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

Montana winter wheat growers broadcast surface apply urea to their crops in late fall, or early spring to supply the crops with needed nitrogen. Urea is known to be susceptible to ammonia (NH3) volatilization losses, no studies have specifically targeted the measurement of these losses from cold soils ( 10°C). This study is being conducted to quantify NH3 emissions from surface applied urea to no till winter wheat; and to evaluate the efficacy of NBPT (N-(nbutyl) thiophosphoric triamide) to mitigate potential ammonia volatilization losses made during colder weather periods.


16 Apr 2014

2013 Annual Interpretive Summary

Urea fertilizer hydrolyzes to release volatile ammonia gas when broadcast on the soil surface under certain moisture and temperature conditions. It had been generally accepted that such losses are accelerated under conditions of a moist soil surface under warmer temperatures in spring and summer, and when additional precipitation (e.g. 0.5 to 1 in. of rain) was not received within a number of days following application. Under Montana weather conditions, significant ammonia losses from urea fertilizer were not expected in late fall, winter, or early spring if soil and air temperatures were 50 °F (10 °C) or lower, and snow cover was minimal (less than 2 in.). This research initiative was designed to measure and compare volatile ammonia losses from surface-applied urea fertilizer and from urea treated with a urease inhibitor under cool late fall, winter and or early spring temperatures.

A micrometeorlogical technique was used that made actual measurements from circular areas 130 ft. in diameter (approx. 40 m) where urea was broadcast without incorporation. This research has generated valuable information showing that there are potential volatile losses of ammonia after late fall, winter, or early spring broadcast applications. Conditions for cold weather loss are similar to conditions for warm weather losses. There is an interaction between surface soil moisture and the length of time to when enough precipitation to dissolve the urea granules and move it into the topsoil is received. Moisture received after urea application is more important than soil temperature. However, if the soil surface remains dry until sufficient rainfall is received, volatile losses are minimal.

In recent studies, ammonia losses have been measured from 10 to 20%, of the urea N applied, occurring over a few-week period. To determine the crop uptake of applied urea-N, 15N-enriched urea fertilizer, with and without NBPT, was applied to microplots, in close proximity to the micro-meteorological measurement sites. The increase in fertilizer N uptake for urea treated with NBPT compared to untreated urea uptake was approximately 10% for late-fall and winter applications in both years. This research is planned to continue for one additional year.