'Losses can be significantly reduced by using suitable fertilisation methods'

Mr Zaehle, what are the main sources of nitrogen emissions?

According to a report by the Federal Environment Agency, every year, agriculture releases 435 gigatonnes of nitrogen into the atmosphere in the form of ammonia in Germany alone. About the same amount is washed out of the fields as ammonium and nitrate. Around 360 gigatonnes of nitrogen oxides are released by traffic and industry; 60% of this is from traffic.

What consequences do the emissions of the various nitrogen compounds have for humans and the environment?

There are several aspects. One is the burden on human health. Emissions of ammonia and nitrogen oxides are detrimental to health because of their irritant effect and the formation of particulate matter. Agricultural discharge also pollutes groundwater and adjacent ecosystems, thereby leading to nutrient accumulation. This has negative effects for biodiversity and the quality of the groundwater. Finally, nitrous oxide emissions damage the climate. This aspect must not be forgotten. The particular problem with nitrogen is that each additional gram of nitrogen in the environment is part of a cascade of biochemical transformations and can thus cause several of these effects in succession.

Because of the diesel scandal, nitrogen oxide emissions from road traffic are now in the spotlight. Is that justified?

Also from the perspective of the nitrogen cycle, there is still something that can be done here. These emissions also contribute to nutrient enrichment in ecosystems. Because they occur primarily in conurbations, they are probably also more relevant than agricultural emissions in terms of health policy. However, the latter are much more important in terms of quantity.

How can emissions from agriculture be effectively controlled?

It must first be understood that the use of nitrogen is necessary in the production of cereals and meat. The problem is that about 80% of the nitrogen used does not end up in agricultural products because it cannot be used by plants and animals. It thus remains in the environment. These losses can be significantly reduced by using suitable fertilisation methods. Farmers could, for example, adapt nitrogen fertilisation to weather and soil conditions, take greater account of how the availability of nutrients changes over time, use technical methods to apply slurry close to the ground, or use green manure. Farmers cultivate plants such as rape, clover, or lupins, which bind nitrogen directly from the air. The advantage of this is that the nitrogen is organically bound and not washed out so quickly or released into the atmosphere. However, it is also important to reduce the total input of nitrogen (e.g. by using artificial nitrogen fertilizers). As already envisaged in the Fertilizer Ordinance of 2017, such a reduction is possible (e.g. via a nitrogen cycle management).

What's that all about?

In particular, farms that are engaged in livestock farming (e.g. those in north-western Germany and Bavaria) have a large nitrogen surplus. They use artificial fertilizers for feed production but also produce a lot of nitrogenous animal waste (e.g. liquid manure), which they spread on their own fields as fertiliser. They should try to reduce this surplus by using nitrogen more efficiently and reducing losses. The slurry could also be dried and made more transportable so that it can be used as fertilizer in regions where there is less nitrogen surplus. This would reduce the burden at the point of production as well as the use of artificial fertilizers in more remote regions.

Should the state regulate this?

It already is but only on the basis of recommendations. The maximum operating surplus in the area is 50 kg per hectare. In Lower Saxony, where I come from, the surplus is currently still over 80 kg. However, I would also like to mention that the production of meat requires significantly more nitrogen than that of cereals. If we ate less meat, agriculture would also need less fertilizer.

You are researching how natural ecosystems – especially forests – are linked to the nitrogen and carbon balance. What results have you achieved?

The amount of CO₂ that plants can absorb depends not only on the climate but also on the CO₂ content of the air and the nitrogen content of the soil. Because the CO₂ concentration is increasing, the plants want to grow more. But if there isn’t enough nitrogen, they aren’t able to. We are investigating how much the lack of nitrogen slows down plant growth in different regions. The forests in the northern latitudes are very limited. Those in the tropics, hardly at all. Simply put, the nodular bacteria that bind nitrogen from the air can work better when it’s nice and warm.

Would it then make sense to spread liquid manure in natural ecosystems that lack nitrogen?

No. This also has a negative impact on biodiversity here – especially the ecosystem structure and nitrous oxide emissions from forests. Such considerations do exist in connection with geo-engineering because forests then grow faster and bind more CO₂. However, forests are not only useful for storing carbon but also for species conservation and drinking water purification.

This interview was conducted by Peter Hergersberg