Planetary boundaries: balancing nutrient flows
Life could not exist without nitrogen and phosphorus. Like other important nutrients, these chemical elements circulate in cycles between land, water, air and living things – in quantities that ecosystems have adjusted to over the course of evolution. But humans have caused serious imbalances in these cycles. There is too much nitrogen and phosphorus in circulation. At the same time, there is a growing shortage of phosphorus.
Nitrogen and phosphorus are essential. All living things need these nutrients to produce important building blocks for their bodies, for example proteins and the genetic material DNA. But too much in one place is not good either. We have far exceeded the planetary boundary for both nutrient flows. This is causing changes to ecosystems and biodiversity.
Industry and agriculture release more nitrogen and phosphorus into the environment than it can absorb while still maintaining a stable balance. In particular, the large amounts of fertilizer we apply to our fields for food, animal feed and energy crops are a problem because the plants we grow only use some of the fertilizer. The rest ends up in the environment. In addition, industrial processes and combustion engines also disrupt these cycles.
Nitrogen occurs in various compounds such as nitrates, ammonia and nitrous oxide. When there is excess nitrogen, nitrates find their way into groundwater and the ocean, where oxygen-depleted death zones can result. Like nitrates in drinking water, ammonia in the air has an impact on human health. And agricultural activity produces nitrous oxide, a greenhouse gas that reinforces climate change.
More nitrogen than in millions of years
Most of our air – 78 percent of it – is nitrogen, but it was in short supply for plants for a long time. The nitrogen in the air is non-reactive and unavailable to plants; only a few organisms can use it. Among the cleverest organisms in the ecosystem are nitrogen-fixing bacteria, which can extract nitrogen from the air and convert it to a more reactive form that can be used by plants. These bacteria are only able to do this in symbiosis with plants. They live on the roots of legumes: lupines, peas, vetches, beans and clover. This symbiotic relationship between bacteria and plants enriches the soil with valuable nitrogen. Since there are few organisms that can do this, nitrogen was rare for a long time.
That changed when humans began to keep livestock such as cattle, sheep or pigs. There is nitrogen in the grass and straw they eat. Their urine and dung fertilize the fields where they graze. At first glance, this seems a useful arrangement. But today’s factory farming often concentrates a large number of animals in a small area, which leads to an enormous surplus of dung and manure. This waste ends up on the fields in the surrounding area because transport over larger distances isn’t profitable. In addition, animal feed is often imported from far away and its nutrients accumulate in the vicinity of the stalls. This also causes imbalances on a global scale.
Artificial fertilizers and a green revolution
At the beginning of the 20th century, scientists achieved a revolution by imitating what nitrogen-fixing bacteria do. With the Haber-Bosch process developed in Germany, they were able to produce ammonia and artificial fertilizer by supplying energy to atmospheric molecular nitrogen. From then on, higher yields became possible from the same amount of farmland and more people could be fed. In addition to food, the new nitrogen fertilizer could also be used in war as an explosive.
Today about half of humanity is fed indirectly through artificially produced nitrogen fertilizer. A side effect is that we have made more nitrogen from the atmosphere available to plants and other organisms than all natural processes on land taken together did previously. In addition to artificial fertilizer and liquid manure, biogas plants also contribute to the excess of digestate that ends up in the fields. Industry, vehicles with combustion engines, and shipping also emit nitrogen oxides. Now nitrogen is no longer in short supply, there is too much of it.
Risks to biodiversity, human health and the climate
The consequences are serious, for the climate and our health. Some of the ammonia that can be released as a gas during the use of nitrogenous fertilizers –especially liquid manure and dung – reacts with other air pollutants there to form particulates that can be hazardous to our health. When these nitrogen particulates fall with the rain, they overfertilize and pollute waterways, coasts and seas, a process experts call eutrophication. Plants adapted to a low nitrogen level lose their competitive advantage over nitrogen-loving species, which then gain the upper hand.
How can we return to the safe zone?
Some of the things we can do to bring the cycles back into balance include:
- Keep fewer farm animals
- Toughen laws on the use of fertilizer
- Comply with limits on nitrates and ammonia
- Enrich soils with nitrogen naturally
- Waste less food
- Consume fewer animal products
- Improve agricultural technology
- Stop the use of internal combustion engines
- Reduce emissions from the industrial, energy, waste and wastewater sectors
This article includes more information about these approaches.
Fertilizer that is not immediately used by plants can be bound by soil particles to a certain degree. When this buffer capability is exhausted, the nutrients are washed into the groundwater, polluting it with nitrates. High nitrate concentrations in drinking water can, in contact with various foods, lead to the formation of nitrosamines, which are suspected of causing cancer. These excesses also affect the climate. In moist soils, excess nitrogen can lead to the production of nitrous oxide, a greenhouse gas 300 times as strong as carbon dioxide.
Like the nitrogen cycle, the phosphorus cycle is also out of kilter. Phosphorus is important for our teeth and bones. In the past, guano (phosphate-rich bird droppings) was traded worldwide for use in making soils more fertile. Today raw phosphates are extracted in mines. Some are processed into more soluble phosphorus fertilizers or directly spread on fields.
Only a fraction of the phosphorus fertilizer that is spread on fields ends up in plants. Some of this is bound in the soil, but some is washed into waterways and seas, especially by heavy rains. In lakes, the results can be quite poisonous, with beaches being closed temporarily because of “algal blooms” caused by cyanobacteria.
If the phosphorus reaches the ocean, it fertilizes algae and along with them the bacteria that live on algae. Since this uses up almost all of the oxygen in the water, “dead zones” almost devoid of life can arise in the ocean: After an algal bloom, microorganisms eat the remains and use up all the oxygen that marine organisms need to survive.
People who fish for a living are especially hard hit, for example in the Gulf of Mexico. Rivers like the Mississippi carry too many nutrients from the fields in the American Midwest into the gulf, where the shrimp catch suffers. Much of the fertilizer used worldwide ends up in the ocean, where it threatens the health of marine ecosystems.
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Paradoxically, phosphorus could become scarce at the same time because new phosphate minerals only form very slowly over millions of years. Retrieving phosphorus from the environment to recycle it is a technical challenge, especially if potential pollutants such as heavy metals should not be recycled with it.
Bringing the cycles back into balance
These nutrient flows are complex and extend across diverse ecosystems. This makes it impossible to rebalance them with a single action. Instead a variety of measures needs to be implemented, especially in agriculture.
In general, phosphorus and nitrogen ought to be distributed more effectively so that one region does not get very few nutrients while another gets too many. To this end, it would be reasonable to integrate livestock farming and arable farming more effectively. That would also allow food production to increase. Initial efforts are underway to reclaim the rare phosphorus from sewage sludge. Phosphorus recycling will be obligatory for large sewage treatment plants in Germany starting in 2032, and in Switzerland eight years earlier. It will also be important to avoid recycling heavy metals, i.e. pollutants, in the process.
The many faces of nitrogen and phosphorus
Over 78 percent of our air is nitrogen. It occurs there in its molecular form as N2 and has strong chemical bonds that make it unavailable for most organisms.
Nitrogen compounds include ammonia (NH3), ammonium (NH4+), nitrogen dioxide (NO2), nitrous oxide (N2O), and nitrate (NO3-).
Phosphorus occurs in soils mainly as a component of minerals, from which it only becomes available to plants through weathering processes.
Stricter fertilizer legislation could result in lower overall fertilizer use. In comparison with its neighbors, Germany is relatively lax and has been successfully sued by the European Commission.
Land-based livestock farming would prevent too much fertilizer being used on fields. In other words, the number of animals kept in a certain area should be limited so that the amount of manure they produce can be used effectively as fertilizer. That could also curb the spread of multiresistant germs.
There are agricultural methods that keep soils alive and receptive and enrich them with nutrients naturally so that less artificial fertilizer is needed; these methods include planting legumes – with their nitrogen-fixing bacteria – in the fields.
It is also important not to waste food that is produced using fertilizers, and to consume less meat and fewer animal-based products like cheese and eggs, so that the amount of fertilizer used for animal feed can be reduced.
The EU limits for nitrate pollution in drinking water, ammonia in the air, and nitrous oxide emissions would have to be complied with; this is where policymakers need to act. Ammonia emissions by the agriculture sector could be reduced with the use of new and modern technology and infrastructure.
About 10 to 15 percent of nitrogen comes from transport, industry, energy generation, and waste and wastewater disposal. Further measures are needed here, like phasing out combustion engines.
It should also be noted that nitrogen and phosphorus are not the only substances whose cycles could be disturbed by human activity. So the planetary boundary for nutrient flows may also need to account for other substances in the future.
Scientific editing: Hans-Jörg Vogel, Doris Vetterlein, Helmholtz Centre for Environmental Research – UFZ
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