What are the harmful effects of fertilizers

The problem with nitrogen from agriculture

Only part of the nitrogen in fertilizers is absorbed by crops. The rest evaporates in the environment and thus influences the climate, biodiversity and human health. Remedy is possible, but difficult.

All over the world, arable farmers are fertilizing their fields with more and more nitrogen. As a result, large amounts of it end up in the environment. And cattle farmers are also contributing to increasingly significant nitrogen emissions: They feed their livestock with more and more so-called concentrated feed with a high protein content, which is made from grain or other forage crops from the world market. Large amounts of nitrogen fertilizers are usually used when these plants are grown. Agriculture - alongside traffic and industry - has contributed in the past decades to the fact that the concentrations of reactive forms of nitrogen in the air and in water have reached a level that experts consider problematic. These nitrogen compounds influence the climate, contribute to an increase in harmful substances such as ozone and fine dust and, through the over-fertilization of bodies of water and natural ecosystems, lead to a decrease in biodiversity.

Reduction necessary

Experts agree that the consumption of nitrogen fertilizers must be reduced for ecological reasons. Because health problems that can be traced back to this and a decrease in biodiversity ultimately also cause economic costs, economic considerations lead to the same conclusion. This is suggested by the “European Nitrogen Assessment”, a recently published report on which almost 200 European scientists worked and which was financed, among others, by the European Science Foundation and the EU Commission. There are individual approaches and also successes of such nitrogen fertilizer reduction. Because more and more food has to be produced for more and more people around the world, the road to this goal is rocky.

It is about the appropriate amount of nitrogen for agricultural crops. The element is eminent for living nature. If it were missing, neither plants, animals nor humans could grow or survive. Nitrogen is a central component of amino acids and thus of proteins. Likewise of chlorophyll, the molecule that gives plants their green color and with which they can obtain energy from sunlight.

Basically, nitrogen is a common element on earth: four fifths of the atmosphere consists of it, but of its molecular form (N 2), with which most living things cannot do anything. Plants, animals and humans can only use nitrogen in its reactive form, such as nitrates, urea or amino acids, and these in turn are comparatively rare. In most ecosystems on earth, reactive nitrogen is the most important limiting factor for growth alongside water, temperature and possibly a few other nutrients. In our fields, too, nothing would grow without sufficient reactive nitrogen.

If a field is fertilized with nitrogen, however, this is never completely absorbed and used by the cultivated plants. Nitrogen remains in the soil, where it is naturally converted into molecular nitrogen by microorganisms. This creates a by-product of nitrous oxide, a greenhouse gas that is released into the atmosphere. The absolute amounts of these are small, as the report shows. However, as a greenhouse gas, nitrous oxide is around 300 times more powerful than CO 2.

Forests grow faster

Nevertheless, when looking at the effects of nitrogen fertilization on the climate, one should not only look at this one negative effect, says Wim de Vries, soil scientist and ecologist at Wageningen University, who worked on the report. Nitrogen from agriculture also has a positive, i.e. cooling effect on the climate: the nitrogen compound ammonia escapes into the air from stables, open manure pits or when the manure is spread on fields. It is shipped and is later deposited again on the first surface that can be found. In this way, ammonia also reaches the leaves of trees and falls into the forest floor. It is also washed out with the rain directly into the forest floor and other ecosystems. In both cases this has a fertilizing effect.

At the end of the 1990s, De Vries and colleagues analyzed 400 forest areas spread across Europe in a large-scale project. They also found that European forests grow around 5 to 10 percent faster than they did 50 years ago because of the fertilizer effect. As a result, more of the greenhouse gas CO 2 is absorbed by the trees and stored in the increasing biomass of the forest.

In addition, ammonia from agriculture, as well as nitrogen oxides from the combustion of fossil fuels, form aerosols in the air, which have a cooling effect on the climate. Ammonia and nitrogen oxide molecules act as condensation nuclei for the aerosols, which diffuse sunlight and thus increase the photosynthesis of plants. This also leads to increased CO 2 storage via increased plant growth. This effect and that of the nitrogen fertilization of forests almost make up for the climate-warming effect of laughing gas according to the "European Nitrogen Assessment". Despite the undisputed negative effects of nitrous oxide, the increasing nitrogen flows do not have a negative impact on the climate as a whole.

At least that applies for the moment, specifies Klaus Butterbach-Bahl from the Karlsruhe Institute of Technology in Garmisch-Partenkirchen, who also worked on the nitrogen report. But one must remember that the aerosols are short-lived particles. Should the environmental legislation be revised in the future with the aim of reducing ammonia and nitrogen oxide emissions, the cooling effect via the aerosols would disappear very quickly. The high rates of forest growth due to the fertilizer effects would probably only last 20 or 30 years. By then, the forests would have reached equilibrium and the biomass would no longer increase. All that remains is the climate-warming effect of laughing gas.

But even if the warming and cooling effects of nitrogen emissions on the climate cancel each other out, at least for the moment, the harmful effects on human health and biodiversity still remain. This is because nitrogen from the soil is also washed out as nitrate into bodies of water, where it leads to over-fertilization. Studies show that biodiversity is decreasing as a result.

New forms of fertilizer

The nitrogen problem is particularly great where farmers fertilize too much. In agriculture, therefore, efforts are made to reduce nitrogen emissions. The most important approach is to reduce the amount of nitrogen fertilizer used. One possibility is new types of fertilizer, which release the nitrogen in the soil only slowly and make it available to the plants. With conventional fertilizers, there is a large excess of nitrogen in the soil at the time of fertilization, which is converted by the soil microorganisms, producing nitrous oxide. The effect of a slow nitrogen release can also be achieved through mixed cultures with so-called nitrogen-fixing plants such as clover. These can continuously convert molecular nitrogen from the air into available forms via microorganisms that live in their roots.

Political Programs

Another method is so-called precision farming, which is already widely used, especially in the USA. Here, fertilizer is not evenly distributed over a field, but the need is technically determined practically square meter for square meter. An entire field does not have to be fertilized in the same way as the section that needs the most fertilizer. The method leads to a saving in fertilizer. It works, for example, with a green sensor attached to the tractor that controls the amount of fertilizer. The greener a part of a culture, the more chlorophyll the corresponding plants have, the better they are supplied with nutrients, and the less additional nitrogen they need.

In addition, nitrogen consumption in agriculture is controlled by agricultural policy in most countries. In Switzerland, for example, farmers have to keep records of their nitrogen balance in order to receive direct payments from the state. In addition, there are programs in some cantons to promote new methods of spreading liquid manure. If, for example, this is brought directly to the ground with so-called drag hoses, the losses due to the volatilization of ammonia are much smaller than with the conventional method in which the liquid manure is sprayed. Only two thirds or even less of the nitrogen contained in the liquid manure gets into the soil with the conventional method and can act as fertilizer there. Drag hoses offer advantages: If more nitrogen can be used from the liquid manure, the farmer can reduce the amount of artificial fertilizer.

The potential to reduce nitrogen emissions from agriculture by technical means is available, but still limited, says Albrecht Neftel from the Agroscope Reckenholz-Tänikon research station. The most effective way is to ensure that not so much nitrogen fertilizer is used around the world. But this is difficult to implement politically and in agricultural practice. If it is not possible to increase the utilization efficiency of the fertilizer, fertilizing with less nitrogen would also mean accepting less yield. However, this is in contradiction to the increasing production pressure on the arable land. Because more and more people populate the earth, the need for food and thus for agriculturally produced plants is increasing. Due to new needs, such as increasing meat consumption around the world (and thus a greater need for animal feed) and agrofuels, the need for plants is even increasing disproportionately to population growth.

Consumers also have a duty, says Neftel. Because they could influence agriculture's nitrogen demand through their eating habits. The several hundred participants at the science conference “Nitrogen and Global Change”, which took place in Edinburgh in April, also made a statement in a final declaration. Among other things, they propagate a «demitarian» diet, the consumption of only half the typical amount of meat.