Urease Inhibitors

Nitrification inhibitors are chemicals that slow down one particular step in the natural nitrogen cycle that happens in soil. To understand what they do and why they matter, it's helpful to first know a bit about how nitrogen from fertilizers behaves in soil.

Most farmers apply nitrogen-containing fertilizers to help their crops grow. The most common form of nitrogen fertilizer is ammonium (NH4+) or urea, which quickly converts to ammonium once in the soil. Plants can take up some nitrogen in this form, but there is a natural process called nitrification carried out by specific soil bacteria that transforms ammonium into nitrate (NO3-). This process happens in two major steps: first, ammonium is converted into nitrite (NO2-) by bacteria called Nitrosomonas, and then another group of bacteria called Nitrobacter convert nitrite into nitrate.

While nitrate is also a form that plants can readily absorb, it has disadvantages. Nitrate is water-soluble and moves easily with water through soil. If heavy rain or over-irrigation happens before plants use the nitrate, it can wash out of the soil in a process called leaching. This means valuable fertilizer is lost before the crop can benefit. Worse, the nitrate that escapes the field often ends up in rivers, lakes, and groundwater, where it can cause water pollution, algal blooms, and drinking water concerns.

Another problem arises in wet or compacted soils where instead of being used by plants, nitrate can be transformed by soil bacteria into nitrous oxide (N2O), a potent greenhouse gas, through a process called denitrification. Nitrous oxide contributes to climate change and is much better at trapping heat in the atmosphere than carbon dioxide.

This is where nitrification inhibitors come into play. These are substances, usually added to fertilizers or applied to soil, that slow down the activity of the Nitrosomonas bacteria. Specifically, they block the first step in the nitrification process: the conversion of ammonium to nitrite. When this process is slowed, ammonium remains in the soil for a longer period, and the buildup of nitrate is delayed.

There are several commonly used nitrification inhibitors. One is nitrapyrin, another is DCD (dicyandiamide), and a third is DMPP (3,4-dimethylpyrazole phosphate). These chemicals do not kill soil bacteria but rather inhibit the specific enzymes needed for ammonium oxidation. This temporary slowdown can last several weeks, depending on the environmental conditions (such as temperature and moisture), after which the bacteria regain their activity and the nitrification process resumes.

The environmental benefits of using nitrification inhibitors are significant. First, by keeping nitrogen in the ammonium form for a longer time, plants have a better chance of taking it up before it is lost. This means that farmers can use their fertilizer more efficiently, getting better crop yields with the same or even lower fertilizer input. Efficient fertilizer use is good for the economy and for global food production.

Second, delaying the buildup and potential loss of nitrate dramatically decreases the risk of water pollution from leaching. Less nitrate in waterways means less risk of algal blooms, which can deplete oxygen levels in water and kill fish. It also helps prevent nitrate contamination of groundwater, which can make drinking water unsafe for humans and animals.

Third, reducing the amount of nitrate in the soil at any given time also lowers the chance for nitrous oxide emissions. Since denitrification (the process by which nitrate is converted into gases like N2O) only happens when there is excess nitrate and low oxygen in the soil, keeping the nitrate pool smaller lessens the likelihood of these greenhouse gases being released. This can help agriculture become more climate-friendly by lowering its greenhouse gas footprint.

It’s also worth noting that keeping nitrogen in the ammonium form has some added agricultural benefits. Ammonium is less likely to move through the soil with water, so it stays where it’s needed. Additionally, some crops prefer ammonium over nitrate, and having a mix of nitrogen forms in the soil can be more supportive of healthy crop growth.

However, the use of nitrification inhibitors is not a cure-all. Their effectiveness depends on careful application: they need to be mixed well with the fertilizer or soil and applied at the right time. Inhibitors are more effective under conditions where nitrification could go rapidly—warm, wet soils—whereas in cold or dry soils, nitrification is naturally slowed anyway. Furthermore, nitrification inhibitors are only part of a suite of good nutrient management options, such as matching fertilizer application to crop needs and timing applications to avoid heavy rain.

To summarize, nitrification inhibitors are valuable tools for managing nitrogen in agriculture. By slowing the conversion of ammonium to nitrate in soil, they help reduce environmental risks like water contamination and greenhouse gas emissions, while also improving the efficiency of fertilizer use. This is a win-win scenario that supports both productive farming and environmental protection, making nitrification inhibitors an important part of sustainable agriculture.