There is a built-in dilemma between soil fertility and building a stable carbon pool in arable land. Plants build up the soil's carbon pool by extracting carbon (CO2) from the atmosphere and passing it on to the soil via the roots and plant remains, thus counteracting climate change.

A fertile soil can supply plants with all the necessary nutrients required for growth, but since nutrients in the soil are bound to carbon, this means that the release of nutrients to plants simultaneously releases carbon into the atmosphere.

To achieve both high soil fertility and efficient storage of carbon, we must not only have a high supply of carbon from plants, we must also have a balanced composition of nutrients in the soil.

In addition to degraded plant residues, the earth's carbon pool also consists of living and dead microorganisms, which make up about half of the total carbon pool. The living microorganisms form a temporary reservoir of nutrients, while dead microorganisms constitute the key to long-term storage of carbon in the soil.

“So far, the results show that follow-up crops with legumes provide an overall greater carbon input compared to similar follow-up crops without legumes.”

The balance between carbon and nutrients affects whether the microorganisms primarily use carbon to survive - i.e. as an energy source - or whether the microorganisms eat themselves large and fat and multiply. The latter is important because it increases both the short-term soil fertility and in the long run the number of dead microorganisms that represents a long-term storage of carbon.

Legumes presumably have a special ability to stimulate microorganisms because the carbon deposited from the roots of legumes is accompanied by nitrogen. Carbon and nitrogen in the right balance is like sweets for microorganisms, but there is a lack of knowledge about the underlying mechanisms and the amount of carbon stored. Legumes - and plants in general – contribute with carbon to the soil pool via plant residue and through continuous deposition during growth.

The research project 'Stable or Fertile' examines the ongoing deposit of carbon and nitrogen; there is a focus on the exchange between legumes and microorganisms on a very small scale just around the roots. We would like to learn how plants and microorganisms work together and talk together at the molecular level to understand how carbon from legumes is incorporated into microorganisms.

In parallel, in another research project 'CCRotate', we are taking the first step towards determining how large amounts of carbon follow-up crops bring into the soil pool. CCRotate focuses on whether after crops with legumes, through plant residues and ongoing depositing, provide particularly high soil fertility and carbon storage.

So far, the results show that after crops with legumes provide an overall greater carbon input compared to similar after crops without legumes. Thus, the results indicate that after crop mixtures with legumes increase both the nitrogen and carbon supply for the benefit of both soil fertility and the climate.

The project ‘Stable or fertile - solving the soil C-N dilemma by exploiting the legume rhizosphere’ has received a grant from the Danish Independent Research Foundation.

The ‘CCRotate’ project is part of the Organic RDD 5 program, which is coordinated by ICROFS (International Center for Research in Organic Farming and Food Systems). It has received a grant from the Green Development and Demonstration Programme (GUDP) under the Ministry of the Environment and Food.