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.

IPNI-2008-USA-MT17

17 Oct 2008

Project Description


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.


Justification

Although ammonia volatilization from surface urea has long been acknowledged as an important mechanism of N loss, few micrometeorological studies have ever been conducted to quantify this loss for cropping systems in the semiarid Northern Great Plains. Therefore, this study will provide some of the first data on ammonia losses from urea in this region.


Objectives
The two main objectives of this study are as follows:
  1. Quantify the extent of ammonia loss from top-dress urea applications applied in the early fall, late fall, and early spring to winter wheat using a mass-balance micrometeorological approach referred to as the integrated horizontal flux (IHF) method.
  2. What level of protection is achieved from treating urea with NBPT (Agrotain, at 0.10% w/w).


Methodology

We will evaluate the efficacy of NBPT coated urea to mitigate these losses. Field sites will be established at one field site in 2008, and two field sites in 2009. Passive flux samplers or shuttles treated with oxalic acid will be placed on a mast at five heights (0.25, 0.50, 1.00, 1.50, and 2.50 m) above the soil surface. A mast will be placed in the center of urea and urea +NBPT treated circular plots (20 m radius). Nitrogen will be applied at a rate of 100 kg N ha-1. A third mast will be placed at a location upwind or at least 100 m from the treated area to estimate background emissions. Shuttles will be exchanged at least once weekly, transported to the lab, and the ammonia collected will be eluted with distilled water. The ammonia concentration in the extracts will be measured with an ammonia analyzer (Timberline Instruments, Boulder, CO, USA). Gas sampling campaigns following N applications will continue until the ammonia flux from the urea and urea + NBPT treated areas return to background levels.