Nitrogen fertilizers contribute to the major share of fertilizer costs, especially in corn production. At present, nitrogen use efficiency (NUE) of the applied N-fertilizers is often below 40% and has great scope to improve it.
The NUE tends to be even lower in the hot and humid climates such as Louisiana where the conditions are congenial for nitrogen losses. Soil nitrogen can be lost through volatilization, gas emissions (denitrification process), leaching and runoff.
Astronomical increase in the N-fertilizer prices in the last year makes it more critical to improve NUE of high nitrogen using crops such as corn.
Use of N-stabilizer compounds is one of the best management strategies to improve NUE and potentially minimize fertilizer costs. N-stabilizer compounds are generally grouped into urease inhibitors and nitrification inhibitors.
Urease inhibitors delay the urea hydrolysis that releases ammonium ions into the soil. Even though soils with a high cation exchange capacity can absorb ammonium ions, they can be lost to the atmosphere through volatilization, especially in alkaline soils. Nitrification inhibitors prevent the oxidation of ammonium ions to nitrite and nitrate that are relatively more prone to losses.
Slowing down the urea hydrolysis and oxidation of ammonium ions can minimize nitrogen losses, increase the duration of nitrogen availability, and could provide agronomic and environmental benefits. However, the benefits of N-stabilizers vary appreciably depending on N-stabilizer type, whether nitrogen fertilizer is surface-applied or soil-injected, soil pH, time of application, climate, and interval between nitrogen fertilization and rainfall or irrigation.
Many studies were conducted by various scientists throughout the US to evaluate the benefit of N-stabilizer compounds in improving corn NUE and ultimately improving grain yields. However, the findings were not consistent with some studies finding improved grain yields and others finding no yield improvement.
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Multiple studies conducted by LSU AgCenter scientists at the Red River, Northeast and Central Research stations showed benefits of N-stabilizers are significant when used for surface applied of N-fertilizers compared to the soil injected nitrogen fertilizers. Nitrogen fertilizers could be surface-applied because of granular fertilizer use after the germination of crop or UAN applied using Y-drops or applied at pre-tassel.
When the nitrogen fertilizers are surface applied, they lose appreciable amount of nitrogen through NH3 volatilization and nitrogen gas emissions. A two-year study conducted at St. Joseph and Bossier City in 2019 to 2020 showed that application of N-stabilizer (NBPT, a urease inhibitor) coated urea compared to uncoated urea improved corn grain yield up to 23% under a single surface band application at 4-5 leaf stage (Fig.1).
The longer the gap between the surface application of N-fertilization and a rain or irrigation event, the higher the benefit of N-stabilizer use. The same study showed that urea coated with N-stabilizer (urease inhibitor) had 86% lower nitrogen loss through volatilization than uncoated urea in the initial 5-days before a rain event as well as improved leaf nitrogen content by 15%.
In another study conducted in Red River (Latanier clay with pH 7.4) and Mississippi alluvial soils (Sharky clay with pH 6.01) from 2018 to 2019, we evaluated the benefit of different N-stabilizers with soil injected UAN (knifed in) applied at 120 or 240 lbs N/ac rate. In the Red River alluvial soil, UAN plus N-stabilizer increased grain yields in both years by up to 14% (Fig. 2).
However, in 2018, yield improvement was observed only at 120 lb N/ac rate while in 2019, a yield improvement was observed for both rates. This could be due to poor crop growth during drier conditions in 2018, thus eliminating any benefit of minimized N-losses.
On the other hand, in the Mississippi alluvial soil, grain yield was increased only in 2018 at the lower N-rate. In the Red River alluvial soils, alkaline soil pH (above 7) is more congenial for nitrogen loss through volatilization; hence, these soils could benefit from the use of N-stabilizer with urease inhibitor that slows down the release of ammonium ion from urea.
Generally, there is a potential benefit from the use of N-stabilizers in Louisiana even with the soil injected UAN application, however the benefits could vary depending on the given year’s weather conditions. These findings are different from many studies conducted in northern parts of the U.S., where they did not see any yield benefit from the use of N-stabilizers with soil injected UAN.
Potential yield gain observed in our studies is attributable to Louisiana’s hot and humid climate that is more congenial for N-losses. Further, application of high rates of nitrogen than the crop can use takes away the benefits of N-stabilizers. So, if N-stabilizers are used, decreasing the planned fertilizer application rate by 5 to 10% is desirable.
Overall, use of N-stabilizer benefits the most under the following conditions:
- Nitrogen fertilizers are surface applied.
- Nitrogen fertilizers are applied late in the season, such as pre-tassel application, when the temperatures are high.
- The soil is light textured such as very fine sandy loams. Use of N-stabilizers containing both urease and nitrification inhibitors may provide the most benefit.