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Overuse of nitrogen fertiliser can have nasty environmental consequences. eutrophication&hypoxia/Flickr

Agriculture’s hunger for nitrogen oversteps planetary boundaries

Planet Earth has boundaries for its biophysical subsystems. By 2009, we had already exceeded three of the boundaries – climate change, biodiversity loss, and the nitrogen cycle.

Climate change is a top-of-mind issue for governments and there has been a UN convention on biological diversity since 1993. But dramatic changes in the nitrogen cycle have received much less attention.

Humans have more than doubled the rate of atmospheric nitrogen being fixed since 1950. Fixing nitrogen makes it chemically active and therefore useful for making fertilisers and explosives.

Nitrogen is necessary, but handle with care

Fixed nitrogen is a necessary fertiliser, but its environmental impacts are serious, especially when used to excess. It contributes to environmental problems from climate change to coastal dead-zones.

Nitrogen fertiliser is vital for plant growth: plants are approximately 4% nitrogen. For this reason about 100 Mt (megatonnes) of nitrogen fertiliser is applied to crops each year. This is quite a lot of nitrogen considering that natural nitrogen fixation (from the atmospheric form to forms usable by plants and animals) is only just over double this at 225 MtN/yr.

Traditionally nitrogen fertiliser came from animal manures or guano, or nitrogen-fixing legumes. Legumes have a symbiotic relationship with bacteria in their roots that can fix nitrogen from the atmosphere.

But these natural sources of nitrogen aren’t enough to support today’s high-yielding agriculture. Almost all of our nitrogen fertilisers contain nitrogen fixed to ammonia from atmospheric nitrogen via a method called the Haber-Bosch process.

Increase in fertiliser use since the invention of the Haber-Bosch process. Trevor Garnett

The Haber-Bosch process was invented by Haber in 1909 and commercialised by Bosch in 1913. The process became particularly important during the First World War when Germany was cut off from supplies of nitrate from Chile.

The process was used in explosive manufacture during the war, but its greatest use in the past half century has been in fertiliser production.

The impact of the Haber-Bosch process on the nitrogen cycle can be seen in the nitrogen isotopes in sediments in fresh-water lakes. Humans began to affect the nitrogen cycle during the industrial revolution, through nitrogen oxides released by burning fossil fuels.

But the main change has been in the last 50 years since the green revolution in agriculture. Nitrogen fixed by the Haber-Bosch process now makes up almost half as much nitrogen input to the biosphere as natural inputs (100 Mt N/year from Haber-Bosch versus 230 Mt N/year from natural sources). Pretty much all of this has been added since the 1950s.

Environmental impacts of nitrogen in agriculture

One of the most visible impacts of nitrogen is in aquatic ecosystems. Excess nitrogen runs off into fresh water and marine environments, causing algal bloom, loss of oxygen from the water and death of aquatic animals.

Nightflyer/Wikimedia Commons

More than of $500 million worth of nitrogen fertiliser runs into the Mississippi each year. Aside from the financial loss, the nitrogen leads to phytoplankton blooms which result in major dead zones and subsequent fish kills. One such bloom recently in the Baltic Sea was half the size of Germany.

In Australia, the great Barrier Reef now receives 5.8 times more nitrogen than it did before European settlement.

Less visible are greenhouse gases resulting from nitrogen fertiliser use and misuse. Fixing nitrogen to make fertiliser is an energy-intensive process accounting for 2% of the world’s energy use. Even worse for climate change are the nitrous oxides released when excess nitrogen fertiliser is broken down by soil bacteria. These gases are 300 times more potent than CO₂ as greenhouse gases.

Steps that could reduce agricultural nitrogen use

Better targeted timing and placement: Fertilisers can be better absorbed by plants if they are applied at the right stage of growth and to the roots of plants rather than the shoots. Traditional practices - such as one application of fertiliser when seeds are sown before winter snow - are much less efficient than multiple smaller applications.

Farmers worldwide could save both money and the environment by more careful application of nitrogen fertilisers. The Food and Agriculture Organisation of the UN has looked at this problem for various regions and crops and come up with a fact sheet on how developing countries can improve soil health and use nitrogen fertiliser more efficiently.

European farmers now have to account for the nitrogen fertiliser they apply to crops, identifying what goes into plants, soils, water and so on.

The UN provides guides for more efficient use of nitrogen. IRRI

Breeding nitrogen-efficient plants: Another way of improving the efficiency of nitrogen fertiliser use is to improve the plants themselves. While considerable effort has gone into breeding crops for yield and disease resistance, little effort has been put into improving their nitrogen use, since nitrogen fertilisers have traditionally been cheap.

Researchers around the world are now working to breed plant varieties that use fertiliser more efficiently. The plants will be more efficient at both taking up nitrogen from soil and using it once it is taken up.

Understanding our nitrogen budget

Nitrogen has a long way to go to achieve the level of public awareness that problems like biodiversity loss and climate change have reached. We’re becoming used to looking at CO₂ and other greenhouse gas emissions in terms of budgets.

A similar system for nitrogen could be very useful in tracking how we’re treating this planetary boundary. A nitrogen accountability system has already started for agriculture in Europe, and could be implemented elsewhere.

What can a consumer do to make a difference? The European Nitrogen Assessment has approached this question with a video which introduces the idea of a nitrogen footprint. Like reducing your carbon footprint, it’s all about lower consumption of energy, transport, water and certain foods. This reduces the amount of nitrogen produced as a by-product of combustion processes and the amount used in agriculture if you eat foods that are more nitrogen efficient (you can achieve this at a basic level by increasing your consumption of plant foods rather than animal protein).

The nitrogen cycle has been strongly affected by human actions. The Haber-Bosch process has allowed us to increase agricultural productivity and sustain the human population at a much larger size than we could without it.

But these activities have strong effects on the environment, and crossing planetary boundaries can lead to rapid environmental change if there is a non-linear response in the earth’s systems.

The nitrogen cycle is only one of the planetary boundaries that humanity is overstepping, but it’s one we can’t afford to ignore.

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