Better global nitrogen fertiliser balance in grain needed

More sustainable fertilisation of land on which grain is cultivated is possible, if the yields are the same on a global scale. Rich Western countries should then use less nitrogen fertiliser and poor countries more, according to an international consortium of scientists – including researchers from Wageningen University & Research in the Netherlands.

25 long-term field trials with grain

The researchers based their findings on an analysis of 25 long-term field trials with grain, a research commissioned by the United Nations Environment Programme (UNEP). The data from these field trials were used to create a model that considers the benefits and costs for both agriculture and society. The research was published Monday 31 January in the scientific journal Nature Food.

“Such a global analysis of long-term studies had not been done before,” says co-author Renske Hijbeek of Wageningen University & Research’s Plant Production Systems chair group. Indeed, the optimal nitrogen application for grain, the most important food crop worldwide, is usually determined with short-term field trials. “In the short term, the history of an arable field and the history of previous fertiliser application plays a strong role in crop yield. The calculated optimum is then not representative for the longer term. In longer-term field trials, you actually look at the balance between fertiliser application and soil fertility and you get a more balanced picture.”

Worldwide, researchers provided data from 25 long-term studies. Among them was the famous wheat study in Rothamsted (UK) that has been running for 180 years.

“Fortunately, all those field experiments were easy to compare. We were able to establish one formula – a generic response curve – and then use economic models to calculate the optimal nitrogen application.”

Nitrate leaching, soil depletion and ammonia emissions

Using a simple economic approach, the researchers determined the optimum not only for grain producers, but also for society as a whole. They did this by looking not only at the need for a reliable food supply, but also at the costs that arise from environmental pressures, such as nitrate leaching, soil depletion and ammonia emissions. How heavy these environmental costs weigh on a society depends on a country’s prosperity. In less affluent countries, sufficient food supply has a higher priority.

Too much and too little fertiliser

According to the researchers, if you look at sustainability in this way and look for an optimum in which the costs and benefits for both grain producers and society are in balance, you now see on a global scale areas that use too much fertiliser, but also areas where too little fertiliser is used.

In general, according to the researchers, the surplus countries are in the rich West, while farmers in poor regions, such as Sub-Saharan Africa, would actually have to spread more fertiliser to reach the agronomic and social optimum.

In many poor areas, soil depletion is now taking place due to underfertilisation

What is striking here is the difference between the conclusions of short-term field trials and field trials over a much longer period. “In many poor areas, soil depletion is now taking place due to underfertilisation. As a result, crops are now withdrawing more nitrogen from the soil than is warranted.”

Optimal nitrogen application rate

For wheat production in richer countries, provided the social costs of environmental damage as a result of over-fertilisation are taken into account, the optimum nitrogen application rate is actually lower than the current standard. If such social costs are factored in, the optimal nitrogen application rate for wheat in the Netherlands, for example, would be reduced by 40%: from about 157 to about 90 kilograms of nitrogen per hectare. However, Hijbeek emphasizes that the study concerns a food system analysis. It cannot be translated into advice for one farmer or one region.

So what solution(s) are there to address this problem?

Renske Hijbeek: “Our research mainly shows that there are social benefits to a different distribution of fertilizer-N use and grain production worldwide. How a different distribution can be achieved is actually a political and economic issue; think of trade policies and national subsidies on fertiliser and grain. We hope that our generic approach to N response and cost-benefit will lead to better discussion and reflection in this area.”

“A new approach that is also playing in the Netherlands is to pay farmers for providing more ecosystem services with less intensive use of fertilisers (or pesticides). But in Sub Saharan Africa, there is no food security now and fertiliser usage and grain yields need to go up; our generic yield relationship shows that improving knowledge to increase potential yield is better for the environment than greatly increasing the use of fertilizer-N (as previously done in India and China), these things will have to go together. Improving infrastructure (for transportation and storage of fertilizer and grains) in those rural areas can also play a role. And we should not forget that the about half of the grain is used as cattle feed. Which measures are chosen and accepted is also a political question.”

Can new (precision) techniques contribute to this?

“Our research was based on 25 long-term trials worldwide, some up to 180 years, to better understand equilibrium situations in nitrogen balances. These trials did not include (precision) techniques, but they could certainly contribute to reducing the environmental burden of N poisons in arable farming. We looked at optimal N donations (for farmer and society) and it would be relevant to see how these optimal N donations change when using precision techniques. If the technique leads to less environmental impact for the same gift then the optimal gift could be higher. On the other hand, precision techniques will probably also allow higher yields to be achieved, putting you in the part of the N response curve with a weaker N response. Perhaps these things cancel each other out, but in any case a farmer will benefit from a better N utilisation.”