Fertile Ground

C&I Issue 1, 2014

Morocco is a beautiful country with astounding landscapes and a rich cultural heritage. Its current political system, based on constitutional monarchy, appears to be stable and unaffected by the troubles found practically everywhere else in Northern Africa.

And still, if you heard that the food security of the entire human population was critically dependent on the situation in Morocco, you might be just a bit worried. But this may well be the way the world is heading.

Phosphorus is an essential nutrient for all plants, and thus for agriculture. Our bones, our genes and our cellular energy all rely on phosphate chemistry. Estimates from 2007 suggested phosphorus production might peak by 2035. A recent revision of the estimates of geological phosphorus reserves came to the result that reserves may last several decades longer than anticipated. However, more than 90% of these reserves are in Morocco, including the West Sahara territory under Moroccan control.

Thus, when reserves of other providers like China, Syria, and Algeria run out, world supply may end up depending on only one country, which could easily become an uncomfortable situation.

Nobel laureate Paul Crutzen, one of the scientists who discovered the threat to the stratospheric ozone layer originating from halocarbons, is among those who have warned of impending phosphate problems. In 2010, he became ambassador of the Global Phosphorus Research Initiative. And in a keynote interview ahead of the Sustainable Phosphorus Summit in Sydney in 2012, Crutzen commented: ‘There is an urgent need to take action now to ensure we will have sufficient phosphorus to feed humanity into the future.’

James Elser and colleagues at Arizona State University in the US have also warned that the current approach of mining geological phosphorus reserves in quantities of over 200m t/year, bringing them out onto the fields and allowing runoff from agricultural lands to cause the eutrophication of surface waters is highly unsustainable. ‘This is no way to run a biogeochemical cycle,’ Elser comments. He now leads the Phosphorus Sustainability Research Coordination Network (RCN).

The obvious remedy to address both supply shortages and environmental impact would be to close the cycle, but there is very little of that going on so far. More efficient use of the phosphorus applied in agriculture would also help. In a recent review paper,1 Elser states: ‘Best practices for fertiliser application combined with no-till cultivation may help considerably in reducing erosion losses, but it would also help if crops were more efficient in exploiting the soil volume and in extracting phosphorus from “unavailable” chemical pools in the soil.’

Farm animals, as Elser points out, are also inefficient at obtaining the phosphorus they need from normal food. Biotechnology might conceivably come up with ways of improving this situation.

However, efficiency improvements alone will not solve the phosphorus problem in the long term. The fundamental problem is that, as with fossil fuels, we are consuming within a few decades the resources that took millions of years to form. Efforts must be made to close the cycle, and to recycle and reuse the phosphorus that has already been produced from mining. Human excrement is an important source. As much of the phosphorus is excreted via the urine, the concept of the separating toilet, which captures urine separately from faeces, could help produce clean and easy to reuse recycled phosphate. The so-called NoMix toilet project, for example, supported by the Gates Foundation, promises to improve both hygiene and food security in developing countries.

Alternatively, where the infrastructure exists, phosphate can be reclaimed from wastewater treatment plants with existing technology, but the presence of a wide spectrum of poorly characterised pollutants, including excreted pharmaceutical substances in wastewater streams, is a growing concern as separating them from the recycled fertiliser could prove costly.

Complete recovery of phosphate from human excretions would make up 20% of the amount needed for agriculture. Other important sources are found all along the food chain, from crop losses and livestock rearing through to food waste. Technologies to process large volumes of organic waste with a view to recapturing phosphorus are only beginning to be developed. Some of this waste material could also be used for energy and thereby make the process more economical.

Elser argues that the challenge of developing a sustainable, essentially closed-cycle phosphorus provision will require a complete rethink of global food production. He concludes that ‘concerted efforts in research, technology transfer, and regulatory and institutional innovation’ are urgently required to address the phosphorus challenge. 

Part of the problem is limited awareness. Sweden has already set itself high targets for phosphorus recycling, and the German Environment Ministry decided in 2012 to focus on phosphorus as one of four substances associated with raw material shortages. In the Netherlands, a ‘Nutrient Platform’ has brought together 33 organisations from business, academia and politics to work together on creating a sustainable value chain. Many governments, however, seem unaware of the issue or stuck in short-termism. The EU has only recently started to set the wheels in motion for the European Commission to come up with strategic plans.

In March 2013, the first European Sustainable Phosphorus Conference was held to raise awareness of the problem and seek to create a sustainable market for recycled phosphorus. The conference led to the launch of the European Phosphorus Platform, which aims to work towards a more sustainable use of phosphorus based on closed value chains. 

One row above phosphorus in the Periodic Table, another element essential for all life is nitrogen. Unlike phosphate minerals, this resource is very evenly distributed around the world, as it makes up the bulk of our planet’s atmosphere. The challenges surrounding nitrogen are, first, to make it biologically usable, and second, not to let excess amounts of nitrate fertilisers accumulate in the environment.

Industry produces more than 100m t/year of ammonia using the Haber-Bosch process, which was first implemented on an industrial scale at BASF in Oppau, Germany, in 1913. Although its invention has successfully averted a global food crisis and now provides at least half of the nitrogen atoms found in all human bodies, many chemists still believe there must be something better.

If root nodule bacteria can fix nitrogen at ambient pressure and temperature, why does the industrial process need 20,000kPa and 500°C as well as an iron catalyst? Studies of the nitrogenase enzyme, whose crystal structure was reported in 1993, gave some clues to what nature’s secret might involve, but the big breakthrough failed to materialise. There is an iron-molybdenum complex at the heart of the nitrogenase, and for a long time chemists have looked at the molybdenum for inspiration.

Now, however, the US group of John Anderson at Caltech has moved the spotlight to the iron in the enzyme, by reporting a tris-phosphine-borane-supported iron complex that can catalyse the reduction of molecular nitrogen to ammonia under mild conditions.2 The precise mechanism of this new catalyst remains to be elucidated, but the authors speculate that the flexibility of their iron complex may be a key requirement for nitrogen reduction under mild conditions.

In an unrelated effort based on very different, non-natural chemistry, the group of Zhaomin Hou at the Riken Center for Sustainable Resource Science in Saitama, Japan, succeeded in ambient nitrogen reduction with the help of a titanium hydride cluster.3 While this study does not yet provide a complete process that leads from dinitrogen input all the way through to the output of ammonia, it covers the most challenging step of breaking up the triple bond, and it provides mechanistic details that may help to make further progress.

Whether any of these approaches will ever be able to replace the century-old brute-force method of the Haber-Bosch process remains to be seen. But even if the production of nitrogen compounds becomes easier, we still have to consider the consequences of adding more reactive nitrogen to the natural biogeochemical cycle.

By the end of the 20th century, the annual output of ammonia from Haber-Bosch plants exceeded the natural production, so it has more than doubled the amount of reactive nitrogen that comes into circulation in a given time. Since then, the industrial production has grown further. As the global cycles of nitrogen compounds are more complex than those of carbon dioxide, for instance, scientists are only beginning to understand where this dramatic change might lead us.

As all biological use of nitrogen is transient and we don’t want reactive nitrogen compounds accumulating in the environment, the extra amounts we put into the cycle will have to be converted back to molecular nitrogen at some point. This process, known as denitrification, is carried out by soil bacteria. But can they cope with the doubling of the amount?

Lex Bouwman from the University of Utrecht, in the Netherlands, has modelled the  development of global denitrification from 1900 and 2000 and found an increase from 68 to 95m t/year, which is impressive but not sufficient to keep up with the much larger increase of the input. Due to the shortfall in denitrification capacity, nitrogen compounds accumulate and lead to the eutrophication of rivers and coastal waters. 

There are further complications to be considered. For instance, excess nitrates will eventually be washed into the oceans, where their fate depends on the local availability of oxygen. Denitrification in the oceans can, under certain conditions, lead to the release of N2O into the air, and make a significant contribution to the greenhouse effect of our atmosphere, as Maren Voss from the Leibniz-Institute for Baltic Sea Research at Warnemünde, Germany, has warned.4

‘And even more critical is the fact that denitrification ceases when oxygen is lacking because the ammonia generated from degradation of organic material cannot be converted to nitrate,’ explains Voss. ‘Thus the substrate for denitrification is not available and ammonia is eventually accumulated in the waters.’

While nitrogen as a resource is far from being limited, there are limits to how much of its compounds we can add to the natural cycle. It is also connected to the biogeochemical cycles of other elements in manifold and complex ways. It is linked with phosphorus through a stoichiometric relation – the oceans require these two elements in a relation of 16:1 for a balanced diet. As we can no longer avoid interfering with these biogeochemical cycles, a major rethink of the ways we manage them may be a good idea.


1. J.J. Elser, Curr. Op. Biotechnol., 2012, 23, 833.
2. J.S. Anderson et al, Nature, 2013, 501, 84
3. T. Shima et al, Science, 2013, 340, 1549
4. M. Voss et al, Philos. Trans. Roy. Soc. London, 2013, 368, 121

Michael Gross is a science writer based in Oxford, UK

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