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Earth's nine life-support systems: Nitrogen and phosphorus cycles

By Fred Pearce

24 February 2010

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Human use of fertiliser is messing with the natural flow of nitrogen

(Image: View China Photo/Rex Features)

Boundary 1: No more than 35 million tonnes of nitrogen fixed from the atmosphere per year

Current level: 121 million tonnes per year

Diagnosis: Boundary far exceeded and effects worsening

Boundary 2: No more than 11 million tonnes of phosphorus to flow into the oceans per year

Current level: 9 million tonnes per year

Diagnosis: Boundary not yet exceeded

Nitrogen is an essential component of all living things, yet only a small amount of the planet’s stock of nitrogen is in a form that living things can absorb. This is “fixed” out of the air by bacteria in a range of leguminous plants. But you can have too much of a good thing. So other microbes “denitrify” ecosystems, converting the element back into forms not available for living things. This is the nitrogen cycle.

Farmers have always interfered with the cycle, because nitrogen availability often limits the fertility of soils. They have boosted production by planting more leguminous crops, like clover.

Then, a century ago, the nitrogen cycle changed forever when Fritz Haber, a German chemist, invented an industrial process for fixing nitrogen from the atmosphere to make chemical fertiliser. Today, 80 million tonnes of nitrogen is fixed from the atmosphere in this way each year and poured onto the world’s fields.

But farming inefficiencies mean that most of this nitrogen runs off the land into rivers and oceans. Much of the nitrogen that does get into crops is later excreted by humans into sewers. We further fix nitrogen by cultivating legumes and burning fossil fuels, timber and crops. Put all that together, and we fix around 121 million tonnes of nitrogen a year, far more than nature does – and nature cannot cope.

The excess nitrogen is acidifying soils, killing vulnerable species and saturating ecosystems so that they lose the ability to recycle the nitrogen back into the air. Meanwhile, some over-fertilised lakes and seas in heavily farmed regions fill with “blooms” of aquatic life which then die and decompose, sucking all the oxygen out of the water in the process. The legacy of such blooms is anoxic “dead zones”. At the last count there were more than 400 such zones in the oceans, covering 250,000 square kilometres, including parts of the Gulf of Mexico, the Baltic Sea and waters between Japan and Korea.

Rockström tentatively sets the safe level for human additions to the nitrogen cycle at about 35 million tonnes a year, one-quarter of the current total. Reaching that figure while continuing to feed the world is, to say the least, a tough ask.

Phosphorus, also used as fertiliser, is potentially part of the same problem. Around 20 million tonnes of phosphorus is mined from rock deposits annually and about half of this ends up in the ocean – about eight times the natural influx – where it contributes to blooms and dead zones. Rockström’s team estimates that we can add up to 11 million tonnes of phosphorus per year without serious repercussions.

Read more: From ocean to ozone: Earth’s nine life-support systems

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