The stakes are larger than one Wisconsin cornfield. Agriculture is responsible for more than 81 percent of global ammonia emissions, and livestock production alone accounts for roughly 60 percent of those emissions in the United States. Once in the atmosphere, ammonia reacts with sulfur and nitrogen oxides to form fine particulate matter, or PM2.5, the microscopic pollution that penetrates deep into the lungs and contributes to respiratory disease, cardiovascular illness, and premature death. A portion of the gas also rains back down onto natural ecosystems, driving soil acidification and nutrient imbalances. Meanwhile, nitrate that escapes below the root zone contaminates aquifers and fuels algal blooms in lakes and coastal waters. Nitrous oxide, though a smaller share of the nitrogen budget, is a greenhouse gas roughly 273 times more potent than carbon dioxide over a century.

Researchers Juma Bukomba, Matthew D. Ruark, and Rebecca A. Larson set out to quantify the full nitrogen ledger under realistic field conditions. At the Arlington Agricultural Research Station on Plano silt loam, they compared surface broadcasting of liquid dairy manure, broadcasting followed by incorporation within one hour, subsurface injection at 15 centimeters, and versions of each treated with the urease inhibitor NBPT, alongside an unfertilized control. Plots stretched nine meters wide and 76 meters long, and the team buried ion exchange resin samplers at 90 centimeters to catch nitrate draining past the corn rooting zone. Ammonia and nitrous oxide were measured with a Fourier transform infrared gas analyzer mounted on a mobile cart, using chamber-based flux measurements that followed the USDA-ARS GRACEnet protocol. Sampling was deliberately concentrated in the first days after application, when ammonia losses peak.

The ammonia results were striking. Surface broadcasting lost the most nitrogen to the air, averaging 7 kilograms of nitrogen per hectare in 2024 and 18.2 kilograms per hectare in 2025 over the first 96 hours. Injection and injection with the inhibitor were statistically indistinguishable from the unfertilized control, meaning essentially no measurable ammonia escaped from the injected plots. Incorporation fell in between. The pattern held in both years, though emissions ran hotter in 2025, when the manure carried more total ammoniacal nitrogen, 87 versus 70 kilograms per hectare, and air temperatures in the critical first four days were about 4 degrees Celsius higher. Warmth accelerates the conversion of ammonium to ammonia gas and speeds its escape, a well-documented but vividly demonstrated effect here.

The urease inhibitor, NBPT, was the study’s most instructive disappointment. Added to manure at the manufacturer’s recommended rate, it failed to significantly reduce ammonia emissions whether manure was broadcast or injected. The reason lies in dairy manure chemistry. NBPT works by blocking the urease enzyme that converts urea into ammonium, but by the time manure leaves storage, most of the urea excreted in urine has already been hydrolyzed. In this study, total ammoniacal nitrogen made up 57 to 61 percent of the total nitrogen applied, suggesting little residual urea remained for the inhibitor to protect. The finding suggests that if inhibitors are to work for liquid dairy manure, they may need to be applied earlier in the management chain, in the barn or storage lagoon, rather than in the tanker on the way to the field.

Then came the trade-off. Nitrate leaching, measured over entire growing seasons, was highest in 2024, a year when rainfall from April through October totaled 836 millimeters, nearly double the 32-year average. That year, incorporation produced the greatest cumulative nitrate loss at 108.4 kilograms of nitrogen per hectare, 37 percent more than surface broadcasting and 89 percent more than the unfertilized control. Injection and injection with the inhibitor also leached substantially more nitrate than surface application. In 2025, a drier year, injection-based treatments again showed elevated leaching relative to broadcasting. The mechanism is intuitive: keeping nitrogen in the soil rather than the air means more of it is available for nitrification and downward transport when heavy rains arrive.

The crop data added another layer of nuance. In 2025, when residual soil nitrogen was low and the field responded to added fertilizer, injection produced the highest corn silage yield at 18.2 megagrams per hectare, with nitrogen uptake of 165.2 kilograms per hectare, significantly exceeding surface broadcasting and the control. Surface-applied manure, having lost so much nitrogen to volatilization, yielded barely more than unfertilized plots in that year. A companion nitrogen rate experiment showed that injected manure reduced the agronomic optimum nitrogen rate to 103 kilograms per hectare, compared with 177 for surface broadcasting and 181 for the unfertilized control, a clear signal that subsurface placement preserved more plant-available nitrogen. Silage quality metrics, including crude protein, fiber fractions, and digestible nutrients, were unaffected by treatment in both years.

Perhaps the most sobering finding concerned the nitrogen already in the ground. The unfertilized control plots leached 57.3 kilograms of nitrate-nitrogen per hectare in 2024, roughly four times the 2025 figure, and the researchers estimated that pre-existing soil nitrogen pools accounted for about half of the total nitrate leached that year. Fields with a history of heavy manure and fertilizer use carry a legacy nitrogen burden that no application method can undo. Managing that reservoir, through cover crops, adjusted application rates, or fallow-season strategies, may matter as much as refining how manure is placed.

Taken together, the study reframes the manure management debate. Injection and rapid incorporation genuinely deliver on their promise of slashing ammonia emissions, and in nitrogen-limited years they translate that conserved nitrogen into higher yields and lower fertilizer requirements. But the conserved nitrogen does not simply disappear into the crop; in wet years and leaky soils, a substantial fraction migrates downward instead. Nitrate leaching, not ammonia volatilization, was the dominant environmental loss pathway in every treatment and both years, sometimes exceeding 60 percent of applied manure nitrogen when measured against the control. The authors argue that ammonia-reduction practices must be paired with complementary leaching controls, such as cover crops or timing applications to avoid saturated conditions, to be truly sustainable.

For farmers and policymakers alike, the message is that there is no single best way to apply manure, only context-dependent trade-offs. Subsurface placement offers real benefits for air quality, human health, and fertilizer value, and it reduces nutrient losses in surface runoff. But on silt loam soils prone to drainage, it shifts the pollution problem from the atmosphere to the aquifer. The next generation of best management practices will need to think in whole-system terms, balancing the air, the soil, the water, and the crop in a single accounting, because this study shows that nitrogen saved from the sky has a way of finding its way into the ground.

Subject of Research: Effects of liquid dairy manure application methods on ammonia emissions, nitrous oxide fluxes, nitrate leaching, and corn silage nitrogen use in silt loam soil

Article Title: Effects of Manure Land Application Practices on Ammonia Emissions and Nitrate Leaching in a Silt Loam Soil

Article References: Bukomba, J., Ruark, M. D., & Larson, R. A. (2026). Effects of Manure Land Application Practices on Ammonia Emissions and Nitrate Leaching in a Silt Loam Soil. Journal of Agriculture and Food Research, Article 103364. https://doi.org/10.1016/j.jafr.2026.103364

Image Credits: AI Generated

DOI: 10.1016/j.jafr.2026.103364

Keywords: dairy manure, ammonia emissions, nitrate leaching, manure injection, urease inhibitor, NBPT, nitrous oxide, corn silage, nitrogen use efficiency, silt loam soil, groundwater contamination, Wisconsin agriculture

The study was conducted by Mohamed Boullouz, Mohamed Louay Metougui, and Ngonidzashe Chirinda at the Agricultural Innovation and Technology Transfer Center of Mohammed VI Polytechnic University in Ben Guerir, Morocco. Their experimental site sits in a semiarid zone where wheat, the region’s staple cereal, is grown under drip irrigation, a water-saving technique that delivers water and dissolved nutrients directly to the root zone. Drip systems are often promoted as environmentally friendly because they reduce water waste and can improve nitrogen use efficiency. But as this study demonstrates, the climate accounting of such systems depends heavily on which fertilizer goes into the irrigation lines.

The team set up a field trial comparing four mineral nitrogen sources at an identical application rate of 125 kilograms of nitrogen per hectare: conventional urea, ammonium nitrate, a urease-inhibited urea product marketed as Duramon, and a blend of ammonium nitrate and urea. An unfertilized control plot completed the design. Urea is by far the most widely used nitrogen fertilizer on Earth, prized for its high nitrogen content and low cost, but it must first be hydrolyzed in soil to ammonium before plant roots can take it up. That hydrolysis step creates a burst of ammonium and a localized rise in soil pH, conditions that can supercharge the microbial processes, nitrification and denitrification, that produce nitrous oxide as a byproduct.

Over the growing season, the researchers measured cumulative nitrous oxide fluxes using standard chamber-based gas sampling. The results were striking. Plots fertilized with conventional urea emitted 1.84 kilograms of nitrogen per hectare as nitrous oxide, while ammonium nitrate and the ammonium nitrate plus urea blend each emitted just 1.02 kilograms. The urease-inhibited urea fell in between at 1.36 kilograms. The unfertilized control produced 0.28 kilograms, establishing the background emission against which fertilizer-induced losses are judged. Expressed per unit of applied nitrogen, urea emitted 1.82 times more nitrous oxide than ammonium nitrate, even though grain yields across the fertilized treatments were statistically comparable.

Those numbers translate directly into emission factors, the percentage of applied nitrogen that escapes into the atmosphere as nitrous oxide. The Intergovernmental Panel on Climate Change currently instructs national inventory compilers to assume a default emission factor of 0.5 percent for dry climates, a figure meant to represent the average behavior of all nitrogen inputs in water-limited regions. The Moroccan measurements tell a different story. Urea’s emission factor was 1.24 percent, with a bootstrap 95 percent confidence interval of 1.10 to 1.38 percent, meaning it exceeded the IPCC default in every one of the thousands of statistical resamples the team performed. The urease-inhibited urea came in at 0.86 percent, also consistently above the default. Only the nitrate-containing fertilizers, at 0.59 percent each, bracketed the IPCC value, with confidence intervals spanning the 0.5 percent line.

The mechanistic explanation lies in soil microbial ecology. When urea hits moist soil, the enzyme urease cleaves it into ammonium within days, temporarily suppressing nitrite-oxidizing bacteria and creating conditions under which nitrifier denitrification and incomplete nitrification release disproportionate amounts of nitrous oxide. Ammonium nitrate, by contrast, delivers nitrogen already split between its ammonium and nitrate forms, bypassing the hydrolysis spike and smoothing out the microbial activity that drives emissions. Urease inhibitors such as the one in Duramon slow the hydrolysis step, and the study confirmed that they do reduce cumulative emissions relative to plain urea. However, the inhibitor did not significantly reduce yield-scaled emissions, the amount of nitrous oxide released per ton of grain produced, suggesting the technology moderates but does not eliminate the urea penalty.

The implications reach well beyond a single wheat field in Morocco. National greenhouse gas inventories under the Paris Agreement rely on IPCC emission factors, and for countries without extensive local measurement networks, the Tier 1 dry-climate default of 0.5 percent is the working assumption. If urea-dominated irrigated systems in semiarid regions actually emit at rates closer to 1.2 percent, those inventories are systematically undercounting agricultural nitrous oxide, and the mitigation policies built on them are aiming at the wrong target. Conversely, the finding that nitrate-based fertilizers hover near the default suggests that the 0.5 percent figure may be reasonable for some fertilizer types but misleading as a blanket value. The study’s authors argue that non-stratified emission factors, ones that ignore fertilizer source, risk underestimating emissions precisely in the urea-heavy systems that dominate global fertilizer consumption.

What makes the result especially compelling is the absence of a trade-off. Climate mitigation in agriculture often asks farmers to accept lower yields, higher costs, or more complex management in exchange for reduced emissions. Here, switching from urea to ammonium nitrate or a urea-ammonium nitrate blend cut nitrous oxide losses by roughly 45 percent while grain yields remained statistically indistinguishable across treatments. In a semiarid environment where every kilogram of grain matters for food security, and where drip irrigation has already been adopted to conserve scarce water, fertilizer substitution is an immediately actionable lever. The caveat is practical: ammonium nitrate can be more expensive or harder to obtain in some markets, and regulatory restrictions on nitrate fertilizers exist in certain countries due to misuse concerns. But where supply chains allow, the agronomic case is straightforward.

The study also adds an important data point to a growing body of work questioning how well global emission factors capture the diversity of real-world cropping systems. Recent research has highlighted that legacy effects of past nitrogen applications, sampling frequency, and irrigation management can all bias emission factor estimates, sometimes causing systematic underestimation. The Moroccan trial, conducted under the precise water control of drip irrigation, isolates fertilizer source as the dominant variable and shows it is powerful enough to double emissions at a fixed nitrogen rate. For semiarid wheat belts stretching from North Africa through the Middle East and Central Asia, where urea is the default nitrogen source and irrigation is expanding, the message is clear: the molecule carrying the nitrogen matters as much as the amount applied, and climate-smart fertilizer policy should treat fertilizer choice as a first-order decision rather than an afterthought.

Subject of Research: Nitrous oxide emissions and emission factors from different mineral nitrogen fertilizers in drip-irrigated semiarid wheat

Article Title: Mineral nitrogen source controls N2O emissions and emission factors in drip-irrigated semiarid wheat

Article References: Boullouz, M., Metougui, M. L., & Chirinda, N. (2026). Mineral nitrogen source controls N2O emissions and emission factors in drip-irrigated semiarid wheat. Environmental Monitoring and Assessment, 198(10), Article 1099. https://doi.org/10.1007/s10661-026-15963-1

Image Credits: AI Generated

DOI: 10.1007/s10661-026-15963-1

Keywords: nitrous oxide, urea, ammonium nitrate, urease inhibitor, emission factors, drip irrigation, semiarid wheat, IPCC Tier 1, greenhouse gas mitigation, nitrogen fertilizer, Morocco, soil microbiology