Minimizing Volatilization Loss of Sidedressed Nitrogen in Corn

One of the most challenging aspects of successfully managing nitrogen in corn is the fact that nitrogen from fertilizer can be lost from the soil before the crop is able to take it up. Nitrogen loss is not only a waste of resources, it also can have negative environmental impacts. Effective management practices can help reduce fertilizer N losses. This, in turn, increases the crop’s access to applied N, enabling greater crop uptake and yield.

Ammonia (NH₃) volatilization is a gaseous loss pathway that occurs when urea-containing fertilizers are used, especially when surface-applied. Following application, soil bacteria hydrolyze urea into one carbonic acid and two NH₃ molecules (Figure 1). If this reaction occurs within the soil, the NH₃ reacts with soil water to form NH₄⁺, which is then bound to the soil. If it occurs on the soil surface, the NH₃ can be lost into the air. Under the right conditions, a large fraction of surface-applied N can be lost to volatilization.

Nitrogen stabilizers are products applied or mixed into N fertilizers with the aim of reducing environmental N loss. These products work by slowing the rate of key chemical reactions that can make applied N susceptible to loss. There are two main types of stabilizers on the market.

Urease inhibitors are products that slow the rate of urea hydrolysis, which can reduce volatilization losses. These products contain the active ingredients NBPT and/or NPPT. Common trade names for urease inhibitors include Agrotain®, ANVOL™, and PinnitMax® TG.

Nitrification inhibitors limit the rate of nitrification, which can reduce N losses from leaching and denitrification. Examples include the active ingredient nitrapyrin (Instinct NXTGEN® and N-Serve®) or pronitradine (Centuro®).

Some products contain both urease and nitrification inhibitors, known as ‘dual inhibitors’.

A three-year field study at five locations was conducted to measure the amount of N volatilization (N lost as NH₃) from different N sources, placement, and application timings when N was applied in-season to corn. The goal of this study was to provide management recommendations for increasing N use efficiency and minimizing N losses from in-season applications.

Replicated field studies were conducted in the USA near Adair, IL, and Windfall, IN, and in Canada near Elora, Ridgetown, and Winchester, Ontario over three years (2021-2023). In total, 15 site-years of data were collected. A total N rate application of 160 lbs N/acre was split-applied each year, with 30 lbs/acre N applied at planting followed by the remaining 130 lbs N/ acre applied in-season. In-season treatments consisted of urea or liquid urea ammonium-nitrate (UAN; 50% urea, 25% NH₄⁺, 25% NO₃^-) applied on the surface or injected 1-2 inches below the soil mid-row. Surface applied urea and UAN were either treated with a urease inhibitor (Agrotain^® Dri-Maxx; Koch Agronomic Services) or left untreated (Table 1). These treatments were applied at all locations at the V13 growth stage. At the US locations the same treatment was applied at the V5 growth stage in addition to the V13 growth stage.

Volatilization was measured up to 21 days following application using a method developed by Van Andel et al. (2017). Briefly, an ammonia dositube (#3D, Gastech Corporation; Japan) was placed in every plot and covered with a chamber (Figure 2a background and Figure 2b) that allowed for airflow around the dositube. The dositube passively recorded NH₃ volatilization and was checked daily for seven days post-application, then every two days for the remaining 14 days and replaced when necessary. Hourly wind speed data were collected using an anemometer placed within the crop canopy. Because the soil under the chamber stayed dry after a rainfall, the dositube and chamber were moved to a new area after every rainfall to avoid bias (Figure 2a foreground). Combining wind speed data with dositube measurements allowed us to measure daily NH₃ loss.

UAN Consistently Had Lower NH₃ Losses Compared to Urea

NH₃ losses were consistently lower with UAN than urea (Figure 3, Figure 4, and Figure 5). This was true regardless of whether an inhibitor was applied, whether injected or surface-applied, or when N was applied at V5 or V13 Figure 4 and was consistent at U.S. and Canada locations (Figure 4 and Figure 5). Among all N sources and placements tested, surface-applied urea without an inhibitor always had the greatest NH₃ loss (Figure 3).

NH₃ losses from urea could be incredibly rapid without an inhibitor. At Adair (2023), for example, 38 lbs N/acre was lost one day after application with an additional loss of 37 lbs/acre over the next four days. This represents nearly 58% of the application. (Figure 6). N losses, while not as sizeable as measured at Adair in 2023, occurred at Ridgetown during 2021, 2022, and 2023 following surface application of urea (Figure 7).

UAN N losses greater than 20 lbs N/ acre (3rd Quartile) were uncommon, occuring only three times out of 15 site years for surface applied UAN. The highest volatilization was observed at Ridgetown, ON, with a N loss of 35 lbs N/acre during 2023. This result is not surprising as UAN is 50% urea with the remaining 50% NH₃ and NH₄⁺. The urea component is the primary source of volatilization losses, so we would expect UAN losses to be around half those of urea. However, volatilization losses from UAN in this study were less than the theoretical 50% reduction.

Urease Inhibitors Reduced N Losses From Surface-Applied N, Especially With Urea

Urease inhibitors reduced NH₃ losses, especially during the first 8-13 days after N application. Time-course measurement of NH₃ losses at Ridgetown, ON during 2022 and 2023 indicated inhibitors were most effective for 8-13 days post-application (Figure 7). This is in line with previous research showing up to 14 days of activity for NBPT. Because N loss from untreated UAN was already quite low, the benefit of the urease inhibitor was much less. A good example of this effect occurred at Elora, ON where the addition of the inhibitor to UAN reduced volatilization 65%; however, this resulted in a small 6 lb N/acre reduction in loss.

NH₃ Loss From Urea was Sensitive to Application Timing

Total NH₃ loss was not affected by N timing, except for the surface-applied urea. NH₃ losses from surface-applied urea (with or without an inhibitor) were greater at V13 compared to V5 (Figure 4). Warmer air temperatures at the V13 application timing may have driven larger volatilization losses. Since the breakdown of urea into NH₃ is a biological process, higher air temperatures increase the rate at which it occurs.

Soil Conditions and Rainfall After N Application Determine NH₃ Volatilization

While many factors under the farmer's control influence NH₃ losses, weather and soil properties are major determinants. For example, averaged across all N source and placement treatments, NH₃ loss from 130 lbs N/acre applied at V13 varied from 2.7 to 40.0 lbs N/acre depending on the location and year. Ideal weather conditions, in terms of minimizing NH₃ losses, would be for soil to be dry at application and then for a heavy rainfall to dissolve and move the N fertilizer a couple of inches below the soil surface as soon as possible after application.

When applied to dry soil, NH₃ losses typically remained minimal for days until rainfall occurred. The urea hydrolysis reaction requires water to proceed, so the reaction rate will tend to be limited by dry soil at the surface. Application to moist soil resulted in immediate volatization, specifically for surface-applied urea. Moist soil is defined as having a water content at 10 cm (4") depth greater than 29%. This would represent soil conditions that are borderline acceptable for field traffic. In moist soils, NH₃ losses would remain high until a rainfall event occurred to move the fertilizer N down into the soil.

Regardless of the initial soil moisture, a heavy rainfall would reduce but not eliminate subsequent NH₃ losses after an initial spike. Very large rainfall events (>2 inches/day [>50 mm/day]) were required to reduce NH₃ losses dramatically. Generally, the more rainfall, the less NH₃ loss. As a general rule, receiving 3 inches (76 mm) of rainfall over the 21-day measurement period was enough to limit NH₃ losses to 13 lbs N/acre on average (though losses from untreated urea were always high).

Post-application, a light rainfall (<0.15 inches [<4 mm]) was possibly the worst situation regarding NH₃ losses, especially on dry soil with higher clay content (less sand) (Figure 8). It appears that adding a small amount of moisture, but not enough to drive the downward movement of N into the soil profile, favors volatilization. Additionally, in a very dry year, disrupting the soil to inject the urea could cause more volatilization than leaving the urea on the surface because it exposed the urea to a small amount of soil moisture. For instance, at Adair, IL (2023) N was applied at the V5 growth stage to very dry soils followed by a cumulative rainfall of 0.1 inch (2.5 mm) during the 14-day measurement period. In this scenario the injected urea treatment had the greatest volatilization (26.5 lbs N/acre) compared to any other treatment (e.g. urea broadcast lost 4.9 lbs N/acre). Urease inhibitors demonstrated the most value in conditions where high volatilization losses were expected; applications onto wet soils, or after a light rainfall when applied onto dry soils.

Soil texture was an important feature that modified the interaction of volatilization and rainfall. The coarser the soil texture (i.e., as sand content increased), the less rainfall that was required to reduce NH₃ losses. This can be seen in Figure 7, comparing volatilization losses and rainfall at Winchester (20-26% sand) and Ridgetown (51-60% sand). Coarser soils may allow for more rapid downward movement of dissolved N. NH₃ losses can remain minimal for days after application to dry soil until rainfall (>0.1 inch [>3 mm]) occurs (Figure 8). Heavy rainfall could reduce the N losses after an initial spike at Ridgetown (2021), whereas the highest NH₃ peak was observed after the large rainfall event at Winchester (2022). Additionally, at Adair (2022) where soil contains a similar sand content to Winchester (26% sand), heavy rainfall (>1.7 inches [>43 mm]) triggered the NH₃ losses, resulting in the highest peak. This difference arises because the field capacity at Ridgetown is around 20% of soil volumetric water content (SVWC), while that in Winchester and Adair is around 30% of SVWC. Thus, in Ridgetown, the dissolved N experienced rapid downward movement unlike in Adair or Winchester by heavy rainfall, preventing NH₃ from volatilizing.

N loss via volatilization can be measured in fields following N application using fairly simple tools. The outcomes from this work indicated that urea has a greater risk of N loss via volatilization compared to UAN, regardless of application timing. If urea is to be top-dressed or broadcast and incorporation via tillage is not possible, we recommend urease inhibitors, especially at later application timings where NH₃ losses are expected to be substantial.

We also identify weather patterns that can work for or against you in terms of minimizing NH₃ loss. Ideally, soil is dry when surface-applying N followed by sufficient rainfall (3 inches [76 mm]) to dissolve and drive the N below the soil surface. When N is applied to wet soils or dry soils followed by small rainfall amounts, NH₃ losses will be large.

The authors would like to acknowledge the technical expertise provided by Dr. John Lauzon, Dr. Craig Drury and Dr. David Hooker. This research was supported financially by the Grain Farmers of Ontario, the Ontario Ministry of Food, Agriculture and Rural Affairs - University of Guelph Alliance, and the Natural Sciences and Engineering Research Council of Canada.

• Van Andel, M., J.S. Warland, P.D. Zwar, B.J. Van Heyst, and J.D. Lauzon. 2017. Development of a simple and affordable method of measuring ammonia volatilization for land applied manures. Can. J. Soil Sci. 97:541- 551. doi.org/10.1139/cjss-2016-0103.
• Svane, S., J.J. Sigurdarson, F. Finkenwirth, T. Eitinger, and H. Karring. 2020. Inhibition of urease activity by different compounds provides insight into the modulation and association of bacterial nickel import and ureolysis. Sci. Reports 10:8503. doi.org/10.1038/s41598-020-65107-9. 

¹Jongwon Kang, M.S., Ph.D. Student, Department of Plant Agriculture; University of Guelph, Guelph, Ontario, Canada
²Jason DeBruin, Global Biotech Trait Characterization Project Scientist; Corteva Agriscience, Johnston, Iowa, USA
³Rebecca Hensley, M.S., Sr. Research Associate; Corteva Agriscience, Johnston, Iowa, USA
⁴Joshua Nasielski, Ph.D,. Assistant Professor, Department of Plant Agriculture; University of Guelph, Guelph, Ontario, Canada