BASF:The technology behind inhibitors (Part 3)

Can technology provide the key to smarter nitrogen use? Recently three fertilizer experts Dr. Markus Schmid, Dr. Wolfram Zerulla, Dr. Gregor Pasda from BASF received invitation from Agropages to share their professional viewpoints on this question. As mentioned in part 2, urease and nitrification inhibitors can be effective tools in minimizing nitrogen losses and increasing the efficiency of fertilizer applications. They are considered as best management practice, and their use is increasing as more farmers value their benefits.

Part 3: The Technology Behind Inhibitors

The Urease Inhibitor Limus® –The Advantage of Two Active Ingredients

In order to reduce the negative impact of ammonia emissions from urea, the latest technology should be used to enable farmers not only to reduce nitrogen rates but also to simplify nitrogen fertilization in general.

Urea is a high concentrated and cost-efficient fertilizer. The molecule urea itself is hardly taken up by plant roots. It has to be transformed first into the main nitrogen forms for plant nutrition ammonium and nitrate.

During these transformation processes, the alkaline compound ammonia occurs. This increases the pH of the urea granule/prill significantly (see figures 1 and 2). Due to this pH increase, the equilibrium between ammonia (NH3) and ammonium (NH4+) in this chemical reaction shifts to the side of ammonia (red arrow in figure 1). As a gaseous compound, ammonia can easily escape to the atmosphere.

Figure 1: Transformation of urea into plant available nitrogen in the soil

Figure 2: pH increase after urea application

This chemical transformation of urea is catalyzed by the urease enzyme. Urease inhibitors work by blocking the enzyme so that a transformation of urea into ammonia is delayed for a certain period of time. This widens the window of time during which rain can help incorporate urea into the soil so that the transformation of urea ends in ammonium due to cation exchange capacity of the soil.

Managing these emissions are the key to optimizing urea-containing fertilizer. BASF recently developed Limus, a nitrogen management product that inhibits volatilization. Limus combines the efficacy of two active ingredients into a patented formulation that helps reduce nitrogen losses occurring through volatilization.

The presence of two active ingredients in Limus is a significant improvement, because several types of urease enzymes with different structures can be present in soil. A single active ingredient will only work to inhibit activity in some of the urease enzymes. NBPT and NPPT, the two active ingredients in Limus, work in a complementary fashion – providing broader protection by binding to urease enzymes with different active sites.

This dual active innovation was patented by BASF and showed extraordinary performance in lab and field trials in various parts of the world. Research demonstrated that the amount of active ingredient could be reduced by at least 40 percent to get the same performance as conventional products. Alternatively, with the same amount of inhibitor, the biological performance of Limus was significantly improved. In many trials, the relative yield increase could be doubled compared to the market standard NBPT as a result of more effective inhibition of the urease enzymes in the soil.

In cases over-fertilization, which is not rare in China today, the level of nitrogen input can be reduced in without yield impact. This could be demonstrated in field trials very impressively. While using Limus farmers would be able to reduce the amount of urea based fertilizers by at least by 20% and get additionally higher yields compared to common practice (see figure 3).

Figure 3: Field trial results China

Using a urease inhibitor could also have positive implications on expenditures of time, energy, fuel and water use – which is particularly relevant for farmers in areas where water is a scarce resource.

In some regions of the world, farmers irrigate their fields after urea applications to wash the fertilizer into the soil. In other regions, they incorporate urea with high energy input into the topsoil. A urease inhibitor like Limus can add efficiencies to both practices, virtually eliminating the need to employ either practice.

Limus can also enable farmers to reduce the number of application rounds due to minimized ammonia losses. With a better and more constant delivery of nitrogen to crops, applications are more precise and yields maximized.

BASF also researched how Limus could contribute to agriculture from a sustainability point-of-view. It used AgBalance™, an approved tool developed by BASF, to objectively evaluate sustainability measures related to Limus use. The AgBalance review considers the ecological, economic and social impacts and contributions of the product.

The AgBalance study, conducted in China, showed that Limus offered an 8% better sustainable performance and improved nitrogen fertilization. Figure 4 shows the detail: performance regarding greenhouse gas emission. 

Figure 4: Limus AgBalance study – results regarding Greenhous gas emission

Another unique aspect of Limus is its formulation. All urease inhibitors of the class of thio-phosphoric acid triamides are not stable on urea-based fertilizers. They decompose rapidly, particularly under high temperatures, if they are not stabilized by a suitable formulation. It already happened that farmers bought a product that did not contain any more an active ingredient because the coated urea was stored too long at too high temperatures – storing conditions which accelerate the decomposition.

The Limus formulation was developed to provide for a long formulation stability itself and to guarantee especially a long stability of the active ingredients on solid urea based fertilizers (see figure 5). Whereas some formulations not even stabilize the urease inhibitor on urea at lower temperatures Limus keep it on the granule or prill also on high temperatures for at least one year.  

Figure 5. Storage stability of different urease inhibitors on urea

 

Additionally, the Limus formulation allows for the coated urease to quickly dry, with excellent flowability of the coated area and no tendency to become compacted. These are properties that help fertilizer blenders or fertilizer producers to coat, store and transport treated urea without problems, and guarantee a high throughput in their blending facilities.

With the addition of Limus to urea and UAN, the negative properties of these two fertilizers are compensated. They reach the effectiveness of ammonium nitrate without losing their advantages (urea: high nutrient content, cheap production, UAN: mixture with pesticides, easy application). With its two active ingredients and its unique formulation, Limus represents the latest development in urease inhibitors. It will be the benchmark for future developments.

DMPP – Benchmark of Nitrification Inhibitors

Nitrification inhibitors bear on the principal that nitrate leaching could be avoided if the positively charged ammonium could be stabilized and bound to the negatively charged soil particles for a certain period of time. Nitrification inhibitors are appropriate for use with organic fertilizers like liquid manure, ammonium or urea-based fertilizers but not to nitrate fertilizers (urea is seen in this context as an ammonium source).

Nitrification inhibitors influence the metabolism of the bacteria Nitrosomonas ssp. which are responsible for the transformation of ammonium into nitrite (see figure 6).

Figure 6: mode of action of a nitrification inhibitor

DMPP (dimethylpyrazole phosphate) is the most effective nitrification inhibitor on the market. Application rates of 0.5 to 1.5 kg ha-1 are sufficient to stabilize ammonium for 4-10 weeks. These rates won’t kill the bacteria (bactericide), but will reduce their activity (bacteriostatic action) significantly and specifically.

DMPP can significantly reduce nitrate leaching without being liable to leaching itself. In pot trials with a loamy sand, where leaching was provoked by excessive water supply, nitrate losses were reduced to approach that of the control. In these trials, spinach was grown in Mitscherlich pots, fertilized with ASN without or with the nitrification inhibitors DCD or DMPP, with excess of 20 mm water 3 times at 7, 18 and 22 days after fertilizer application (see table 1).

Table 1 NO3-leaching under spinach after application of different nitrification inhibitors (Zerulla et al. 2001)

Similar results were achieved in lysimeter studies at different locations. It could be shown that even under high temperature conditions, nitrate leaching can be reduced significantly (SERNA et al. 2000).

DMPP also clearly reduces N2O emission, without a negative effect on the methane sorption capacity in the soil. Smaller N losses, probably together with temporary ammonium nutrition, frequently leads to increased crop yields where a stabilized fertilizer has been applied. This allows farmers a more flexible fertilizer application timing and the possibility to combine or save application rounds (reduced workload).

Besides DMPP, which was developed by BASF in the late 1990s, only two other nitrification inhibitors are commercially used. Nitrapyrine (2-chloro-6-(trichloromethyl) pyridine (commercially known as N-Serve®), which was introduced in 1962 by Dow into the U.S. market (GORING 1962), and DCD (dicyandiamide), which has been marketed in a number of European countries since the early 1980s (SOLANSKY 1982).

While these compounds have their merits, they also have some unfavorable properties.

Nitrapyrine has a high vapor pressure, which prevents its combination with solid fertilizers for a long time. Moreover, the time period over which it remains effective is highly dependent on temperature. Therefore, the use of N-Serve was restricted to anhydrous ammonia for many years, usually when applying ammonia before winter. Nitrapyrine belongs to the group of organochlorine chemicals that are increasingly considered critical for environmental and toxicological reasons. It also needs careful handling since it is corrosive and explosive (TRENKEL 2010). New formulations of the active improved these characteristics but today, the main market for that compound is in the U.S.

For DCD, the low specific activity implies high application rates, which is why the use of DCD is sometimes difficult to justify economically. DCD to a certain extent is itself mobile in the soil. In situations of high rainfall it is at risk of being disassociated from the adsorbed ammonium it is expected to safeguard (TESKE and MATZEL 1988). Under certain weather conditions, DCD can be phytotoxic to crops, which generally does not result in reduced yields but excludes its use from several crops, e.g., leafy vegetables, as necrotic leaf margins would render them unmarketable (REEVES and TOUCHTON 1986).

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