Costs and Benefits of Nitrapyrin

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In December 2020, our research group (as part of the Food Integrated Science Team) published a review of nitrapyrin costs and benefits in the journal Environmental Science & Technology.

nitrapyrin graphic

The Issue:

Nitrapyrin is a chemical that can be added to ammonia-based fertilizers, as well as urea and manure. The purpose is to minimize nitrogen leaching and emissions and retain ammonium in the soil to maximize crop yield and enhance farm profits. In our review, we asked if these benefits are actually realized and if there are environmental or health costs with nitrapyrin that either limit its usefulness or create new hazards (Fig. 1).

 

Nitrapyrin fate

Figure 1. Overview of nitrapyrin fate, potential agronomic and environmental benefits, and environmental effects. Items in bold represent nitrapyrin research gaps.

(Environ. Sci. Technol. 2021, 55, 3, 1345-1353)

Addressing the Issue:

We concluded that the anticipated benefits of nitrapyrin are sometimes, but not always obtained and that new hazards or environmental impacts are potentially created. Notably nitrapyrin does not always increase yield and can even reduce yield. Additionally, nitrapyrin can transport off-field from agricultural lands into waterways, where it can persist and potentially disrupt the natural nitrogen cycle. There is some evidence that nitrapyrin can also impact the carbon cycle.

Nitrapyrin is not a new fertilizer additive; it has been on the market since 1974. In 2017, nitrapyrin or related products were added to 24% of the total tons of nitrogen sold in the United States. Nitrapyrin is most widely used with corn, wheat, sorghum, and tree crops such as almonds; it is also used on horticultural crops like beets and strawberries.

 

How nitrapyrin works and its intended benefits

Nitrapyrin is categorized as a “nitrification inhibitor,” meaning that it prevents natural soil bacteria from converting the ammonia in fertilizer into nitrate (Fig. 2).  This keeps fertilizer in its ammonium (NH4+) form. Ammonium uses its positive charge to cling to soil particles where it is available to plants. Nitrate is also used by plants but has a negative charge that helps it leach out of soils into drinking water, groundwater, and waterways. Therefore, nitrapyrin use aims to reduce nitrate leaching.

nitrification inhibitor figure

Figure 2. Nitrapyrin inhibits the conversion of ammonium to nitrite and nitrate.

(Environ. Sci. Technol. 2021, 55, 3, 1345-1353)

Nitrate formation also lays the groundwork for soil microbes to make nitrous oxide, a greenhouse gas with greater warming potential than carbon dioxide and methane. So, the use of nitrapyrin also aims to limit nitrous oxide emissions from agricultural fields.   

 

Nitrapyrin and crop yield

Depending on environmental conditions, nitrapyrin reduced crop yields by up to 20% or enhanced them up to 207%, with an average improved yield of 7%. Nitrapyrin’s effectiveness at increasing yield depends on soil temperature, pH, moisture, aeration, microbial community, and organic and clay content. Weather conditions during and after application, such as rainfall and wind also affect fertilizer stability and nitrapyrin efficiency. Finally, farm management practices, like fertilizer type, rate, placement, and timing of application(s), as well as the crop being grown all influence the ability of nitrapyrin to limit N loss and enhance crop yield. In summary, the ability of nitrapyrin to increase crop yield is highly variable.

 

Nitrapyrin tends to decrease nitrous oxide emissions, but effects still variable

The Intergovernmental Panel on Climate Change recommends nitrification inhibitors, like nitrapyrin, as a tool to limit greenhouse gas emissions. Like yield, effects of nitrapyrin on emissions are variable, ranging from no benefit to reducing emissions by 70%. Again, this variability seems to be due to differences in soils, temperatures, moisture levels, agricultural practices, the microbial community, and crop specifics. However, of all the expected benefits of nitrapyrin, reduced emissions are most often realized.

 

Nitrapyrin can increase or decrease nitrate leaching

Nitrapyrin does not reliably reduce nitrate leaching and can even increase leaching under some circumstances. Published observations range from 43% less leaching to 32% more leaching. There is also evidence that applying fertilizer in the spring, rather than fall, is just as good at limiting nitrate leaching as adding nitrapyrin.

 

Nitrapyrin most often increases ammonia volatilization

It is estimated that, on average, 18% of applied nitrogen is lost to the atmosphere as ammonia. Because nitrapyrin keeps ammonia-based fertilizers in their ammonia form, does adding nitrapyrin cause even more nitrogen loss through volatilization? The answer is yes. Nitrapyrin addition increases ammonia volatilization to the atmosphere by 3% to 150%. Additionally, when ammonia levels in the soil are high, there is greater opportunity for nitrification/denitrification by soil microbes (Fig. 2). These processes can produce nitrous oxide and contradict other potential benefits of nitrapyrin related to nitrous oxide emissions from soils. In summary, nitrapyrin tends to promote loss of applied nitrogen through volatilization and enhance greenhouse gas formation in the form of nitrous oxide, especially when soil ammonia levels are high.

 

Environmental fate of nitrapyrin

When nitrapyrin is applied to farmland, it can move into adjacent waterways as spray drift through the air, or through water runoff. Entering waterways is important because it creates potential for nitrapyrin to inhibit the natural nitrogen cycle and drive increases in toxic ammonia in aquatic systems. Investigating this potential effect of nitrapyrin is a critical data gap.

Iowa stream

Figure 3. Iowa stream where nitrapyrin was measured.

(Credit: Dana Kolpin, USGS. Public domain.)

Some important questions are – how much nitrapyrin is in waterways, does it degrade, and is it disturbing the nitrogen cycle? We have very limited data to test this. The Environmental Protection Agency (EPA) mostly uses mathematical models to make predictions, based on the physical characteristics (solubility, etc.) of nitrapyrin. The EPA models assume a half-life of 4.4 years for nitrapyrin in freshwater. The European equivalent of the EPA, called REACH, reports that nitrapyrin has a half-life of 2.74 years in freshwater sediments at 25 °C. In research conducted by USGS, nitrapyrin persisted in Iowa streams for up to 5 weeks after application (Woodward et al., 2019) (Fig. 3). That study reported nitrapyrin levels up to 1.2 µg/L in agricultural streams.

In water, soil, and food crops like strawberries, nitrapyrin can degrade to 6-chloro-2-picolinic acid (6-CPA) directly or form 6-CPA via the intermediate 2-chloro-6-(dichloromethyl)-pyridine (DCMP). 6-CPA is more mobile and water soluble than nitrapyrin. It is worth noting that nitrapyrin and 6-CPA can temporarily accumulate in fish and plant foods, but the dynamics of accumulation, like how quickly it happens and how long it lasts, are not well-characterized.

The bottom line for understanding environmental fate of nitrapyrin is we need more data to evaluate the full picture of environmental nitrapyrin.

 

Benchmarks for nitrapyrin exposure

In 2019, the EPA revised nitrapyrin exposure limits, and REACH completed similar revisions in 2020. Table 1 of our paper summarizes the new environmental benchmarks, which range from 0.1 – 8.7 mg/L in water or 1.8 – 209 mg/kg in soil, depending on the organisms or types of tests used. These benchmarks are well above nitrapyrin concentrations currently measured in the environment.

To determine benchmarks, the EPA and REACH both depend on short term testing, ranging from 4-56 days and looking for mortality or decreases in growth or reproduction of test animals. Current testing paradigms do not include more sensitive outcomes like changes to the immune system, nervous system (e.g. behavior), hormone production, or microbiome, so research on these outcomes is needed.

Nitrapyrin and healthy nitrogen cycling

Because nitrapyrin is designed to inhibit microbes that process ammonia into nitrate, part of our review focused on how nitrapyrin might impact wild microbial populations that maintain the nitrogen cycle. Nitrogen cycles in the environment and also in the bodies of organisms. In fact, in humans, microbes in our mouth and digestive system assist with nitrogen cycling that supports healthy vascular, nerve, and metabolic functions (see Edwards and Hamlin, 2018 for additional information and references).

The take home message at this point is that environmental or health testing has not addressed impacts of nitrapyrin on microbes beyond farm soils. In fact, most testing has investigated only a few bacterial strains, grown as pure cultures in the lab. Major microbial groups that participate in nitrogen cycling have not been studied at all with regard to nitrapyrin impacts. These include ammonia oxidizing archaea and the nitrogen cycling symbiotic microbes inside animal bodies. It is plausible that nitrapyrin affects these processes but the relationship between dose and effect remains a data gap.

Nitrapyrin and the carbon cycle

Although nitrapyrin is intended to disrupt the nitrogen cycle in agricultural soils, there is evidence that it also inhibits methane oxidation and carbon dioxide uptake by methane oxidizing bacteria in soils. This side effect enables carbon dioxide to linger in the atmosphere where it contributes to climate change.

 

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