Mette Vodder Carstensen

Photo: Anne Kring.

Nitrogen reductions must be considered together with nature and climate

Mette Vodder Carstensen researched how to use drainage systems as a buffer between fields and streams to avoid nutrients from the fields ending up in nature.

By Filip Graugaard Esmarch

Danish agriculture is undergoing a transition to ensure less leaching of nitrogen and phosphorous from fields into streams. However, it can be difficult to find suitable solutions among the means currently available. In her PhD project, biologist Mette Vodder Christensen has studied how to expand the range of drainage systems and increase their multifunctionality.

“Many of Denmark’s natural wetlands that serve as the kidneys of the landscape have been eliminated. They have a natural ability to remove nutrients. At the same time, around half of Danish agricultural land has subsurface drainage pipes that serve as a motorway for the nutrients into the streams. However, using drainage systems as ponds, you can replicate the filter function of natural wetlands,” explains Mette Vodder Carstensen.

So far, Denmark has only approved two types of drainage systems, however, new systems are being tested to give the farmers better opportunities to select solutions that match the specific landscape. This is where Mette Vodder Carstensen has contributed with research studies of a new type of system called intelligent buffer zones.

Versatile methods

Part of the secret behind the drainage systems is the natural presence of denitrifying bacteria that convert nitric nitrogen in the water into nitrogen gas. However, the drainage systems also pose some challenges:

“Under deoxidised conditions, the denitrifying bacteria in the drainage system are able to breathe using nitrogen, which is thus removed from the water. However, when the nitrogen is gone, the bacteria start using other substances to breathe, which will ultimately lead to the formation of methane. In addition, inadequate denitrification can lead to the production of nitrous oxide,” explains Mette Vodder Carstensen.

A large part of her project involved describing these challenges through work that covered comprehensive literature studies of new statistical methods and more detailed measurements of greenhouse-gas emissions in the field and programming in front of the screen.

Negative side effects

A key objective is naturally to design the drainage systems to reduce the emission of greenhouse gases as much as possible, while achieving satisfactory nitrogen removal at the same time.

“This can for instance be done by avoiding too long retention times during summer so as to avoid creating conditions in which methane-forming bacteria thrive,” says Mette Vodder Carstensen.

When selecting and designing drainage systems in the future, you will need to consider a multitude of aspects, and this is where Mette Vodder Carstensen’s results can be used. And that is also why she has focused on phosphorous, biomass and biodiversity. She did her PhD as part of the Nordic Center of Excellence Biowater financed by Nordforsk, where she is still doing research as a postdoc with focus on the green transition to bioeconomy in the Nordic region.