Urease Activity and Inhibition – Principles and Practice

Keywords: Ammonia volatilisation, N-(n-butyl) thiophosphoric triamide, urea, urease, urease inhibitors

Urea has become the most used N fertiliser in the world, accounting for 46% of total world nitrogen consumption. Its market share is increasing because it is the cheapest form of solid N fertiliser available and its high N content (46%N) offers transportation advantages over other sources. The efficiency of urea is decreased by losses of N through NH₃ volatilisation after the urea is hydrolysed at the soil surface by reaction with the enzyme urease. Many compounds have been evaluated as urease inhibitors, which slow urea hydrolysis potentially allowing the urea to move away from the soil surface to where it is not as susceptible to ammonia loss. However, few of these compounds meet the requirements of being effective at low concentrations, non toxic, stable, inexpensive and compatible with urea. The most promising class of inhibitors are the phosphoryl di- and tri-amides, which are structural analogues of urea. The most widely tested tri-amide is N-(n-butyl) thiophosphoric triamide (nBTPT), where its urease inhibitory activity is associated with the formation of its oxygen analogue. It is highly effective at low concentrations, reducing NH₃ volatilisation from surface-applied urea and increasing yield over non-inhibited urea in a range of crops including maize, rice, Kentucky bluegrass (Poa pratensis L) and temperate grassland. It can also prevent the adverse effect of ammonia toxicity on seed germination and seedling growth and has the potential to reduce NH₃ loss from livestock waste.

N-(n-butyl) thiophosphoric triamide is less successful in lowering NH₃ loss in flooded soils. This is because under anaerobic conditions the conversion of nBTPT to its active form is slow. Mixtures of urease inhibitors in conjunction with algicides have been more successful, although further research is required on the forms and amounts needed in tropical systems.

Maximum benefits of urease inhibitors will occur when crop yield potential is high, soil N levels are low and soil and environmental conditions promote extensive NH₃ volatilisation losses. As nearly half of all N actually used in agriculture is in the form of urea with a large proportion being surface-applied or used on flooded rice, there is considerable potential for urease inhibitors.

N-(n-butyl) thiophosphoric triamide is the only urease inhibitor that is currently commercially available, having passed extensive toxicological and environmental tests. It was first introduced (by IMC-Agrico) for the American maize market in spring 1996, under the trade name of Agrotain . Agrotain can be used to impregnate urea granules or be added to urea-ammonium nitrate solutions prior to surface spreading in the field. The expanding acceptance and commercialisation of nBTPT as a urease inhibitor world-wide may lead to its availability within the EC in the near future. However, its acceptance will be dependent on legislative and economic considerations.

Dr Catherine J Watson, Department of Agriculture & Rural Development, N Ireland, and The Queen’s University of Belfast, UK.

39 pages, 9 figures, 9 tables, 129 refs.