Nitrification (DMPP) and urease (NBPT) inhibitors had no effect on pasture yield, nitrous oxide emissions, or nitrate leaching under irrigation in a hot-dry climate

Warwick J. Dougherty , Damian Collins, Lukas Van Zwieten and David W. Rowlings - Soil Research

Type: Research Paper
Knowledge level: Advanced

Farm Table says:

A well designed study that came to no hard and fast conclusion.

What is the problem?

Nitrogen (N) is a crucial input in Australian dairy farming enterprises for pasture growth, with much of this applied as inorganic N. N is lost through ammonia (NH3) volatilization, gaseous losses of nitric oxide, nitrous oxide (N2O) and dinitrogen via nitrification and denitrification, and leaching of nitrate (NO3–), representing a cost to the farmer. Inhibitors used in the Australian dairy industry are the nitrification inhibitor 3,4-dimethyl pyrazole phosphate (DMPP) and the urease inhibitor N-(n-butyl) thiophosphoric triamide (NBPT), which work to reduce the losses mentioned above. This study investigated the effect of these two inhibitors on pasture yield, N2O emissions and N leaching relative to standard urea.

What did the research involve?

The site was south west of Sydney with an average annual rainfall of 788 mm and had been under permanent pasture used for dairy products for over 20 years. Twelve plots were laid out in a randomized block design with three treatments;

  • standard urea
  • urea + nitrification inhibitor (0.16% w/w DMPP)
  • urea + urease inhibitor (0.045% w/w NBPT).

Each plot was fertilized, harvested and irrigated. Pasture was a mixed ryegrass and Kikuyu. N was applied immediately after every simulated grazing (harvest) in spring and autumn and every other harvest in summer and winter, as urea, at a rate of 46 kg N ha–1 per application (100 kg ha–1 of urea). The following components were measured;

  • Pasture yield and quality measurements
  • N2O fluxes
  • Soil mineral N
  • N leaching

What were the key findings?

There was no effect of nitrification or urease inhibitor (DMPP and NBPT respectively) coated urea on soil mineral N, pasture yield, N2O flux or NO3– leaching. The authors think at their site, gaseous losses were highly episodic and that there was a lack of coincidence of N application and conditions conducive to lose, and so the effectiveness of the inhibitor products was minimal and did not result in an increase in pasture yield

Final comment

It is thought that better matching plant demand with N supply from fertilizer and from mineralization of organic N to minimize soil mineral N, will provide an effective means of minimizing the loss of N2O.

2015 - Australia - Warwick J. Dougherty , Damian Collins, Lukas Van Zwieten and David W. Rowlings - Soil Research
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