of carbon thanks to the strong presence of roots.
Here is an example where the transition from a permanent grassland to a no-till system sees a strong decrease in root density and thus, in the long term, a decrease in the organic matter content.
Should products with low or high C/N be applied?
Similarly, green cover crops and crop residues with a low C/N will have a proportionally high humification rate, but a low total carbon stored. Indeed, these organic materials contain little carbon.
On the other hand, straw which contains a lot of carbon will be too deficient in minerals to have a good humification rate. Thus, it is advisable to mix straw and cover crops to have an optimal humification rate and compensate for the two types of inputs.
What impact does the mineral composition of the soil have?
Not all soils are equal from a mineral standpoint for storing carbon. To stabilize organic carbon, it must be inserted into the clay-humus complex. Therefore, clay and calcium are needed. A sandy soil will have a low storage potential. Scientific literature shows that the carbon storage potential increases in clay soils, regardless of practices.
To create a clay-humus complex, calcium is also needed to bind humus and clay. Scientific literature shows that without soluble calcium in the soil, carbon storage remains very difficult.
What impact does climate have?
On the map below, the darker the color, the more organic matter in the soil; conversely, the closer to white, the less there is:
We immediately see that mountain and northern zones appear in black, thus rich in organic matter. This is not due to agricultural practices but to the climate. These zones are characterized by long cold periods limiting mineralization, thus favoring organic matter accumulation.
How to detect abnormal variations?
To observe a variation in soil organic matter content, the first thing is to properly perform the sampling for soil analysis. Below 15 samples in a homogeneous area for a single analysis, the uncertainty increases too much and totally distorts the results. Here is an example below. The variations are too large to be solely due to agricultural practices. There is therefore a sampling defect, either from too few or poorly distributed samples.
Similarly, if a 1% increase in OM is observed in one year on a plot without external input, it is very likely related to a sampling error. For example, to increase OM by 1% in one year on one hectare, about 150 tons of straw would need to be added.
How to store carbon?
An ACS system without livestock or exogenous OM inputs stores very little carbon, but concentrates it on the surface. Over time, levels will eventually increase, but slowly.
It should not be forgotten that it is very important to properly perform soil sampling to obtain a reliable measurement.
In conclusion, how to store organic matter?
- Favor the presence of roots.
- Have a balanced C/N of residues, to provide carbon and nitrogen.
- Ensure the presence of earthworms to promote mixing between soil and organic matter.
- Have a humid and cold climate during the winter period to limit mineralization and thus loss of organic matter. Nothing can be done about this as it depends on the climate.
- Ensure sufficient calcium and clay to aid humification from a mineral standpoint. Clay content cannot be changed as it depends on soil type. For calcium, it can be corrected by inputs.
Finally, to accelerate the system, it is also necessary to make external organic matter inputs, such as livestock manure, compost, or RCW, and integrate perennial plants into the rotation.
Sources
This article was written thanks to the kind contribution of Martin Rollet, agronomist engineer at the National Center for Agroecology.
Appendices
- ↑ Clive A. Kirkby, Alan E. Richardson, Len J. Wade, Graeme D. Batten, Chris Blanchard,John A. Kirkegaard. Soil Biology and Biochemistry. https://www.sciencedirect.com/science/article/abs/pii/S0038071713000229
