The nutrient cycle is not closed and the Netherlands has more manure than it can use on the land. Photo: Hans Engbers / Shutterstock
How can you make sure crops get enough nutrients without harming the environment and the climate? Professor Wim de Vries is trying to answer this question with research conducted around the world. Based on his findings, he looks for solutions that can help close the nutrient cycle. This is needed perhaps more urgently than ever given the current nitrogen crisis.
For years hardly anyone heard about nitrogen, but now the issue is a permanent feature in the Dutch news. That started with the ruling by the Council of State in 2019 that the Netherlands would need to drastically reduce its nitrogen emissions. Last summer, the government announced its plans, which had far-reaching consequences for agriculture in particular – the largest source of nitrogen emissions with a share of 60 per cent.
Adverse impact due to excessive concentrations
Nitrogen is spread on the land when it is fertilised, explains Wim de Vries, professor of Environmental Systems Analysis at Wageningen University & Research (WUR). “More nitrogen ends up in the soil than is taken up by the crops, which leads to a surplus of nitrate, a compound of nitrogen and oxygen. That leads to acidification, which makes the soil less fertile. When nitrate leaches into the groundwater, this can contaminate drinking water sources.”
Emissions released into the atmosphere – in which nitrogen has become bonded to hydrogen to form ammonia – have adverse effects on the surrounding nature. De Vries: “Nitrogen boosts the growth of some plants but not others, so biodiversity declines. Nitrogen is also harmful for the climate as nitrous oxide is a greenhouse gas that leads to climate change.” With his doctoral research on the effects of nitrogen and sulphur deposition, De Vries laid the foundation for the critical deposition value for nitrogen.
Leaky nutrient cycles
Nitrogen and phosphate are the key nutrients in manure for crops. De Vries: “The nutrient cycle is leaky in many countries. In the Netherlands, for example, that is because we have a huge livestock population on a relatively limited amount of land. As a result, we import a lot of animal feed and we end up with vast quantities of manure.”
“
Ideally, you should use the nutrients in the place they came from
Nitrogen and phosphate are the key nutrients in manure for crops. Photo: Kletr / Shutterstock
The manure quota means farmers can only use some of the manure on their own farm, and so some has to be exported. Artificial fertilisers are then used to make sure crops still get enough nutrients. A lot of CO₂ is emitted in the production of that fertiliser. De Vries: “Which is not what you want either. Ideally, you should use the nutrients in the place they came from.”
Looking at the system as a whole
De Vries has been studying the effects of nutrients on soil, air and water quality for almost forty years and he has spent twenty years researching the efficient utilisation of nutrients and how to close nutrient cycles. In addition to nitrogen and phosphate, he also looks at other nutrients such as potassium, calcium, magnesium and sulphur. De Vries, who is affiliated with the Wageningen Institute for Environment and Climate Research (WIMEK), says: “The essence of the field I work in is to use our knowledge of the various nutrient inputs and outputs and determine the optimum for crop cultivation while limiting the negative impact on the environment. That overall picture is essential. For a long time, politicians dealt with the legislation for the various aspects of the problem – for instance how to curb ammonia, reduce the supply of phosphate or combat nitrous oxide and methane emissions – in isolation without looking at how they are interlinked. That meant farmers were obligated by law to take measures that reduce ammonia emissions, for example, but increase nitrous oxide emissions.”
De Vries and his fellow researchers use field trials, experiments and literature studies to collect data on the use of fertiliser and manure, the uptake of nutrients by crops and the quantities that accumulate in the soil or are lost to the air and water. That is a huge operation because the data differs per area.
As the world’s largest agricultural producer, China is an interesting country for research, for example looking at the effects of phosphate application. Photo: chinahbzyg / Shutterstock
“You need to look at not just the crops but also the local climate, land use and soil type. We are evaluating some long-term experiments in China, for example. As the world’s largest agricultural producer, that country is incredibly interesting from a research perspective. We’re looking at what happens if you use a lot of phosphate in an area over a thirty-year period and what happens if you only use a little phosphate.”
De Vries is also working with a research group on experiments in Zambia with the cultivation of maize. “We’re considering various nutrient ratios and what happens if you leave out one particular nutrient. When do you get problems for the crop or the soil, and when is the balance just right?”
Model calculations for tens of thousands of combinations
All these data are then incorporated in models. De Vries: “This lets us perform calculations and make predictions. What happens to crop yields when you fertilise the land in that area with those quantities of nutrients? And what effect does it have on the soil and air and water quality?”
Model calculations are performed for some tens of thousands of combinations of land use, soil properties and climate. The resulting outcomes and insights have been used to determine the planetary boundaries for nitrogen and spatially delimited areas within Europe, among other things.
The model predictions can also offer pointers in the evaluation of innovations in agriculture, says De Vries. He distinguishes five types of measures. “The first concerns feed. With the right kind of feed, you can make sure cows excrete less nitrogen and phosphate and at the same time stay healthy and continue to produce enough milk and meat.”
“
The essence of my field is to use our knowledge of all the inputs and outputs and determine the optimum for crop cultivation while limiting the negative impact on the environment
In low-emission barns, faeces and urine are separated so that less ammonia is formed. Photo: Shutterstock
Low-emission barns are also a solution, continues De Vries. “Within those barns, the faeces and urine are separated, so less ammonia is formed. Another measure is precision agriculture, which increases the uptake of nutrients by the crop. The efficiency depends in part on the type of fertiliser, the timing and the placement of the fertiliser.”
The final two categories of measures are tillage and crop rotation. De Vries: “Crop rotation means you grow a different crop on a given tract of land each year. Both measures can improve the quality of the soil. They reduce the discharge and leaching of minerals, and they lead to less erosion and more biodiversity.”
Agricultural measures alone will not be enough
While these five measures are key to cutting the loss of nitrogen and emissions of greenhouse gases, De Vries acknowledges that more is needed. “We will also need to focus more on circular agriculture so that we import less nitrogen and phosphate in the form of animal feed. The livestock population will also need to become smaller. The talk now is of halving the population, but a much smaller reduction is possible if you also introduce innovations and circular agriculture.”
De Vries stresses that it is important to look at the level at which the nutrient cycle is closed. “A closed nutrient cycle doesn’t mean much in itself. In theory, you could say we already have a closed cycle at the global level. But that creates major problems regionally, with nutrient excesses in some places and shortages in others.”
He thinks it would be best to have a closed cycle in a relative limited area, for example a province or district. “Within that area, you can grow food, keep livestock and spread the manure on the land. The ideal solution would be a closed cycle at the farm level, but that would mean every farm being a mixed farm. That’s a pipe dream.”
Online
Read more about sustainable nutrient management
Read more about the research of the Wageningen Institute for Environment and Climate Research (WIMEK)
Share this article