What does it cost to remove nitrogen from farmers’ fields?

Nitrogen from farmers’ fields must not end up in the aquatic environment but what is the most cost efficient offsite solution? This question was discussed at an international workshop organised by Aarhus University. 

2017.06.02 | Janne Hansen

Several factors must be taken into consideration when calculating which landscape filter system has the best cost efficiency. Photo: Colourbox

There is no doubt that nitrogen must be removed from water coming from agricultural fields before it ends up in the aquatic environment. With this in mind, the Ministry of Environment and Food of Denmark has approved catch crops and a range of collective measures that can contribute to preventing nitrogen loss to the aquatic environment, and has set aside funds for supporting implementation of these measures.

The big question is which of the approved measures is the most optimal to use when looking at costs and effects. Senior researcher Brian H. Jacobsen from the Department of Food and Resource Economics at the University of Copenhagen shed light on this issue at an international workshop in Slagelse, Denmark, on April 20, 2017. The meeting was organised by Aarhus University with the aim of exchanging knowledge and experience with international experts in this area. 

The answer to the question is not so clear.

- The choice of measure depends to a great extent on how much nitrogen the system removes, how much the system costs to establish and maintain, and how big the catchment area is. Another factor to consider is that differences in local conditions mean that the same solution is not necessarily the one that should be used everywhere, said Brian H. Jacobsen, who had calculated the cost efficiency of mini-wetlands based on preliminary estimates of project costs.

The cost of removing one kg nitrogen

The cost efficiency of removing one kg nitrogen varies a great deal. With catch crops it costs 31 kroner to remove one kg of nitrogen from the root zone provided that the catch crops on average remove 9.6 kg nitrogen per ha per year and that they cost 300 kroner per ha. If you include the compensation of 700 kroner per ha then the cost of removing nitrogen with the aid of catch crops is 73 kroner per kg nitrogen.  

Catch crops cannot solve the task on their own and therefore collective measure have to be used. Collective measures are various offsite systems that contribute to reducing nitrogen loss from agricultural land and that the public supports financially. Among the possibilities here are re-established wetlands, constructed surface flow mini-wetlands and constructed subsurface flow mini-wetlands. 

Re-established wetlands can remove one kg nitrogen at an average cost of 52 kroner while surface flow mini-wetlands can do it at an average cost of 55 kroner per kg removed nitrogen. The cost efficiency here is based on the costs that are included in the government’s Food and Agricultural Package.   

Subsurface flow system is more expensive

There are, however, some ”ifs, buts and maybes” in the calculations. The cost of removing one kg nitrogen with the aid of mini-wetlands can vary from 15 kroner per kg nitrogen to as much as 156 kroner per kg nitrogen.  

If the catchment area is small or the nitrogen load is not very high, then it costs more per kg removed nitrogen than if the system removes nitrogen from a large catchment area or there is a high nitrogen load. The cost per kg removed nitrogen also depends on the system’s establishment and maintenance costs. If you take all these factors into consideration, then subsurface flow mini-wetlands are typically more expensive than surface flow mini-wetlands.   

- If you look at it from a pessimistic viewpoint and calculate with a catchment area that is only 5 ha, the system only removes on average 6 kg nitrogen, and the lifespan of the mini-wetland is only 15 years, then the cost of removing one kg nitrogen is 156 kroner, Brian H. Jacobsen explains.

- On the other hand, if you take the optimistic viewpoint and calculate with a catchment area of 100 ha, a nitrogen removal effect of 26 kg removed nitrogen per ha, and a system lifespan of 25 years, then the cost of removing one kg nitrogen falls to 11 kroner per kg nitrogen with a mini-wetland. A subsurface flow mini-wetland would under these premises be able to remove one kg nitrogen for 15 kroner per kg nitrogen. 

Mini-wetlands can also provide other benefits. The workshop showed that besides a nitrogen removal effect there is also a phosphorous effect as well as other potential positive effects that can improve the benefits of mini-wetlands. 

Size matters

The figures show that cost efficiency in mini-wetlands is best when they can remove nitrogen from a catchment area that is larger than 30 ha. If the area is smaller, then the cost per kg removed nitrogen will increase unless cheaper ways to establish the wetlands can be found.

- Most fields are smaller than 30 ha, so it will be quite a challenge if we have to stick to fields that are larger than 30 ha, Brian Jacobsen points out.

Farmers show great interest in wetlands and authorities and advisers are also eager to get going with implementation of this measure. However, before investing billions of the public’s kroner, it is important to ensure that we invest in the most cost efficient solutions, including selecting the right locations.  

With regard to costs, the investment costs mean a lot for small projects but there might be other possibilities, e.g. if the farmer carries out some of the work himself. We can also expect contractors and engineers to prepare concepts that will lead to a greater degree of standard solutions. 

- We are still missing answers to several questions, including more knowledge about the nitrogen removal effect under various conditions, and establishment, management and maintenance costs under various conditions. The goal of minimum 1000 projects over the next 3-4 years is ambitious keeping in mind that more wetlands must be established per year than before. With regard to location, measures in the field and collective measures have to be seen in context with each other as the effect of one measure can affect the efficiency of the other measures, says Brian H. Jacobsen.

Contact: Senior researcher Brian H. Jacobsen, Department of Food and Resource Economics, University of Copenhagen, email: brian@ifro.ku.dk


More information:

Cost efficiency of mini-wetlands depends on several factors:

  • How efficient the system is at removing nitrogen
  • How much the system costs to establish and maintain
  • How big a catchment area the system affects
  • How long the lifespan of the system is

Facts and figures

  • In Denmark we must remove a total of 13,000 tons nitrogen before the end of 2027.
  • With the aid of collective measures, i.e. offsite measures, Denmark must remove 3,800 tons nitrogen per year from 2016-2020.
  • The Ministry of Environment and Food of Denmark has budgeted that constructed mini-wetlands must remove 900 tons nitrogen per year. Given that mini-wetlands on average can remove 9 kg nitrogen per ha, this will require establishment of 1000 mini-wetlands (catchment area 100 ha).
  • Surface flow mini-wetlands can cost a total of 250,000 kroner per ha to establish.
  • Subsurface flow mini-wetlands can cost a total of 400,000 kroner per ha to establish. The costs can vary greatly from location to location. The government’s Food and Agricultural Package has set aside 550,000 kroner per project (with 100 ha catchment).  
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