An expert group gathers to discuss this elemental problem. The John Innes Centre’s Allan Downie reports on problems and progress.
What is the nitrogen crisis? It is clear that we have introduced major global shifts in production and use of reactive nitrogen without really knowing what happens to the ammonia and nitrogen oxides released to the environment.
The production of nitrogen fertiliser and combustion of fossil fuels doubles the amount of reactive N entering the nitrogen cycle annually.
At a meeting entitled ‘The Nitrogen Crisis: what are the solutions?’ held recently in Oxford, UK, David Fowler (Centre for Ecology and Hydrology, Edinburgh) explained how the resulting atmospheric and marine pollution has an estimated cost of Є70-300Bn (mostly due to human health effects) in Europe and that the problem is likely to get worse with climate change.
Mark Sutton (Centre for Ecology and Hydrology Edinburgh and co-chair of the UNECE task force on reactive nitrogen) made the case for introducing international protocols to improve the use of the reactive N we currently produce. To put this into perspective, based on current use, the survival and feeding of about half the world’s population depends on the application of nitrogen fertiliser to crops. However, in Europe, only a small fraction of this N (less than 5%) ends up in our mouths as food!
One issue is that there are no costs to users of effectively losing (or dumping) reactive N into the atmosphere or oceans; but society will pay in the end as nitrogen pollution increases. As Charles Godfrey (population ecologist at University of Oxford) pointed out, if this is our position now, consider the consequences if, as predicted, the human population increases by 50% to 10-11 billion.
There are actions we can, we must, take to ameliorate these issues.
However there is very little political will for a global approach to deal with this among the main polluters of North America, Europe and China. For example, replacing 25-50% of European consumption of animal-derived foods with plant-based foods could cut nitrogen emissions by 40%, greenhouse gas emissions by 25-40% and per capita use of cropland by about 23%.
Paradoxically, lack of nitrogen fertiliser is one of the main limitations on food production in several countries, especially in sub-Saharan Africa, due to a lack of money and infrastructure for distribution of fertilisers.
Theoretically, nitrogen-fixing leguminous plants such as peas, beans and lentils can resolve part of this problem, due to their symbiosis with bacteria (rhizobia) that can reduce N2 to NH3; this can supply all the nitrogen required by the plants with relatively low losses to environmental pollution.
However, as pointed out by Alfred Gaythorne-Hardy (Oxford India Centre for Sustainable Development), unlike the increased production of almost all other crops in India, the growth of legume grains has remained unchanged over 60 years. So production per capita has fallen by about a third, even though in terms of ecology and human (and animal) nutrition, legumes are an excellent complement to growing cereals.
Lack of increase in growth of grain legumes is due to subsidised supply of fertilisers, difficulties in managing legume crops and to economic factors – legume crop yields are lower than for cereals. John Howieson (Murdoch University, West Australia) explained that even in West Australia, where leguminous lupins became a useful break crop, over the last 15 years growers have replaced lupins with canola (oilseed rape). Low prices, problems with both weed control and introducing legumes in poor soils all contributed to the change. So, even in highly advanced agronomies legumes may be falling out of favour, while in the meantime pollution with reactive nitrogen gets worse.
Fixing the future
So what can we do with crops? Ken Giller (University of Wageningen) described some genuine success stories in Africa with introductions of appropriate legumes and their inoculants. A key message for transfer of technology to Africa was that a lot of effort is needed: knowledge transfer was essential, as was tailoring the legume crop and inoculant for each situation.
Other options for improving the situation worldwide include: a) breeding crops with optimal nitrogen use efficiency; b) generating legumes that can be managed/grown more easily; for example in the Global Food Security programme BBSRC are looking at breeding ‘superlupins’ as animal feeds, c) identifying crop varieties that associate with nitrogen-fixing bacteria (as with sugar cane and as described by Jean-Michel Ane at University of Wisconsin, with an unusual variety of maize); and d) possibly to enhance nitrogen transfer to plants by root-associated and/or endophytic nitrogen-fixing bacteria.
In the longer term it may be possible to genetically engineer cereal plants to fix nitrogen. Ray Dixon and John Peters discussed what genes would be required for production and assembly of the nitrogen fixing complex and its cofactors, in organelles like chloroplasts or mitochondria. As explained by Giles Oldroyd, an alternative approach could be to engineer non-legumes such as maize to be able to interact with nitrogen-fixing rhizobia.
Even a very inefficient symbiosis could have a significant impact, particularly in Africa. However, the problems with achieving such genetically engineered solutions are enormous and we do not yet know whether they will be surmountable.
In the meantime we should be taking actions now to optimise our use of fertiliser and identify ways of ameliorating pollution caused by reactive nitrogen.
As a first step, perhaps we should consider reducing the amount of animal-derived foodstuffs we consume, while the scientists and agronomists try to optimise use of existing nitrogen-fixing crops and explore improvements in nitrogen use efficiency in all crops.
About Allan Downie
Allan Downie has been working on rhizobial-legume nitrogen-fixing symbioses for over 30 years. He is now an Emeritus Fellow at the John Innes Centre in Norwich and Honorary Professor at the University of East Anglia.