Aarhus University Seal / Aarhus Universitets segl

Four per 1000 – is that a good idea?

Soil can store carbon, which is a significant element in dealing with the human contributions to the greenhouse gas balance. The question is whether Danish farm land can store sufficient amounts of carbon to reduce our greenhouse gas emissions significantly.

[Translate to English:] Professor Jørgen E. Olesen tvivler på, at målet med 4 per 1000 kan nås. Foto: Jesper Rais

When talking about ”4 per 1000” we are not talking about blood alcohol levels. Instead, we are talking about a concept launched by France – more specifically the French Ministry for Food and Agriculture – to reduce greenhouse gas emissions.

Briefly stated, the concept “4 per 1000” introduces the notion that if all soils in the world are capable of increasing soil carbon stocks by 4 o/oo of carbon (C) per year, this would be sufficient to halt the annual global increase in atmospheric CO2. The initiatives proposed through this concept includes reduced soil tillage, increased plant cover, more intermediate crops, more forests, improved grassland management, longer grazing periods and restoration of exhausted soils.

The idea is intriguing but is it feasible?

We have asked our expert on climate and agriculture, Professor Jørgen E. Olesen of the Department of Agroecology at Aarhus University. He has carried out extensive research on soil carbon dynamics and he led the EU project SmartSOIL. This project examined different initiatives that can help increase soil organic carbon content to the benefit of soil structure and fertility.

- Straw retention, catch crops and permanent grassland can make a difference in connection with carbon storage, says Jørgen E. Olesen. 

Carbon must first be absorbed by plants

The question is whether the Danish soil carbon storage is sufficient to reduce greenhouse gas emissions to the extent required.

- This new initiative claims that it will be possible for the soil to absorb the amount of CO2 caused by our fossil fuel consumption if we manage to increase soil carbon storage by 4 o/oo carbon (C) per year. Is it possible? I do not consider it realistic – at least for Danish soils, he says.

Carbon does not simply go from being atmospheric CO2 to being stored in the soil. First, the plants have to absorb and convert CO2 into carbohydrates, and subsequently this material goes to the soil where it is degraded and stabilized by soil fauna and microbes. Plants need to absorb approximately seven kg of carbon in CO2 from the atmosphere for each kg carbon stored in the soil.

The top 50 cm of Danish farmland contains about 100 tons of carbon per ha. If Danish soils are to increase storage by 4 o/oo (0.4 %) carbon, this will mean that an additional amount of 0.4 tons of carbon per ha per year should be stored.

- If this is the case then we would have to produce a further six tons of plant dry matter per ha for adding to the soil. This would be in addition to the amount of biomass that we already harvest for feed, food and bioenergy purposes, Jørgen E. Olesen explains. We already have the ambition is to increase agricultural productivity, but this increased production will be used for food or bioenergy, not for soil carbon storage.

Several new initiatives

The French concept proposes several actions that can be taken to achieve the goal. One of these is reduced soil tillage.

- Reduced soil tillage is not a long-term solution in this context. This is documented by several international studies one of which has been published in Nature, says Jørgen E. Olesen. 

Studies performed at Aarhus University reach the same conclusions – based on modelling as well as field experiments.

Increased plant cover is another actions proposed in the French concept.

- In Denmark this would imply growing more cover crops in our arable systems or enhancing the area with grassland. It would also require additional applications of organic material (e.g. straw) to our fields, says Jørgen E. Olesen and takes out his calculator. He states:

The total Danish farmland area comprises approx. 2.1 million hectares, about 60 percent of which is cultivated with cereals. If we were to store an additional 0.4 tons of carbon per year, this would imply storage of 0.8 million tons per year.

Without having to resort to radical changes there are actions that the agricultural sector can implement. These contribute to soil carbon storage as follows: 

- We cannot go from 0.14 to 0.8 million tons without radical changes. This would imply an increase in grassland area of 700,000 hectares, which is about half of the current farmland used for grain production. Jørgen E. Olesen considers this to be completely unrealistic.

Grass instead of grains

If the Danish agricultural sector decides to grow grass instead of grain the soil carbon storage could increase by about 1 ton per hectare per year nationally. This is more than the proposed 0.4 tonnes per hectare – but what would we do with all that grass?

- Grass is best suited for cattle feed. But if the grassland area is to be significantly increased we must find new ways to use the grass, for instance for pig feed or energy. However, this will require new technologies that will likely not be available within the next decade, Jørgen E. Olesen points out.

The French plan also includes a proposal to extend the grazing period.

- It is always sensible to manage your grassland well, but it is important to focus on maximizing the yield. This is not the same as increasing utilization. In some areas it may actually be better to shorten the grazing period in order to avoid over-grazing, explains Jørgen E. Olesen.

Restoring exhausted soils

Another French suggestion is to restore degraded soils. This initiative is especially relevant in arid climate regions, but may also be considered in Denmark.

- There is a significant CO2 emission from drained peatlands. A solution would be to stop draining these, thus reducing CO2 emissions, and to use the area for paludiculture. In Denmark there are about 100,000 hectares of lowlands and much of this would be suitable for paludiculture. This might help reduce emissions by as much as 0.7 million tons CO2, says Jørgen E. Olesen. 

- I do not think there are many more options in Denmark. We have carried out research in the application of biochar to the soil. The conversion process is very slow, but a relatively small amount of carbon is retained in the soil as most of it is used for energy. Another suitable action is to apply animal manure to the soil. We already do this, so it would not increase carbon retention, he says.

- The bottom line is that it is important to increase the total amount of biomass produced. We must ensure that there is enough biomass for food, feed and energy and for sufficient amounts to be retained in the soil. There is no doubt that it is important to improve soil carbon content to ensure more fertile and well-functioning soil. The actions suggested by the “4 per 1000” concept fully comply with this goal. However, I doubt that it will be possible to achieve the “4 per 1000” goal, says Jørgen E. Olesen.  

Further information

Read more about climate on the theme page 


Professor Jørgen E. Olesen
Department of Agroecology
E-mail: jeo@agro.au.dk
Telephone: +45 8715 7778
Mobile: +45 4082 1659

Climate-Smart Agri-Food Systems is one of the research areas in which the Department of Agroecology is particularly strong and from which results are delivered in line with national and global societal challenges and goals.