Enhancing efficient use of nitrogen fertilizer
Options for combatting N loss from volatilization, leaching and denitrification.
Figure 1: The fate of urea fertilizer after application may include volatilization, denitrification or leaching losses before crops have an opportunity to take up nitrate nitrogen. Image courtesy of Ross H. McKenzie.
Prairie farmers primarily use urea (46-0-0), anhydrous ammonia (82-0-0), or liquid urea-ammonium nitrate (UAN) (28-0-0) as their nitrogen (N) fertilizer sources. Nitrogen fertilizer can be lost due to volatilization, denitrification or leaching, depending on how the N is applied and the weather conditions after application.
In recent years, several different types of controlled-release N fertilizers have been developed to reduce losses by protecting or delaying the release of N. The two primary goals of these controlled-release fertilizers are to minimize N fertilizer loss and to control N release to try to better match N availability with crop uptake requirements.
Mechanisms of N loss
First, it is important to understand the mechanisms that cause N fertilizer loss. Figure 1 shows the potential pathways urea fertilizer can take after application.
When urea fertilizer is broadcast applied onto the soil surface or shallow banded (less than five centimetres deep), it can be subject to significant volatilization when surface soil or air temperatures are greater than 3 to 5 C. Urease is an enzyme that occurs naturally in soil and catalyzes the breakdown of bonds in urea [CO(NH2)2] fertilizer to form carbon dioxide (CO2) and ammonia (NH3). Volatilization occurs when moisture activates the urease enzyme on or near the soil surface, converting the urea to gaseous ammonia that is lost to the atmosphere. Significant N loss to the atmosphere may occur depending on environmental conditions. UAN fertilizer, a liquid blend of urea and ammonium nitrate, is subject to the same potential volatilization losses as granular urea when sprayed or dribble banded onto the soil surface or when shallow banded.
Normally, when urea is banded or broadcast-incorporated into warm, moist soil, the urea converts to gaseous ammonia and then rapidly attaches to hydrogen (H+) ions found in water to convert to ammonium (NH4+). Ammonium is quite stable in soil, as it is positively charged and normally will not leach.
Nitrate is the primary form of N that is taken up by cultivated crops. Typically, when urea fertilizer is banded into a warm, moist soil, soil bacteria including Nitrosomonas and Nitrobacter will convert the majority of the urea to nitrate (NO3-) within about three weeks through a process called nitrification. However, the conversion time is very dependent on soil temperature, moisture and other soil factors.
Once the N is in nitrate form, it is subject to leaching losses because nitrate is negatively charged and cannot be held on the negatively charged soil exchange complex. Leaching occurs when excess water from precipitation or irrigation moves the soluble nitrate downward in the soil profile, below the crop rooting depth. Coarse-textured sandy soils are most prone to leaching losses of nitrate. Soil clay particles and organic matter can only hold positively charged ions such as NH4+; they cannot hold negatively charged ions such as NO3-, which are subject to leaching under excess moisture conditions.
Denitrification of nitrate occurs most often in medium- to fine-textured soils (loam to clay loam) when soil pores, particularly the soil micropores, are filled with water. The result is a lack of oxygen (O2) in the soil for microbes. Under very wet soil conditions, anaerobic bacteria strip the oxygen from nitrate, converting it to nitrite (NO2) and eventually gaseous nitrous oxide (N2O), which is then lost from the soil to the atmosphere. Nitrous oxide is a very harmful greenhouse gas with significant global warming potential. Nitrous oxide can trap almost 300 times more heat in the atmosphere than carbon dioxide.
Soil nitrate is mostly taken up by actively growing crops, but it is also taken up by soil microbes, which require N to grow and to breakdown crop residue material. A handful of healthy topsoil can contain five to seven billion microbes. When fields are under-fertilized with N, soil micro-organisms compete with crops for nitrate. When microbes tie up soil N, it is called immobilization. This is not a loss of N from soil, however the soil N becomes unavailable for crop uptake nonetheless. As microbes die and decay, the N is released and recycled back into the soil.
It is important to develop a good understanding of when and how N may be lost from your fields. Understanding N dynamics in soil helps you to understand which products or practices might be best to manage N losses and improve N fertilizer efficiency on your farm.
The next step is to become familiar with controlled-release N fertilizer products and how they function to reduce N losses. The three main types of products available in Western Canada to control N release are: polymer-coated urea, urease inhibitors and nitrification inhibitors.
Polymer-coated urea: Urea coated with a synthetic polymer is an excellent slow-release N fertilizer called Environmentally Smart Nitrogen – or ESN – (45-0-0). When banded or broadcast into soil, water in the soil diffuses through the micro-thin polymer coating, dissolves the urea pellet, allowing the urea to slowly diffuse out into the surrounding soil. Under typical soil moisture and temperature conditions, it takes about 60 days for full release to take place; about half of the N is released at roughly 30 days. Once the urea has moved into the surrounding soil, it is converted to ammonium and then nitrate. Some websites suggest the polymer coating must breakdown to release the urea, however this information is not correct.
One advantage of ESN’s slow-release characteristic is that it allows for as much as three to four times the rate of ESN to be seed-placed compared to urea. ESN works very well in banded or broadcast-incorporated conditions in late fall or spring. It also works well to protect and slow N fertilizer release, reducing potential leaching and volatilization in a wide range of crops.
Urease inhibitors: Urease inhibitors stop the urease enzyme in soil from catalyzing the breakdown of urea. Inhibitors are effective when urea is broadcast onto the soil surface or shallow banded into soil (less than five centimetres deep). As mentioned earlier, soil moisture causes unprotected urea to hydrolyze and convert to ammonia, which may then be lost through volatilization. Urease inhibitors protect urea when placed on or near the soil surface. The urea will not breakdown as quickly, allowing time for rainfall to move it into the soil.
Agrotain is one example of a urease inhibitor that can be used to protect granular urea or liquid UAN. The active chemical in Agrotain is N-(n-butyl)-thiophosphoric triamide (NBPT). Agrotain generally protects urea from volatilization for 10 to 14 days after fertilizer application. The length of this protection period is affected by soil temperature and moisture conditions.
Urease inhibitors are useful in no-till or reduced tillage systems when surface application of urea is necessary or when urea is broadcast onto forage or winter crops in spring. It also allows flexibility for broadcast application timing. It is important to note that when there are limited conditions for volatilization, urease inhibitors have limited value.
Nitrification inhibitors: Nitrification inhibitors prevent Nitrosomonas bacteria from starting the conversion of ammonium to nitrate nitrogen. Inhibiting the Nitrosomonas bacteria slows the conversion of ammonium to nitrate.
N-Serve is a nitrification inhibitor used with anhydrous ammonia (82-0-0) and eNtrench is an example of a nitrification inhibitor used with urea. The active chemical in both products is nitrapyrin [2-chloro-6-(trichloromethyl)-pyridine]. Instinct II is also a nitrification inhibitor used with urea or UAN and the active chemical is dicyandiamide (DCD). The length of the protection period is not clearly stated in promotional material, but should be at least two weeks and up to four weeks. Length of protection will be affected by soil pH, temperature and soil moisture conditions.
Nitrification inhibitors offer the greatest value when potential nitrate losses are high, either from leaching or denitrification. When field conditions are wet in spring or soils are poorly drained, nitrification inhibitors should be considered. They are also useful when applying N fertilizer in the fall, in no-till or reduced tillage systems in wetter regions of the Prairies and when leaching potential on tile-drained soils is high.
Urease + nitrification inhibitors: Some products provide multiple protections by inhibiting both the urease enzyme and inhibiting nitrification. An example of this type of product is SuperU, which contains NBPT and DCD.
Before using any bio-inhibitor product, always read and follow the label instructions to ensure the product is correctly used and only with the crops for which it is registered.
When a nitrogen control product is needed
All products mentioned here work when the specific conditions for N losses occur. However, when conditions for loss are not present, there may not be any crop yield benefit to using a nitrogen control product. Farmers must look at their potential risk of N loss: How frequently do you have conditions that result in significant N loss from volatilization, leaching or denitrification? The higher the potential risk, the more serious the consideration that should be given to using a product that protects N fertilizer from being lost. Look at the cost of the N fertilizer you use, then the cost of the various control products. For example, if urea is your N fertilizer of choice, know the cost per tonne of urea, then determine the cost of the control product you are considering per tonne. Calculate the difference in cost per acre between the urea alone and the controlled urea product, then look at your risk of N loss to decide if the additional per acre cost warrants the benefit of using a controlled-release N product.
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