Ammonia volatilization after application of urea to winter wheat over 3 years affected by novel urease and nitrification inhibitors

• Fully replicated NH3 loss trials under practical canopy and management conditions.
• First field investigation on effects of two novel inhibitors on NH3 emissions.
• Highly variable urea derived NH3 emissions deviating from a single emission factor.
• Reduction of NH3 emissions by urease inhibitor to level of calcium ammonium nitrate.
• No unequivocal effect of nitrification inhibitor on NH3 loss under study conditions.

Ammonia emission from urea application negatively affects both environmental quality and human health, and so it is desirable to minimize nitrogen loss by ammonia volatilization and to improve nitrogen use efficiency. This field study aimed to assess the effects of recently introduced urease (N-(2-nitrophenyl) phosphoric triamide, 2-NPT) and nitrification inhibitors (mixture of dicyandiamide and 1H-1,2,4-triazol) on NH3 emissions following urea application as compared to calcium ammonium nitrate (CAN) in Northern Germany. The measurements were carried out in winter wheat (Triticum aestivum) in the years 2011–2013 covering in total 12 urea application dates. Urea was applied as unamended granulated urea, or combined with urease or nitrification inhibitor or with both inhibitors. Fertilizers were applied in multi-plot field trials with four replications and ammonia losses were measured simultaneously by a combination of a calibrated dynamic chamber and passive samplers. Application date strongly affected relative NH3 loss (% of applied N) due to seasonal variation of soil moisture, temperature, and rainfall. Initial soil moisture showed a strong effect on NH3 emission. Averaged over the three vegetation periods, relative NH3 losses from unamended urea amounted to 8%, with mean emissions of 5%, 4%, and 17% for split N applications in March, April, and early June, respectively. Compared with treatment without urease inhibitor, the urease inhibitor addition reduced emissions by 26–83%, resulting in emissions similar to that from CAN. Analyzing the total data set, no significant effect of the nitrification inhibitor on NH3 emission was observed while at specific applications significantly higher as well as lower emissions compared to unamended urea were detected. The results highlight that NH3 emissions after field application of urea are highly variable under north German climate conditions and simple emission factors should be reevaluated. Urease inhibitor and appropriate application timing are effective measures to reduce NH3 emission from field applied urea.