Since the end of the Second World War the quantity of nitrogen circulating globally has doubled. The main reason is the use of artificial fertilizers, which have enabled the production of food to feed the world’s growing population. But one major problem is nitrogen losses from soil, which has an adverse impact on climate and the environment. Sara Hallin, professor of soil microbiology at the Swedish University of Agricultural Sciences (SLU), elaborates:

“Large quantities of nitrogen leach into the environment, and it ends up where we don’t want it: in water as nitrate and in the atmosphere in the form of nitrous oxide.”

Eutrophication is a threat to biodiversity, and the climate is negatively impacted by increasing quantities of greenhouse gases.

Research is ongoing into ways of reducing the amounts of artificial fertilizers by increasing the nitrogen use efficiency, e.g. by introducing other crops or refined cultivation methods. 

But there may also be other strategies for addressing the problem, and an in-depth examination of the nitrogen cycle can help in finding solutions.

“The key to a greater understanding lies in the soil. Soil microbes account for almost 60 percent of nitrous oxide emissions,” says Hallin.

Soil ecosystems involve an intricate interplay between different nitrogen-transforming microbes. They engage in both cooperation and competition. And it is these processes that determine how much nitrogen remains in the soil and how much dissipates into air and water.

Unexpected discovery

For a long time, there was a fixed view of the nitrogen cycle, but over the past 30 years multiple new processes and organisms have been discovered. Just over ten years ago Hallin and her colleagues unexpectedly discovered new soil organisms capable of reducing nitrous oxide.

“They had never been studied before, but turned out to be really common everywhere.”

This is one of the findings indicating that the nitrogen cycle is more complex than had previously been thought.

Hallin’s research team is using molecular techniques, genetic information and new analytical methods to study networks of microbes in soil ecosystems. They are gathering data from soils all over of the world, including the arctic tundra, boreal forest, deserts, savanna and agricultural land.

By analyzing a large number of DNA sequences from the various soil ecosystems, the team aims to identify and isolate more unknown microbes and processes contributing to nitrogen transformations.

“We will be pursuing new lines of research into unknown organisms and processes.”

Among other things, the researchers want to figure out how common it is for microbes to be specialized on specific reactions that are part of a process in the nitrogen cycle, and they want to study the details of cooperations that ultimately results in complete processes.

Simulating climate

Part of the research entails conducting laboratory experiments, as Hallin explains:

“We can perform small-scale experiments in climate chambers. Here on Ultuna campus we have good facilities for cultivating plants in soil and simulating different kinds of climate.”

The team will be eliminating certain groups of microbes and observing how new cooperation occurs or changes. Hallin hopes this will make it easier to understand the mechanisms controlling the nitrogen cycle.

She also wants to see how the microbial network is impacted by different stress conditions. For instance, in the lab they can simulate extreme drought followed by rain. The aim is to understand how extreme weather affects microbial functions and survival. 

“The key to a greater understanding lies in the soil. Soil microbes account for almost 60 percent of nitrous oxide emissions.”

The stress is actually caused not by drought, but when the rain comes.

“Salinity in the cell increases during dry periods. When it gets wet again a difference arises between salinity in the cell and the external environment. This may cause the cell to rupture.”

The research is of great interest because there are many signs that the adverse effects of nitrogen compounds will accelerate as climate change continues, and in particular because of more frequent and more extreme dry periods, combined with increased precipitation.

Hallin points out that the climate debate often focuses on the carbon cycle, but she thinks it is important to consider the nitrogen cycle as well. Changes in the nitrogen cycle also impact aspects of the carbon cycle, such as carbon storage in the soil, for example.

More efficient agriculture

At present the team is engaged in basic research, but exciting applications may lie ahead. They hope it will be possible to develop measures that can make cropping systems more efficient and reduce adverse impacts of agriculture on ecosystems.

One idea is to stimulate microorganisms that reduce nitrous oxide emissions, which would help to increase nitrogen use efficiency in the soil.

“There are already organisms capable of reducing nitrous oxide emissions, and we would like to find more of them and describe them better. And there may also be another process that can remove nitrous oxide from the atmosphere. It’s a huge challenge, but also a fantastic opportunity to make a real difference,” Hallin says.

Text: Nils Johan Tjärnlund
Translation: Maxwell Arding
Photo: Magnus Bergström