A Micrometeorological Study to Quantify Ammonia Volatilization Losses from Surface Applied Urea in the Semiarid Northern Great Plains

Montana grain growers annually seed over 5 million acres of wheat, primarily winter wheat. Nitrogen is the primary nutrient that limits wheat production on this land. Hence N fertilization is essential for sustaining yields as well as ensuring production of high protein quality grain. To meet this challenge Montana wheat growers apply N fertilizer to their fields. Most frequently this is achieved through broadcast applications of urea-N (46-0-0) to the soil surface with applications occurring between October and early May. Surface urea applications are susceptible to ammonia (NH₃) volatilization losses if not incorporated with tillage or by rainfall. A number of environmental and soil related factors interact together to affect this process and define the magnitude of loss. Research has continued on this project for the fourth year.

There have been sufficient sites where NH₃ losses were measured (approx. 20), with different weather patterns experienced, that it is now possible to better understand what weather and soil conditions result in low, moderate, and higher relative losses of NH_3. The loss amounts and associated weather patterns can be separated into three categories: low (when applied urea loss is <10%), moderate (when applied urea loss varies between 10 and 20%) and high (when applied urea loss is >20%). Associated soil and weather patterns for these categories include urea applied to dry soil surface then large precipitation events (>0.7 in.) following fertilization for the low category, urea applied to dry soil surface followed by light scattered precipitation events (<0.3 in.) for the moderate category, and urea applied to wet or damp soil surfaces followed by slow drying without precipitation, daily soil temperatures cold (-2 to 3˚C) for the high category.

At all of the research sites besides a control or zero N treatment, there has been an application of NBPT urease inhibitor (Agrotain^® at 0.01% by weight) to urea for comparison. Generally use of NBPT has been shown to reduce NH₃ losses by around 50%. Additional laboratory experiments are being conducted to better understand the effect of soil pH on the length of effectiveness of the NBPT. This is being done because of observations made of increased length of effect of NBPT on alkaline soils compared to acidic soils. Laboratory research on this will continue over the next year. MT-17