Improving nitrogen use efficiency using urease and nitrification inhibitors
In Portugal, spring rainfall is common and irrigation is often not needed until late April. This has led to an interest in applying granular fertilizers to the orchard in the spring until the irrigation season begins. Unfortunately, granular nitrogen applications aren’t very efficient, with as much as 40-50% of the nitrogen lost either through ammonia volatilization or denitrification.
Nitrogen losses within the field can occur multiple ways. These include run-off, leaching, ammonia volatilization, and denitrification. Run-off and leaching are due to the movement of the nitrogen off the field and below the rootzone, respectively. Run-off frequently occurs with water soluble fertilizers like urea and nitrate. Leaching, however, is more common with nitrate due to its negative charge. These losses can be managed by timing fertilizer applications in-season when the risk of large rain events is low. Leaching can also be reduced by timing the application of nitrogen in the middle of fertigation events to reduce the risk of moving the applied fertilizer beyond the rootzone.Ammonia volatilization occurs when urea is converted into ammonium. If this is done in the soil, the ammonium will stick to the negatively charged sites of the soil. If on the soil surface, however, the ammonium will convert into ammonia and volatilize. The risk of loss from ammonia volatilization is less in irrigated settings because the water provided by irrigation typically moves the ammonium deep enough into the soil to prevent losses (2-3”/5-8 cm). Rainfall does the same.Denitrification is the process in which nitrate is converted to nitrous oxide and nitrogen gas. It is common in saturated soils when oxygen isn’t present, and rarely occurs on the soil surface. Nitrous oxide is created when irrigation and fertilizer is applied within an orchard (fertigation). This is due to the transient soil saturation associated with fertigation, and can be reduced through the appropriate timing of fertilization injection as well as shorter irrigation events. These events also improve nitrogen use efficiency as they reduce the risk of nitrate leaching.
Nitrogen losses within the field can occur multiple ways. These include run-off, leaching, ammonia volatilization, and denitrification. Run-off and leaching are due to the movement of the nitrogen off the field and below the rootzone, respectively. Run-off frequently occurs with water soluble fertilizers like urea and nitrate. Leaching, however, is more common with nitrate due to its negative charge. These losses can be managed by timing fertilizer applications in-season when the risk of large rain events is low. Leaching can also be reduced by timing the application of nitrogen in the middle of fertigation events to reduce the risk of moving the applied fertilizer beyond the rootzone.Ammonia volatilization occurs when urea is converted into ammonium. If this is done in the soil, the ammonium will stick to the negatively charged sites of the soil. If on the soil surface, however, the ammonium will convert into ammonia and volatilize. The risk of loss from ammonia volatilization is less in irrigated settings because the water provided by irrigation typically moves the ammonium deep enough into the soil to prevent losses (2-3”/5-8 cm). Rainfall does the same.Denitrification is the process in which nitrate is converted to nitrous oxide and nitrogen gas. It is common in saturated soils when oxygen isn’t present, and rarely occurs on the soil surface. Nitrous oxide is created when irrigation and fertilizer is applied within an orchard (fertigation). This is due to the transient soil saturation associated with fertigation, and can be reduced through the appropriate timing of fertilization injection as well as shorter irrigation events. These events also improve nitrogen use efficiency as they reduce the risk of nitrate leaching.There are tools to help reduce the risk of nitrogen. These include coatings, urease inhibitors, and nitrification inhibitors. To understand the role of each of these, the conversion process from urea to nitrate should be reviewed (figure 1). In this process, urea is released from the fertilizer prill. When it mixes with water, it forms ammonia and carbon dioxide. This process is facilitated with the enzyme urease. Once ammonium, it could form ammonia (NH3) and volatilize if on the surface of the soil, be used by the plant, or converted to nitrate if in the soil. The conversion to nitrate is known as nitrification and is mediated by several different bacteria species. If these bacteria do not have access to oxygen due to saturated soils, they will use energy from the nitrate, reducing it to nitrous oxide and nitrogen gas (denitrification).There are several technologies that impact the nitrogen cycle to improve nitrogen use efficiency. These include controlled release or slow release prills, urease inhibitors, and nitrification inhibitors. Slow release or controlled release formulas do just that – reduce the release of urea into the soil. This reduces exposure of urea to the elements, increasing efficiency. Urease inhibitors block the activity of urease, meaning the urea will remain as urea for a much longer period of time (14-20 days, depending on temperature). Lastly, nitrification inhibitors tend to disrupt or reduce bacterial populations leading to reduced rates of nitrification.The value of coatings, urease, or nitrification inhibitors is impacted by rain. Slow release or controlled release formulations are useful if consistent overhead irrigation or rainfall occurs as they reduce the risk of run-off and leaching. Urease inhibitors provide value when managing inconsistency in rainfall after fertilizer application. Their value is decreased if rainfall occurs shortly after urea application. With nitrification inhibitors, excessive rainfall increases the value as it reduces the conversion of ammonium to nitrate, allowing it to remain attached to the soil particles. Yield responses using these tools would then be dependent on both the frequency and amount of rainfall, but generally all technologies have been shown to increase yields in rainfed cereal crops when comparing to regular fertilizer applications.Within almond orchards, which technology is best? This depends on the farm, weather, and operational goals. Within Portugal, we will integrate urease inhibitors (NBPT) into our fertilizer blends. This technology will allow us to apply an application of granular fertilizer in late February that will provide enough nitrogen to last us until mid-April. Although we don’t know when the rain will fall, we feel confident enough to know that rain will occur within a 2-3 week period at this time. The use of this inhibitor will provide us with a longer window without the risk of losing too much nitrogen through ammonia volatilization, while providing nitrogen to the trees in March and April without the need to irrigate.Both urease and nitrification inhibitors may provide additional value. The use of urease inhibitors allows for any variable rate applications within an orchard. Current irrigation technology prevents us from applying site specific fertilization more prescriptive than the valve level. If granular fertilizers were applied using variable rate technology in the spring, the entire orchard can be brought to a point in which fertigation can complete the remaining fertility needs.Nitrification inhibitors may have a role in reduction of greenhouse gases. Their use can reduce the formation of NOx, which is a potent greenhouse gas – one of the major point sources within agriculture. If carbon markets mature, the inclusion of this product into our fertility programs would be offset by carbon sales, providing us with additional nitrogen use efficiency for free. Nitrification inhibitors, however, can negatively impact certain soil bacterial species, which may disrupt soil health efforts within the orchard.Sources/referenced works:Byrne M., et al. 2020. Urease and nitrification inhibitors- as mitigation tools for greenhouse gas emissions in sustainable dairy systems: a review. Sustainability. 12:6018. doi:10.3390/su12156018Allende-Montalbán, R.; Martín-Lammerding, D.; Delgado, M.d.M.; Porcel, M.A.; Gabriel, J.L. Urease Inhibitors Effects on the Nitrogen Use Efficiency in a Maize Wheat Rotation with or withoutWater Deficit. Agriculture 2021, 11, 684. https://doi.org/ Cantarella, H. R. Otto, J. Rodrigues Soares, and A. Gomes de Brito Silva. 2018. Agronomic efficiency of NBPT as a urease inhibitor: a review. Journal of advances research. 13:19-27. https://doi.org/10.1016/j.jare.2018.05.008. Ferguson, R, B. Maharjan, C. Wortmann, and B. Krienke. 2019. Nitrogen inhibitors for improved fertilizer use efficiency. 2019 Crop Production Clinic Proceedings, Institute of Agriculture and Natural Resources, University of Nebraska.