Scientists have long debated whether erosion of agricultural soil has a positive or a negative effect on the global emission of carbon to the atmosphere. An international group of scientists has now come closer to answering the question by, among other things, using information from nuclear fallout from past nuclear testing.

Can the global erosion of agricultural soil act as an important source of CO2 emission i.e. carbon (C) to the atmosphere? Or does erosion have the exact opposite effect, so that it ”tidies up” some of the CO2 that is in the air and deposited in the soil?

In collaboration with international colleagues, scientists from the Faculty of Agricultural Sciences can now present astonishing results. Among other things, they have used nuclear fallout from nuclear testing that was carried out about 50 years ago to answer the question.

And the result? The answer is that erosion of agricultural soil does not have a significant impact one way or the other on the global C-cycle. The sensational measurements and results were published in the renowned scientific journal Science in October 2007.

The results show, surprisingly, that erosion only modestly promotes deposition of carbon in the soil. Erosion is still something that should be controlled for the sake of plant production and the environment.

- However, carbon deposition cannot be used as a new argument for focusing on erosion protection. Our very concrete measurements can be incorporated in models so correct figures instead of suppositions can be used, says senior scientist Goswin Heckrath from the Department of Agroecology and Environment at the Faculty of Agricultural Sciences, University of Aarhus.

The results are based on models and actual observations in small agricultural landscapes. The novelty is that measurements were carried out in many fields in different climate zones and that carbon and the radioactive trace mineral caesium were used in the studies.

- Other scientists have until now only done a few measurements here and there. We have followed another procedure: We have measured complete agricultural landscapes with both erosion and deposition areas, explains Goswin Heckrath about the group’s new approach to the subject.

The caesium isotope that was used was 137Cs. This radioactive version of caesium does not occur naturally but was deposited throughout most of the world by the nuclear testing that took place in the beginning of the 1960s. 137Cs binds very closely to soil particles. It does not break down, is not leached and is only transported if the soil itself is transported. Therefore, 137Cs can be used to trace soil movement. With erosion soil moves from the spot while with deposition it moves to the spot.

With the aid of caesium and carbon measurements, carbon transport in the landscape due to erosion can be determined. At the same time, the scientists also estimated how much carbon had moved from the atmosphere to the soil or vice versa for each point of measurement.

It was seen that there is a strong positive correlation between the rate of soil erosion and carbon deposition in the eroded area. On the other hand, the deposition areas are stable with regard to carbon absorption and release. This means that for an entire field the soil actually absorbs C from the atmosphere, but only in small amounts.

The fact that the article with the research results was published in the distinguished journal Science goes to prove that they are new, groundbreaking results of current and global interest and that they are the fruits of international collaboration that has been ongoing for several years. The Science article can be found at the following link:

http://www.sciencemag.org/cgi/content/full/318/5850/626?maxtoshow=&HITS=10&hits=10&RESULTFORMAT=&fulltext=soil&searchid=1&FIRSTINDEX=0&issue=5850&resourcetype=HWCIT

For more information please contact: Senior scientist Goswin Heckrath, Department of Agroecology and Environment, Faculty of Agricultural Sciences, University of Aarhus, telephone: +45 8999 1715, e-mail: Goswin.Heckrath@agrsci.dk

Text and photo: Janne Hansen