Effect of urease, nitrification and algal inhibitors on ammonia loss and grain yield of flooded rice in Thailand( Fertilizer research,October 1994, Volume 40, Issue 3, pp 225–233)

Abstract :This paper reports a study, in a flooded rice field in Thailand, on the effects of two urease inhibitors, cyclohexylphosphorictriamide (CHPT) and N-(n-butyl)phosphorictriamide (NBPTO), the nitrification inhibitor phenylacetylene and an algicide treatment, consisting of alternate additions of copper sulfate and terbutryn at ~3 day intervals, on nitrogen (N) transformations and transfers, and grain yield. The addition of algicide reduced the growth of algae and maintained the pH of the floodwater below that of the control for 11 days. Judging from the ammoniacal N concentrations of the floodwater, phenylacetylene inhibited nitrification. The two urease inhibitors markedly reduced urea hydrolysis and CHPT was more effective than NBPTO. Addition of CHPT maintained the ammoniacal N concentration of the floodwater below 2 g m−3 for 11 days and reduced ammonia loss by ~90%. All urease inhibitor treatments in combination with algicide and / or nitrification inhibitor significantly (p < 0.05) increased the recovery of applied N by the plant. Addition of NBPTO or CHPT in combination with phenylacetylene and algicide resulted in a 2 or 3 fold increase of applied N in the grain, and significantly (p < 0.05) increased grain yield.

Agronomic effects of urease and nitrification inhibitors on ammonia volatilization and nitrogen utilization in a dryland farming system: Field and laboratory investigation (ournal of Cleaner Production,Available online 21 January 2017)

Abstract : Reduction of ammonia (NH3) emission in agricultural systems is highly important from agronomic, ecologic, and economic standpoints. We studied NH3 volatilization from surface applied urea and SuperU® (urea containing urease and nitrification inhibitors) and its interaction with tillage system (conventional tillage and no-tillage) in a dryland arable soil. Agronomic efficiency of the fertilizers was also assessed by measuring yield, agronomic N-efficiency, and apparent N-use of camelina (Camelina sativa L.). Results indicated significant variation between the field and laboratory experiments. Due to wet spell around the time of fertilization, cumulative volatilization was significantly lower in the field study. NH3 emission associated with SuperU in the field study was about 45% lower than regular urea, which was attributed to a) activity of urease inhibitor delayed SuperU hydrolysis for about five days and b) a rainfall occurred four days after fertilization that likely carried more fertilizer into the soil from SuperU than urea. In the incubation study, NH3 loss was notable and superiority of SuperU was not seen. The results suggest that just replacing urea with SuperU would not result in mitigation of NH3 emission, but it provides a greater chance for N to enter the soil by one way or another (rainfall in our study). Our results did not show any appreciable interaction between N fertilizers and tillage systems. Application of SuperU in the first year of the study resulted in a significantly higher camelina yield and agronomic N-efficiency, but in the second year, due to unfavorable environmental condition, camelina did not grow well thus did not respond positively to N fertilization. The superiority of SuperU over urea in terms of NH3 volatilization and agronomic efficiency may be achieved if farmers apply the fertilizer timely in accordance with weather conditions.

Some PDFs enclosed for further reading ...

I think N-(n-butyl) thiophosphoric triamide (NBPT) , Trade name, "Agrotain" is an excellent urease inhibitor originate biologically. Beside this, you can also try with Phenyl phosphorodiamidate (PPD), which also tested a very good urease inhibitor for reducing ammonia volatilization.

If 'biological urease inhibitors' means compounds of a natural origin, consider allicin, the garlic active substance. It is reported to be a non-competitive urease inhibitor with a thiol-dependent mode of action [Juszkiewicz A. et al. // J. Enzyme Inhhib. Med. Chem., 2003. - N 5. - P. 419-424.]. The last but not the least: the ability to penetrate through a cell membrane readily seems to be necessary to act on ammonia-producing microflora in soils.

Bundle of thanks to Dr. Srivastava, Dr. Tarafdar, and Dr. Ivnitsky for valuable information and suggestions.

I am of the opinion that neem (Azadirachta indica L.) oil could be the best organic based urease inhibitor and is proven in various studies.