The Association of American Plant Food Control Officials (AAPFCO) defines EEFs as products with characteristics that allow increased plant uptake and therefore reduce potential nutrient losses to the environment (e.g., gaseous losses, leaching, or runoff) when compared to an appropriate reference fertilizer that does not contain additives. When comparing nitrogen EEFs, examples of reference products would be traditional fertilizer such as anhydrous ammonia, granular urea, ammonium sulfate, and urea-ammonium nitrate solutions.
The AAPFCO further breaks down EEFs into two distinct subcategories: (1) stabilized fertilizers and (2) controlled or slow-release fertilizers.
1. Stabilized fertilizers – Products claiming stabilization of nutrients must reduce the transformation rate of fertilizer compound(s), extending the time of nutrient availability to the plant by a variety of mechanisms relative to its unamended form.
2. Controlled or slow-release fertilizers – Products that convert and/or release nutrients that are in the plant-available form at a slower rate relative to a “reference-soluble” product.
When discussing stabilized nitrogen fertilizers, having a basic understanding of the nitrogen cycle is critical. The nitrogen cycle describes the chemical transformations, input processes, loss pathways, and uptake of nitrogen in ecosystems.
Why Are Nitrogen Stabilizers Needed?
The overall nitrogen fertilizer use efficiency in cereal production systems worldwide is estimated to be 33 percent (Raun and Johnson 1999). Nitrogen fertilizer use efficiency is low due to numerous loss pathways that include gaseous losses to the atmosphere via volatilization, as well as denitrification, leaching, and runoff .
In Virginia, all of these loss mechanisms are a distinct possibility in our cropping systems. For example, increased cover crop usage and no-tillage are positive for improving soil structure and increasing organic matter content in agronomic production systems, but they also increase the likelihood of nitrogen loss via ammonia volatilization.
In addition to gaseous losses, water percolating through soil will carry nitrate beyond the root zone and eventually to groundwater through a process called leaching.
The largest row crop production areas of the commonwealth are located on sandy loam soil in the Coastal Plain that has a high propensity for leaching.
Ammonia Volatilization Control With Nitrogen Stabilizers
Nitrogen stabilization products potentially act on two nitrogen transformation processes: mineralization and nitrification.
Mineralization is the process by which nitrogen — in forms unavailable to plants — is converted by microbes into usable forms. These unavailable nitrogen forms include proteins and other chemical compounds from decomposing plant and microbial biomass (such as organic matter). The degradation of these complex nitrogen “organic” molecules results in the formation of ammonium, which is one plant-available form of nitrogen.
Mineralization describes many chemical reactions taking place in the soil environment simultaneously, but of these chemical reactions, urea hydrolysis is the principal reaction targeted by nitrogen stabilizers.
Urea hydrolysis is the conversion of urea — the most common nitrogen fertilizer source worldwide — to ammonium. However, this chemical reaction does not occur without a penalty. The urea hydrolysis reaction produces bicarbonate, which raises soil pH around the reaction zone. The rise in pH results in the transformation of ammonium to ammonia gas, which can be lost to the atmosphere via ammonia volatilization.
Nitrogen stabilizers focus on limiting urea hydrolysis and ammonia volatilization by inhibiting the enzyme urease. When urease is absent, urea hydrolysis proceeds much slower — 10 to 14 times slower — than the catalyzed reaction (Krajewska 2009). Stabilized nitrogen products that claim a reduction in nitrogen loss via ammonia volatilization must slow the transformation of urea to ammonium and the resulting buildup of bicarbonate. This slower conversion allows soluble urea to spread into a larger volume of soil. This in turn, minimizes the pH increase and production of ammonia gas.
Reducing Nitrate Leaching With Nitrogen Stabilizers
Nitrate is a plant-available form of nitrogen, but nitrate is also very mobile in soils. The mobility of nitrate is due to its negative charge; which prevents it from forming bonds with clay minerals and organic matter, also predominantly negatively charged. Think of a magnet: The same poles (charge) will repel one another.
Nitrification is the process of ammonium transforming into nitrate. The nitrification process needs to be mediated by mircroorganisms. In soils, two microorganisms, specifically bacteria, are responsible for driving nitrification. The two bacteria genera are Nitrosomonas spp. and Nitrobacter spp. Each plays a different role in nitrification, with Nitrosomonas spp. responsible for conversion of ammonium to nitrite and Nitrobacter spp. responsible for the conversion of nitrite to nitrate.
Inhibiting nitrification at the right place is critical, and nitrification inhibitors should target Nitrosomonas spp. Accumulation of nitrite in soils could result in nitrite toxicity to plants. Inhibiting Nitrosomonas spp. will maintain nitrogen as plant-available ammonium that is less mobile in soils than nitrate because ammonium has a positive charge that allows it to “stick” to the negatively charged soil particles. Think of a positive side of a magnet (the ammonium) sticking to the negative side of a magnet (the soil particle).
Maintaining nitrogen as ammonium minimizes leaching and groundwater contamination risk while maximizing available nitrogen in agroecosystems. The mode of action for different nitrification inhibitors typically falls into two primary categories: (1) bacteriocide and (2) bacteriostatic activity.
1. Bacteriocides kill Nitrosomonas spp. and are labeled as pesticides.
2. Nitrification inhibitors that have bacteriostatic activity slow the metabolism of targeted species or genera (Nitrosomonas spp.), thereby slowing the transformation of ammonium to nitrite.