Out of thin air

What is the problem with the element that feeds the world? (8 minute read)

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When we take a breath, most of us probably don’t give much thought to what’s in the air we draw into our lungs. If we do, we might think about the oxygen that our bodies need to function. We might think about greenhouse gases like carbon dioxide, which make up only a tiny fraction of the air but have such a huge impact. But we probably don’t think about nitrogen, which actually makes up 78% of the air. In its gas form, it is largely non-reactive. So, we breathe it in and we breathe it out, but it doesn’t really do anything, it’s just there.

When combined with other elements though, nitrogen is essential for life. It is the fourth most abundant element in the human body, after hydrogen, oxygen and carbon. It’s present in our bones, our blood and our brains. It’s present in proteins, it’s present in energy-carrying molecules like ATP (adenosine triphosphate) and it’s present in our DNA. In living things from the rainforests to the poles and the depths of the ocean, nitrogen is there.

How does something so inert in its elemental state get into our bodies? The answer is that it comes from our food, but that doesn’t fully answer the question. How does it get into the food we eat?

The answer to that question is quite complex. It’s difficult to convert nitrogen from the air into any kind of reactive molecule. The process is known as nitrogen fixation, and in nature there are only two ways that it happens – microbes and lightning. Exactly how much lightning contributes is uncertain – estimates range from 1-10%, meaning that most of the work is done by microbes.

Photo by Eugene Triguba on Unsplash

Microbes which fix nitrogen are found in the soil, in the ocean and in the roots of certain plants (there are also a few plants which have these microbes in their leaves – more on them later). Familiar plants which have nitrogen fixing microbes in their roots include beans and clover. Another plant with nitrogen-fixing microbes in its roots is gorse (Ulex europeus), a familiar weed to New Zealanders. Because nitrogen is important for plant growth, a plant with nitrogen-fixing microbes in its roots has an advantage over other plants when growing in environments which are low in nutrients. (Mostly, we talk about these plants as “nitrogen fixers” as if they themselves fix the nitrogen, but this is giving the plants unjustified credit, because it is always the microbes which do the nitrogen fixing).

The cycle where nitrogen is fixed by microbes and lightning, used by plants and animals, and released back into the air is known as the nitrogen cycle (there’s a good video explaining the process here). In natural environments, nitrogen is one of two elements which limit the growth of plants, and therefore the productivity of the whole environment (the other limiting element is phosphorus).

People began adding nitrogen to the soil to help plants grow long before they knew what nitrogen was or understood the nitrogen cycle. Early farmers observed that animal and human waste helped crops to grow. Ancient Romans documented the use of seaweed as a fertiliser. People also grew nitrogen-fixing plants. Perhaps the most remarkable example of this is the use of the fern Azolla in rice paddies. Azolla is not the kind of fern you might have seen in the forest or as a house plant. It’s an aquatic fern that floats on the surface of slow-moving waterbodies, including flooded rice fields. In its leaves, it has tiny pockets which are home to nitrogen-fixing microbes. For millennia, Azolla has been deliberately grown on rice fields, supplying nitrogen to rice plants.

A British chemist called William Crookes was one of the first to recognise that the world’s need for food was in danger of exceeding the natural nitrogen supply. He gave a lecture in 1898 in which he proposed that scientists needed to find a way to convert nitrogen from the air into a form available to plants – on a much greater scale than could be achieved with bacteria. Just over 10 years later, the German Jewish chemist Fritz Haber developed a process which did exactly that. His method was small-scale, but soon afterwards Carl Bosch of the BASF chemical company took Haber’s process and developed it on an industrial scale. Known today as the Haber-Bosch process, the method developed by these two men is one of the most important technological developments of the 20th century.

Initially, the Haber-Bosch process was put to less benign uses than growing food. Until the 1950s, it was mostly used in the development of weapons – certain nitrogen compounds, such as ammonium nitrate, are important explosives. But from the 1950s onwards, nitrogen compounds manufactured using the Haber-Bosch process became increasingly important as fertilisers (including ammonium nitrate, which exploded with such devastating impact in Beirut two years ago). Without these fertilisers, the population limit of the world would be approximately 4 billion people – a figure we reached in 1975.

But the Haber-Bosch process comes at a cost. To understand why that is, I’ll need to explain a little more about how the process works. One ingredient, as I’ve mentioned, is nitrogen from the air. The other ingredient is hydrogen gas. The two are combined under high pressure and temperature, and in the presence of a chemical catalyst, to produce ammonia (there’s a good video explaining the process here).

Hydrogen gas is highly reactive. It can’t simply be drawn from the air as nitrogen can. And here is why the Haber-Bosch process is important and why I wanted to write about it. The hydrogen gas comes from fossil fuels, and it produces carbon dioxide as a by-product. The high temperatures required for the process also use a huge amount of energy, and this energy also comes from fossil fuels. The Haber-Bosch process contributes 1.2% of human-caused carbon emissions. While this doesn’t sound like much, it’s more than any other chemical manufacturing process. And if we really want to find solutions to climate change, we have to look at all of our carbon emissions.

Carbon emissions aren’t the only reason to be concerned about nitrogen fertilisers. We have become used to an abundant and relatively cheap supply, and we have been grossly overusing them. Over 80% of the fertiliser we apply to our fields is not used by our crops at all, but is lost to the environment as waste.

If we apply more nitrogen to a field that a crop can use, two things happen. The first is that microbes in the soil convert some of the nitrogen fertiliser into nitrous oxide, a potent greenhouse gas. Nitrogen fertiliser isn’t the only source of nitrous oxide from agriculture – animal urine is also important – but it’s not insignificant.

Not all of the excess nitrogen from fertiliser is converted to nitrous oxide. Some of it is leached from the soil and into water in the form of nitrate, which creates its own set of problems. Too much nitrate in rivers and lakes causes increased growth in aquatic plants and algae. In some New Zealand lakes, excess nitrate (as well as phosphate, another ingredient in fertiliser) has contributed to the collapse of the lake ecosystems, leaving the water murky and unsafe. The problem isn’t confined to New Zealand either. Around the world, excess nitrogen is pouring into lakes and rivers. From there, it reaches the ocean.

The ocean is a big place, with a lot of water to dilute the nitrogen flooding in. However, even in the ocean, excess nitrate is a problem. The excess nitrate (and phosphate) results in algal blooms across large areas of ocean. As cells from these blooms die, they fall to the bottom of the sea and decompose, a process that uses oxygen. The huge volume of decaying material sucks all the oxygen from the water, creating areas known as dead zones. Fish and other mobile marine animals leave the dead zone. Those that cannot leave die. Every summer, a massive dead zone forms in the Gulf of Mexico, fuelled by nitrate washed down the Mississippi River.

On the surface, the solution to the nitrogen problem seems obvious. We are wasting 80% of the nitrogen fertiliser we produce – if we stopped wasting it, we would solve most of the problem. It’s not that simple of course, but there is a global initiative to halve nitrogen waste by 2030. If countries can stick to their commitments, that will make a big difference. But New Zealand has some way to go. The latest State of the Environment report, from 2021, showed that nitrogen levels continue to increase in our lakes and rivers.

There has never been a better time to do something about our wasteful use of nitrogen fertiliser. Not only does it contribute to climate change and pollution of our waterways, but the price of fertiliser is rapidly rising. It was already rising in 2021, but things have got a lot worse in 2022. The reason? The world’s largest producer of nitrogen fertilisers is Russia. If we didn’t have enough motivation to solve the problem before, perhaps we have it now.

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