A lack of nitrogen in the soil often restricts the productivity of a crop. Nitrogen fertilisers, made by the energy-intensive Haber Bosch process, have hugely improved farm yields around the world. But a new study on the wider impacts of the use of nitrogen compounds on soils suggests that these benefits are much less than the environmental costs. What is called ‘reactive’ nitrogen pollutes water supplies, produces greenhouse gases, cuts air quality and reduces biodiversity. Mark Sutton and colleagues completed a cost-benefit analysis of reactive nitrogen.(1) They found the value of extra yields from the use of nitrogen was €25-€130bn. (An unusually wide estimate, it must be said). But the cost of nitrogen-related pollution was put at €70 to €320bn meaning that roughly speaking the pollution costs of nitrogen are three times the value of the enhanced crop yields. So why we do use so much fertiliser? It is much like the banks: the gains are private (accruing to bankers and to farmers) while the losses are socialised (accruing to taxpayers and citizens). Put another way, the application of fertiliser onto farmlands is far too cheap because its price does not recognise the full costs of using it. The result is that farmers are not incentivised to be efficient in its use and the study authors estimate that about half of the nitrogen added to Europe’s soils ends up as pollution or back into the air as nitrogen gas.

Perhaps more controversially, the report recommends a reduction in meat consumption. About 85% of the nitrogen that isn’t wasted and which ends up in the complex molecules in food crops is eaten by animals, not humans. If we were all vegans, we would need to grow more plant matter but would still reduce the need for artificial nitrogen by 70% from today’s levels, even if we wasted as much nitrogen as we do today.

There’s another point which isn’t mentioned in Mark Sutton’s summary of the full report. One of the causes of nitrogen waste is monoculture. If our croplands are all devoted to huge acreages of a single plant, whether wheat or sugar beet or oilseed rape, then nitrogen uptake is likely to be lower than if we grow a diverse mix of crops in the same area. The precise mechanisms by which nitrogen is better absorbed by a range of different plants in the same area than by just one plant are not well understood. Nevertheless several studies now show that biodiversity reduces the run-off of nitrogen and thus cuts the environmental impact of using fertilisers, whether animal manure or artificial.

By coincidence, one such study was carried in Nature the week before the Sutton summary of the European work.(2) Bradley J Cardinale showed that maintaining the diversity of different species of algae in streams resulting in greater uptake of the available nitrogen that would otherwise have polluted downstream rivers. He concludes that ‘biodiversity may help to buffer natural ecosystems against the ecological impacts of nutrient pollution’.

An earlier piece of research demonstrated a similar result. Whitney Broussard and Eugene Turner found that places in the US with diverse crops had lower levels of dissolved nitrogen in the rivers leaving the area.(3). The authors recommend rotating crops, decreasing field size, increasing the width of field edges and incorporating more native grasses between fields. All worthy objectives, but as Broussard says, ‘The American farmer is caught in a mode of production that has tremendous momentum and cannot be changed on the farm’. In other words, farmers are trapped into monocultures and have neither the knowledge nor the ability to take the financial risks of moving away from reliance on one crop. We therefore need to find a way of incorporating biodiversity’s value in reducing the costs of nitrogen pollution in the calculus of the farmer.

The curious fact is that moving away from single crop agriculture also seems to increase yields. Mixing different plants in a single field, or having animals living alongside the plants, such as ducks living in rice paddies, can systematically improve crop performance. One study in China showed that mixing plants such as maize, sugar cane, wheat, potato and broad bean in a single field might add 30 to 80% to overall production.(4) I have seen similar results from mixing grains and other crops in Australia. Perhaps even if we cannot find a way to reward farmers for increasing biodiversity, and thus reducing nitrogen run-off, we can persuade them to investigate intercropping of different plants in the same field simply because it improves their overall yields.

1, Sutton M.A. et al. The European Nitrogen Assessment, (Cambridge University Press, 2011), available at http://go.nature.com/5n9lsq. A summary can be found at www.nature.com/nature/journal/vaop/ncurrent/full/472159a.html

2, Cardinale, Bradley J, Biodiversity improves water quality by niche partitioning, Nature, www.nature.com/nature/journal/v472/n7341/full/nature09904.html

3, This research is described at http://www.azocleantech.com/Details.asp?newsID=4617

4, Chengyun Li et al., Crop Diversity for Yield Increase, PLoS ONE, November 2009