Myth: It costs too much to reduce nitrogen leaching.

Reality: It is far more expensive to remedy the effects of nitrogen than it is to stop leaching in the first place.

Generally, it is much more difficult and costly to remove contaminants from the environment once they are there than it is to not put them there in the first place [1]. Additionally, the costs will be much higher in the future if we do nothing (or the very little that we are currently doing to stop the problem). Some of the effects may be irreversible and we are putting these costs on future generations.

It has been argued that nitrogen leaching cannot be reduced sufficiently without a reduction in profits. However, the following example from some New Zealand lakes shows a comparison of costs between cleaning up and reducing nitrogen inputs.

90% of wetlands gone To remove total nitrogen (TN) from the Rotorua lakes using wetlands would cost between $14,000 per tonne TN [2] and $4 million/tonne TN [3]. The cheapest cost of $14,000 is to protect existing natural wetlands, which would require natural wetlands to still be around. Unfortunately, 90 per cent of New Zealand’s original wetlands are already gone [4] and some regions are still draining areas of wetlands. Wetlands have many environmental benefits, such as filtering inputs that flow into them, reducing the effects of flooding, and providing habitat for biodiversity.

Constructed floating wetlands, which have been used in the Rotorua lakes, are the least cost effective of all types of wetlands. They cost almost 40 times more than the cost of protecting natural wetlands and at least five times the cost of other constructed wetland types. The cost of a floating wetlands project would be almost $1 million per tonne of TN removed. Estimates of actual nutrient removal achievable through wetlands range between $246,000 and $4 million per tonne of TN [3].

However, on-farm reduction of nutrients may be far cheaper than removing them from the wider ecosystem.

Cost of nitrogen leachingFor example, cutting nitrogen fertiliser on six dairy farms in the Rotorua catchment was estimated to reduce returns by $46 to $428 per hectare per year (ha/year) with an average loss in gross revenue of $173 [5]. The average reduction in nitrogen (N) leaching over the six farms was 26 kg N/ha/yr, yielding a reduction in gross margin of $6.62/kg N or $6620/tonne N.  Thus, removing nitrogen from the lakes using wetlands was 2-600 times as costly, or at least 37 times more costly than the estimated achievable removal rates supplied by scientists (minimum of $246,000 per tonne of TN) [3].

There is widespread agreement among the general public that polluters should pay. However, the costs of cleaning up are generally paid for by other sectors of society (the public) rather than the farmers or industries that are causing the problems. These costs are called externalities – the costs are external to those that produce the problem.

For example, in the Rotorua district the public is paying $19 million to 20 farmers to reduce nitrate pollution by around 40 tonnes of N per year. In Taupo, there is a similar deal costing the public $190 million to reduce N by 170 tonnes per year. This is a cost of approximately $56,000/tonne N/year (if the fund is spread over 20 years).


[1] Foote, K; Joy, M and Death, R (2015). New Zealand Dairy Farming: Milking our environment for all its worth. Environmental Management. Click here for article.

[2] Hamill, K, MacGibbon, R, Turner, J (2010). Wetland feasibility for nutrient reduction to Lake Rotorua. Whakatane, New Zealand: Opus.

[3] Ford-Robertson, J. (2013). TDR symposium. Ford-Robertson Initiatives. Rotorua.

[4] Ministry for the Environment (2007). Environment New Zealand 2007. Wellington, New Zealand.

[5] Ledgard, S, Judge, A, Smeaton, DC & Boyes, M (2010). Greenhouse gas emissions from Rotorua dairy farms: Summary report (Report to Ministry of Agriculture and Forestry). Hamilton, New Zealand: AgResearch.

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