environment sustainability

Planetary boundaries 2 – nitrogen

Last week: ozone
This week: nitrogen

The earth has many natural cycles. Water, with its cycle of rainfall and evaporation, is one of the simpler ones. Nitrogen is a bit more complicated, but you might remember it from school. I vaguely recall it from a junior high life science class, but then didn’t think of it again for twenty years. I should have done, because it’s important. Human activity has disrupted the nitrogen cycle considerably, and most of us are scarcely aware of it.

Nitrogen is vital to life, to the formation of proteins and DNA. But life requires nitrogen in its active form, and most nitrogen is in the inactive form of gas. The air we breathe is 78% nitrogen, and for millennia, there were just two ways that this nitrogen became available. The first was through lightning, with a flash of high temperature and small quantities of nitrates coming down in the rain. The second was nitrogen-fixing bacteria in the soil.

These two processes essentially put a natural cap on the abundance of life on earth. Plants and animals could flourish and expand no faster than the nitrogen fixing bacteria they depended on. That natural limit presented itself to humans as we discovered farming, operating as a ceiling on harvests, sometimes leading to soil exhaustion and famine. We slowly came to recognise the importance of manure, or of intercropping with legumes, long before we understood why it worked.

Just over 100 years ago, we overcame that natural limit to yields. The breakthrough was the synthesizing of ammonia by Fritz Haber and Carl Bosch, at high pressure and at 500°C. This ammonia could be used to create fertiliser, and commercial fertiliser production began in earnest around 1913. Today, the industry creates 120 million tonnes of nitrogen a year. As the planetary boundaries report points out, “human activities now convert more N2 from the atmosphere into reactive forms than all of the Earth’s terrestrial processes combined.”

There’s an interesting comparison with last week’s boundary here. It was the invention of CFCs at General Motors that created the ozone depletion problem. A major corporation was involved here too, the German chemical company BASF. General Motors created a wholly synthetic gas that had never existed in nature before, while Haber and Bosch had accelerated a natural process. Both problems are the consequence of chemistry on an industrial scale. Both had obvious positives, and unforeseen negatives.

The positives to the new sources of nitrogen are really quite immense. In fact, Haber and Bosch have had more influence on the human affairs of the last century than probably anyone else. As fertiliser use rose, yields went up and food supplies stabilised. The periodic famines that had blighted most of the world ceased in many places, though notable exceptions remain. Fewer people were needed on farms, acceleration urbanisation. With better diets, people lived longer and healthier lives. The human population on earth soared. It’s safe to say that without Haber and Bosch, there would not be 7 billion people alive today.

The negatives are less obvious than the positives, but we ignore them at our peril. The problem comes when fertiliser use overloads ecosystems with more reactive nitrogen than they can productively use. On land, it leads to the proliferation of weeds. When it runs off into rivers and lakes, the flood of nutrients into the water causes algae to bloom, blocking out light and sucking up all the oxygen. This makes life impossible for all other aquatic life, but it’s not sustainable for the algae either. Having killed everything else, it then dies itself, leaving a dead zone. There are 400 of these today, often at the point where major rivers meet the sea.

Each of these dead zones is a local problem, but they add up to a global problem, which is why nitrogen is a planetary boundary. Nitrogen pollution, and its close relative phosphorus pollution (from detergents) deplete fish stocks, pollute fresh water and lower plant and marine biodiversity. Nitrous oxide, derived from reactive nitrogen, is also a powerful greenhouse gas. The top end of the risk is that we add so much nitrogen to the oceans that we trigger a global ‘ocean anoxic event’ – a mass marine extinction, although the planetary boundaries report suggests that mined phosphorus reserves will be depleted long before that occurs.

Still, the nitrogen cycle needs to be managed better if we are to solve these problems. The science suggests that a safe level of nitrogen use would be about   25% of current production. That has big consequences for agriculture, which will need to use fertiliser more intelligently. We can use less too. The argument over whether or not organic agriculture can feed the world is a red herring here. We don’t need to be absolutist about it, we just need to use less, targeting fertiliser use to the crops that need it and substituting organic techniques where possible.

Better pollution controls on coal power stations and car exhausts can limit the nitrogen oxide emissions that contribute to the problem. Smog and respiratory illnesses would be reduced too, so that should be prioritised. Another source of nitrogen into soil and water is through sewage, and half the world’s population lives without modern sewage treatment. Providing clean water and sanitation is a top priority already, and lowering nitrogen run-off is an added bonus.

Mark Lynas also points out that reactive nitrogen can be turned back into N2 by microbial processes, and that this takes place in wetlands and swamps. Creating floodplains and river deltas would catch nitrates before they reach the sea, while also restoring wetlands that are often under threat already. It would be “a win-win for at least four planetary boundaries: those on fresh water, biodiversity, land use and nitrogen.”

There are actions we can take to get back within a safe space on the nitrogen boundary. Some of them are easier than others, and a transition towards sustainable agriculture will be a long process. But it can be done. And as we shall see as the series continues, many of the boundaries overlap and progress can reduce ecological impact on multiple fronts at once.

Nitrogen boundary

Safe limit: 35 million tonnes a year

Status: in overshoot


  1. Good explanation of the science. There is like you say at the end a nitrogen cycle which cycles nitrogen between its different forms with bacteria at the centre which we have distorted by using so much fertilizer. However, this fertilizer production is unsustainable due to its high energy dependency using natural gas to since as you say you need a transition metal catalyst and a temperature of 450-500 degrees C. Up to 3% of the worlds gas supply is used every year for this purpose and its hard to see how this can continue.

    1. Yes, the gas use is unsustainable too, but I left that out to keep the focus on the nitrogen angle. Of course, reducing our dependence on fertiliser would reduce our use of gas too, lowering carbon emissions in the process.

  2. I think the phosphorous problem might be even worse – and it is closely linked to nitrogen. If I were asked, I would actually call it the Nitrogen-Phosphorus-Boundary. Phosphorous is a component of the DNA and RNA backbone and hence as essential for life as is nitrogen. Most fertilizer is a mix of phosphates and nitrates. As you mentioned above, phosphorus is likely to run out rather soon, which is not really good news since, in contrast to nitrates, phosphates cannot easily be produced artificially from an abundant source. Phosphate production depends on a small number of very limited geological reserves. Phosphorous is pretty much the major limiting factor for the abundance of life on Earth. A clever choice by nature, I’d say, since it puts an upper limit on the maximum total biomass, sparing the planet the fate of death from Eutrophication – including, it might turn out, artificial eutrophication caused by a species that appeared recently and went technological.

    1. You’re on the same page as the boundaries report itself there. They put nitrogen and phosphorus together and say they had a long debate about whether phosphorus should be a separate boundary or not. Tipping points on phosphorus pollution are a long way off, but certainly from a depletion point of view, phosphorus is the bigger deal.

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