Managing for Better Nitrogen Use Efficiency

• Keeping adequate nitrogen (N) available to the crop all season long involves a systems approach.
• Minimize N loss with optimal N timing, placement and protection with nitrogen stabilizers.
• Urease inhibitors protect against volatilization from surface-applied N.
• Nitrification inhibitors protect against denitrification and leaching by reducing the amount of N in the nitrate (NO₃^-) form.

Environmental nitrogen (N) involves a complex cycle that influences plant availability and susceptibility for loss. The goal of nitrogen fertility in corn is to keep adequate nitrogen available to the plant for season-long uptake and utilization.

Nitrogen Use Efficiency

Improving nitrogen fertilizer use efficiency can be accomplished through the 4R Nutrient Stewardship (right rate, right place, right time and right fertilizer source) approach. Applying the correct amount of N based on the environment and yield goals, placing N near the crop rooting zone, timing N applications to crop uptake and using the appropriate N source to minimize N loss is key to optimizing N availability (Table 1).

Peak N uptake in corn occurs between the V8-VT/R1 growth stages. During this time, corn takes up 7 lbs. of N per acre per day for 21 straight days (Figure 1). Most of the total N amount should be applied during or just prior to this timing. Utilizing slow-release forms of N can help supplement N later in the growing season while minimizing the risk of N loss.

Sources of N Loss

1. Denitrification

• When soils become saturated, bacteria convert nitrate (NO₃^-) into nitric oxide (NO), nitrous oxide (N₂O), or dinitrogen gas (N₂), which is lost to the atmosphere.
• Conditions conducive to N loss through denitrification include high soil temperature and an increased number of days with saturated soils.
• Ways to avoid denitrification include applying N closer to crop uptake, reducing the amount of N in the nitrate form (NO₃^-), and using a nitrification inhibitor.

2. Leaching

• The nitrate (NO₃^-) form of nitrogen is negatively charged and therefore does not attract to negatively charged soils, allowing it to move freely with water through the soil profile and potentially be lost through tile lines.
• Conditions conducive to N loss through leaching include coarse soils (sands), tile drainage and heavy rainfall.
• Ways to avoid leaching include applying N closer to crop uptake, reducing the amount of N in the nitrate form (NO₃^-) and using a nitrification inhibitor.

3. Volatilization

• When urea-containing N fertilizers are not incorporated by rain or tillage, the urea portion can volatilize into the atmosphere as ammonia gas (NH₃).
• Conditions conducive to N volatilization include moist soil, high relative humidity, high soil pH (>7.0), high soil temperature (>70°F) or frozen soil, crop residue, low cation exchange capacity and poorly buffered soils.
• Ways to avoid volatilization include N fertilizer incorporation (rainfall or tillage), banding UAN fertilizer compared to broadcast and using a urease inhibitor to slow the process of urea hydrolysis.

Using Nitrogen Stabilizers to Manage Loss

The inability to control environment and weather most often limits our ability to control nitrogen loss. Under ideal conditions, nitrogen loss can be insignificant. Depending upon the form of nitrogen applied, two different types of nitrogen stabilizers can be used to offset risk of environmentally driven nitrogen loss

1. Urease Inhibitors

Urea-containing nitrogen fertilizers must first go through a natural chemical process to convert to the plant-available form, ammonium (NH₄⁺). During this two-step process, urea is first hydrolyzed to ammonia gas (NH³), which is subject to loss through volatilization if applied on the soil surface and not incorporated with tillage or rainfall. Urease inhibitors work by slowing the activity of naturally occurring urease enzymes that are part of the hydrolysis process converting urea to NH₃. Slowing this process increases the opportunity time for a rainfall event to incorporate the fertilizer into the soil before significant N loss can occur (Figure 2). Some of the more common urease inhibitor product names and active ingredients are shown in Table 2.

2. Nitrification Inhibitors

In the soil, ammonium (NH₄⁺) naturally converts to nitrate (NO₃^-) through a process called nitrification. Nitrate is subject to loss through leaching. Minimizing the nitrification process can reduce the potential for N loss. Nitrosomonas and Nitrobacter are two naturally occurring bacteria where the nitrification process takes place. Nitrification inhibitors work by temporarily reducing the population of Nitrosomonas and Nitrobacter bacteria in the soil and/or blocking binding sites on the enzymes within the bacteria where the reaction takes place (Figure 3).

Nitrification inhibitors help keep nitrogen in the NH₄⁺ form longer, reducing risk of leaching or denitrification. Some of the more common nitrification inhibitor product names and ingredients are shown in Table 2.

Summary

Research has shown nitrogen stabilizers, both urease and nitrification inhibitors, to be effective on reducing N loss. However, if conditions are not conducive to the type of N loss they protect against, a yield response to nitrogen stabilizers is unlikely. In addition, if N is overapplied and conditions are conducive to N loss, there may still be sufficient N available when N is not the limiting factor. It is important to use the correct nitrogen stabilizer for the potential source of loss. A urease inhibitor will not protect against NO₃^- leaching. Similarly, a nitrification inhibitor will not prevent volatilization loss from surface-applied urea. Understanding a grower’s nitrogen program, environment and weather forecast is key to selecting the appropriate nitrogen stabilizer to protect against potential loss and maintain adequate N availability.