Modification of Nitrogen Fertilisers Using Inhibitors: Opportunities and Potentials for Improving Nitrogen Use Efficiency

Keywords: ammonia volatilisation, urease inhibitors, N-(n-butyl) thiophosphoric triamide, nitrification, nitrification inhibitors, DMPP, nitrous oxide, nitrate losses.

When fertiliser nitrogen (N) is applied to soil it is not used efficiently and plant N uptake is seldom greater than 50% of the N applied. One of the reasons for poor N use efficiency is high losses from the plant-soil system by ammonia (NH₃) volatilisation, during nitrification and by leaching, erosion, surface runoff and denitrification. The loss of N represents an economic loss to farmers and has environmental implications for water quality and greenhouse gas emissions. Stabilised N fertilisers (urease and nitrification inhibitors) have received considerable attention recently due to their potential to lower N losses to the environment and increase crop yields. However, the efficacy of urease and nitrification inhibitors can be quite variable depending on the crop, soil properties, climatic and management factors. Many compounds have been evaluated as urease or nitrification inhibitors however, few meet the requirements of being effective at low concentrations, non toxic, stable, inexpensive and compatible with standard N fertilisers. Urease inhibitors delay the rate of urea hydrolysis to ammonium-N (NH₄⁺-N) and hence prevent localised zones of high pH and NH₄⁺-N concentrations in soil, which are conducive to NH₃ volatilisation. They are expected to be most beneficial on soils where NH₃ volatilisation from urea fertiliser is high. The urease inhibitor N-(n-butyl) thiophosphoric triamide (trade name Agrotain ^®) is the most successful commercially, where it has been shown to lower NH₃ volatilisation from surface-applied urea and increase yields on a wide range of crops. Agrotain is now licensed or sold in over 70 countries worldwide, in both agricultural and amenity markets.

Nitrification inhibitors slow down the rate of oxidation of NH₄⁺-N to nitrate-N (NO₃–N) and hence reduce NO₃^– leaching and the production of nitric oxide (NO) and nitrous oxide (N₂O) by both nitrification and denitrification. They are likely to have greatest benefit on soils where N losses due to leaching or denitrification are large. The nitrification inhibitor 3,4-dimethylpyrazole phosphate (DMPP) appears to have benefits over other inhibitors because of its effectiveness at low concentrations, its stability and lack of mobility in soil. For nitrification inhibitors to be more widely accepted for use in agriculture they will need to be priced competitively. As their benefits include increase crop production, improved crop quality, greater management flexibility and reduced environmental losses, there should be increased interest in their use, particularly if carbon credits for lower greenhouse gas emissions can be used to offset costs.

Catherine J Watson, BSc., PhD, Ronnie J Laughlin, BSc., PhD, PGCE, and Karen L McGeough, BSc., MSc., PhD.

Agriculture, Food and Environmental Science Division, Agri-Food & Biosciences Institute, Newforge Lane, Belfast, BT9 5PX Northern Ireland, UK.

40 pages, 9 tables, 3 figures, 128 references.