Montana grain growers annually seed over 5 million acres of wheat with production averaging approximately 15 million bushels (1998-2007). 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 volatilization losses if not incorporated with tillage or by rainfall. This process is well known and is summarized by the reactions below.
A number of environmental and soil related factors interact together to affect this process and define the magnitude of loss. Most research shows that losses are enhanced by high soil pH, warm temperatures, light rains sufficient to moisten the soil surface but insufficient to enable movement of urea below the soil surface, and no till management. No till systems have been reported to enhance volatilization losses of urea because tillage is not present to incorporate urea fertilizer granules into the soil. Also, the urease enzyme necessary for hydrolysis of urea (see reaction above) is particularly active in crop residues that accumulate at the soil surface under no-till.
Although surface-applied 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 losses made during cold weather periods. Ammonia emissions following fertilization are being quantified using an integrated horizontal flux approach at three no till winter wheat farms in northern Montana. This micrometeorological mass-balance approach utilizes circular plots (20 m radius) treated with urea and NBPT-coated urea (100 kg N ha-1), a center mast, and five Leuning samplers per mast arranged in a gradient spacing (0.25, 0.50, 1.00, 1.50, and 2.75 m above the ground). Leuning shuttles are replaced approximately weekly over 8-week gas sampling campaigns following fertilization.