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New method can measure the carbon deposition of growing plants

Plants deposit carbon to the soil from leaves and roots as they grow. A new method makes it possible to quantify more precisely what proportion of the total supply of carbon to the soil originates from the deposition from plants. Measurements and data from a trial in Foulum show a share of up to 10 percent for cover crops.

2021.09.02 | Camilla Brodam

Photo: Colourbox

Agriculture in Denmark accounts for more than 20 percent of Denmark's total greenhouse gas emissions, and for that reason there is a strong focus on reducing agriculture's climate footprint. One way, according to researchers from the Department of Agroecology at Aarhus University, is to store more carbon in the soil by optimising the cultivation of crops and cover crops. As long as the carbon is stored in the soil, it is not converted to CO2 and emitted into the atmosphere.

Video: See and hear Esben Øster Mortensen tell about the research and results. 

“In the autumn, we do not grow feed or food on many of our fields. Instead, we can use cover crops to reduce nitrogen leaching, but at the same time also to store carbon in the soil. Although cover crops are used at large scale in Denmark, we actually do not know much about what they mean in relation to carbon in the soil. In order to quantify and optimise this carbon supply from the plants, we must examine the plants' input of carbon - not only in the form of biomass, but also the depositing that take place while the plants are growing in the field," explains PhD student Esben Øster Mortensen from the Department of Agroecology.

With a group of researchers from the same department, he has used a method to quantify not only the carbon that is added to the soil from the roots and leaves of plants when they are decomposing, but also something called phyllo- and rhizo deposition of carbon to the soil. Or in short: the carbon deposition of plants.

“By using isotopic labeling of the carbon that is absorbed when the plant grows, we can follow it into the soil. That enables us to measure the pools in which the carbon ends up. It gives us an idea of ​​the potential stability of the carbon that is added to the system,” says Esben Øster Mortensen.

Plants deposit carbon in the soil as they grow

When we talk about the deposition of carbon to the soil, it is about the carbon that is deposited from the leaves of the plant (phyllo) and roots (rhizo) while the plant is still growing. The carbon consists of carbon compounds that are easily degraded by microorganisms. In other words, they are easily convertible carbon compounds. And that is precisely why the size of the plants' carbon deposition is important to know.

“The microbial biomass is crucial when it comes to stabilising the soil's organic carbon in the long run. Therefore, using this new method, we have investigated and quantified the pool of carbon that originates from the plants' carbon deposition, just as we have investigated how different cultivation systems affect this particular type of carbon supply,” says Esben Øster Mortensen.

Different treatments of the soil

But is the carbon deposition of plants higher or lower on organically grown fields, and does it matter if the cover crops contain legumes? These were just some of the questions the researchers sought to find answers to in their study. Using isotopic labeling, the researchers examined carbon deposition in six different cultivation systems in a long-term experiment:

  • Organic livestock manure without cover crops
  • Organic livestock manure with cover crops without legumes
  • Organic livestock manure with cover crops with legumes
  • Mineral fertiliser without cover crops
  • Mineral fertiliser with cover crops without legumes
  • Mineral fertiliser with cover crops with legumes

All the experiments were carried out in a field trial in Foulum, which was established in 1997. Here, it has been investigated how varying use of fertilisers and cover crops affect the soil's fertility. 

“It was an optimal place for us to investigate how differences in the availability of nitrogen affect the plants' carbon deposition. We found that the deposition of carbon to the soil from the plants account for 3-10% of the total supply of carbon from the plants,” explains Esben Øster Mortensen. He explains that the proportion may seem small, but the importance of this carbon pool is so great that a proportion of 3-10% is actually of great importance, as it suggests that up to 50% of the organic carbon in the soil comes from microbial biomass.

Carbon deposition varies

But the proportion of plants' carbon deposition to the soil is not constant; it varies a great deal, for example, in relation to cultivation systems. 

“We found a high variation in this particular pool of carbon when we examined the different cultivation systems. In systems with a high availability of nitrogen, we saw that the pool of plants' carbon deposition was relatively low, while in treatments with a low nitrogen availability we saw a significantly higher carbon deposition from the plants,” explains Esben Øster Mortensen.

Previous research has shown that legumes in catch crops are self-regulating in relation to nitrogen uptake. (Read more here) The new figures for how much carbon plants deposit to the soil, while they are still growing, point in the direction that there is also a regulation in relation to carbon deposition.

"This indicates that the deposition can adapt, but we do not yet know exactly why. This is something we would like to know more about in the future. The most important thing right now is that with this study we have come closer to a robust method that can give us some accurate measurements and figures for the plants' carbon deposition to the soil, which opens the doors for new studies in the future. For example, we are in the process of a new study where we are investigating the same pool of carbon down to a depth of one meter. This study only covers the top 25 cm of the soil, and we can see a significant carbon deposition from the cover crops down to a minimum depth of one meter. It is important to include everything in the calculations when we have to quantify the share of this particular pool in the total supply of carbon from plants,” explains Esben Øster Mortensen.

Additional information
We strive to ensure that all our articles live up to the Danish universities' principles for good research communication (scroll down to find the English version on the web-site). Because of this the article will be supplemented with the following information:
Study type:Field experiments and quantitative analysis
Collaborators:Department of Agroecology at Aarhus University
Funding: Fonden Frands Christian Frantsens Legat. The project was also part of the CCRotate project, which is funded by OrganicRDD5 - Green Development and Demonstration Program GUDP. The project was coordinated by the International Center for Research in Organic Food Systems (ICROFS). The study is also part of the CatCap project, which is funded by the Danish Agency for Agriculture
Conflict of interest:None
Read more: The article ”Short-term cover crop carbon inputs to soil as affected by long-term cropping system management and soil fertility” is published in Agriculture, Ecosystems and Environment. It is written by Esben Øster Mortensen, Chiara De Notaris, Leanne Peixoto, Jørgen E. Olsen and Jim Rasmussen.
Contact:PhD student Esben Øster Mortensen, Department of Agroecology, Aarhus University. Mail:eom@agro.au.dk

Research, Agro, DCA