Measuring Volatilization Losses from Nitrogen Fertilizer Applications

Ammonia (NH₃ ) volatilization is a gaseous loss pathway that occurs when urea-containing fertilizers are used, especially when surface-applied. This protocol provides all the necessary information to use the dositube method for measuring ammonia volatilization losses from nitrogen (N) fertilizer applications.

At the simplest level, this method involves placing a round chamber with a dositube inside of it on a representative part of the field after N application (Figure 1). The chamber must be checked at least every 3 days to record the dositube value to get an accurate measurement of N loss. The change in dositube values over time, combined with wind speed data, is used to calculate the amount of NH₃ lost. The dositube method was developed by John Lauzon and his research group at the University of Guelph to measure NH₃ emissions from manure (Van Andel et al., 2017), but it can be readily used for measuring NH₃ emissions from fertilizer.

Dositubes are a passive gas detection devices that measure the concentration of specific gases, such as ammonia. Commonly used in industrial settings, they operate based on a colorimetric principle where ammonia in the air reacts with a reagent (sulfuric acid) inside the tube, causing a color change from purple to yellow. This application uses the GasTec 3D dositube manufactured by GasTec Corporation, based in Fukayanaka Japan. Once the reagent is used up by the reaction, the dositube will need to be replaced with a new one. Expect to use 5-10 dositubes over a 21-day measurement period, depending on the amount of NH₃ loss.

Chambers for each dositube are required. A chamber is a white 4-gallon (15.4 L) bucket with an opening diameter of 10.8 inches (27.5 cm). Drill eight ½-inch holes at the bottom of the chamber and eight ½-inch holes about 1 inch from the lip of the chamber. These holes should be equally spaced. The holes act as chimneys for the chamber. With a plastic bucket, a standard ½-inch drill bit can be used. An example is shown in Figure 2. It is critical that the chamber used is as close as possible to the specified design. Altering the design of the chamber will result in large changes in measured dositube values (e.g. using different size chambers or changing hole configuration).

Wooden stakes and rubber bands keep the dositube centered in the middle of the chamber at a height 6 inches above the soil surface. Standard 12-inch wooden plot stakes can be used. Dositubes should be attached to the stake parallel to the soil surface, with the open tip slightly lower than the closed tip. This ensures that any moisture runs off the dositube (Figure 3).

An anemometer (wind speed recorder) is technically required as wind speed is needed to convert dositube readings to NH₃ loss in kg-N/ha, but workarounds using average estimated wind speed values can be used if an anemometer is not available. The anemometer should be installed at the height of the chamber  in a representative area of the field.

How many chambers are needed? We recommend at least four dositube chambers for every N fertilizer treatment being measured. For example, eight chambers would be needed to compare UAN with and without an N stabilizer. Comparing three different N fertilizer treatments would require 12 chambers.

When should the chambers be installed in the field? Installation should occur as soon as possible after the N fertilizer application, as volatilization is normally greatest in the first few days after application.

Field installation: In the field, break off the dositube end (Figure 3), attach the dositube to a stake, and push the stake securely into the ground. The dositube should be roughly 6 inches above the soil surface. To ensure consistency, draw two horizontal lines on each stake: one to indicate how deep to push the stake and the other to indicate where to attach the dositube. Then, place a chamber directly over the dositube. No plants should be under the chamber, just bare soil. The chamber should be pressed into the soil to make sure there are no large gaps between the chamber lip and the soil (Figure 4). It may be necessary to cut away some plants depending on crop row spacing.

Chamber Placement: Place the chamber in a representative area to measure NH₃ loss appropriately. For broadcast urea, the chamber can be placed anywhere between the crop rows. For injected or surface-banded N, place the chamber directly over the band so that the band bisects the chamber in the middle (Figure 5). Cut away plants if necessary to accomplish this.

The dositube measures NH₃ loss in ppm/hr which stands for ‘parts per million per hour’. The reading is equal to where the line is between the yellow and purple color (Figure 6). While easy to read, note that dositubes use the log scale, so the distance on the tube between 100 and 200 ppm/hr is much greater than the distance between 300 and 400 ppm/hr.

Ideally, data should be recorded from each chamber daily (or every two days) for at least 14 days and up to 28 days after N application. However, visiting the chamber every day may not be feasible. Based on our data, the measurement interval for recording dositube measurements can be stretched to every three days without a large loss of accuracy. The more N loss there is, the more important it will be to record dositube readings frequently to get an accurate measurement.

If there is a heavy rain, move the chamber and dositube to a nearby spot to ensure soil conditions are representative. The chamber acts like an umbrella, keeping the soil beneath it dry. This is not representative of real-world conditions, so the chamber needs to be moved after every rainfall for accurate measurements.

Once the dositube reaches around 350 ppm/hr, we recommend that you replace it with a new dositube. This improves accuracy, especially if your measurement interval is greater than 2 days.  Be sure to note the day the dositube is replaced. Depending on the severity of N loss, a 3-week measurement period will require 5 to 10 dositubes per chamber.

Record the average daily wind speed at the height of the bucket. If an anemometer is not available, there are a couple of options for estimating wind speed. If a nearby weather station measures wind speed, those data can be used. Weather stations typically measure wind speed 2 meters above the soil surface, which isnot representative of wind speed conditions closer to the soil surface. Wind speed is reduced the closer you are to the soil surface. Correction factors are available to convert wind speeds measured at one height to a different one. But once canopy closure is reached (when corn leaves cover up the inter-row area), wind speed data from local weather stations will not be representative. If you have no nearby wind speed data source, our research suggests that using average wind speed values as found in Table 1 do not result in major accuracy loss. You can use a daily average wind speed value of 0.88 m s-1 before canopy closure and 0.39 m s-1 after canopy closure if you do not have access to actual wind speed data.

Table 1. Average daily wind speeds in May, June and July at 30 cm above the soil in corn fields (m/s) in at select locations in Ontario (Winchester, Elora, Ridgetown) and in the US (Windfall, IN and Macomb, IL). Data was collected over three seasons (2021-2023).

The dositube method relies on the change in ppm/hr between two consecutive measurements. If the dositube reading is unchanged between two readings, then no NH₃ was lost. Change in the measurements between dates indicates N was lost as NH₃.

To convert the change in ppm/hr to a change in kg-N/ha of NH₃ loss, there are two steps. First, use a validated equation that uses the change in ppm/hr between measurements, as well as the average wind speed during the measurement interval, to convert ppm/hr to kg-N/ha. Next, use a scaling factor, based on the way N was applied, to properly scale the conversion of N loss measured from a round chamber to an area (hectare) basis. This second step helps account for things like skip-row applications where N is applied every second row. The equation and scaling factors are found below.

Equation:

Where ppm/hr equals the change in dositube readings (in ppm/ hr) between two measurements and wind equals average wind speed (in meters per second), and scaling factor is a number that depends on method of application.

Table 2. List of scaling factor values for the conversion equation, depended on the method of N application.

If N is applied skip-row, divide scaling factor by two.

• ¹Jongwon Kang, M.S, Ph.D. Student, Department of Plant Agriculture; University of Guelph, Guelph, Ontario, Canada
• ²Joshua Nasielski, Ph.D., Assistant Professor, Department of Plant Agriculture; University of Guelph, Guelph, Ontario, Canada
• ³Jason DeBruin, Ph.D., Global Biotech Trait Characterization Project Scientist; Corteva Agriscience, Johnston, Iowa, USA
• ⁴Rebecca Hensley, M.S., Sr. Research Associate; Corteva Agriscience, Johnston, Iowa, USA
• ⁵David Hooker, Ph.D.,, Associate Professor, Department of Plant Agriculture; University of Guelph, Guelph, Ontario, Canada
• ⁶Craig Drury, Ph.D., Soil Biochemist; Agriculture and Agri-Food Canada
• ⁷John Lauzon, Ph.D. Associate Professor, School of Environmental Sciences; University of Guelph, Guelph, Ontario, Canada