Ammonia from pig farms feeds soil microbes that pump out a greenhouse gas 300 times worse than CO2

Nitrogen Cycling journal

Ammonia is not a greenhouse gas. It does not trap heat in the atmosphere. But a study published in Nitrogen Cycling shows that when ammonia from livestock farms settles onto surrounding soils, it sets off a microbial chain reaction that produces something far worse: nitrous oxide, a gas with nearly 300 times the warming potential of carbon dioxide over a century and the dominant ozone-depleting substance being emitted today.

Measuring along the plume

Researchers conducted field measurements near a large pig farm in central China, monitoring soil nitrous oxide emissions at multiple points from 50 to 500 meters downwind. The natural gradient of ammonia deposition across the landscape, heavier near the farm and lighter farther away, created a real-world experiment without needing to artificially apply any chemicals.

The pattern was unambiguous. Soil nitrous oxide emissions decreased steadily with distance from the farm. Areas receiving the most ammonia deposition produced the most nitrous oxide. Within a 500-meter radius, total emissions reached an estimated 69.7 kilograms of nitrogen per year, representing roughly 1.3% of the ammonia nitrogen deposited in the area. That conversion factor is slightly higher than the emission factor commonly used in international climate assessments, suggesting current estimates may undercount this pathway.

The microbes doing the work

To understand the mechanism, the team examined microbial genes in the soil responsible for nitrogen cycling. They found that ammonia deposition increased the abundance of ammonia-oxidizing archaea, microorganisms that convert ammonium into nitrate through a process called nitrification. Nitrous oxide is a byproduct of that conversion.

The relationship is straightforward. More ammonia falls on the soil. More ammonium accumulates. Nitrifying microbes have more substrate to work with. They proliferate and produce more nitrous oxide as a metabolic side effect.

Laboratory experiments confirmed the field observations. When researchers supplied soils with ammonium-based nitrogen, nitrous oxide emissions were significantly higher than when the same soils received nitrate. The reduced form of nitrogen, the kind deposited by ammonia, was the key driver.

An overlooked accounting gap

Livestock farming is already the largest global source of ammonia emissions. The environmental harms of that ammonia, including ecosystem acidification, water pollution, and particulate matter formation, are well documented. What has received less attention is the possibility that ammonia acts as a secondary source of agricultural greenhouse gas emissions by feeding soil microbial processes that generate nitrous oxide.

Current greenhouse gas inventories from agriculture primarily focus on direct emissions from manure management and fertilizer application. The indirect pathway measured here, ammonia traveling through the air and stimulating nitrous oxide production where it lands, is accounted for in assessment frameworks but may be underestimated. The 1.3% conversion factor measured in this study exceeds the default values used by the Intergovernmental Panel on Climate Change, though a single site cannot establish a global correction.

What this means for policy

The finding creates an additional argument for reducing ammonia emissions from livestock operations. Technologies for ammonia capture already exist, including covered manure storage, acidification of slurry, and air scrubbing systems on barn exhaust. These are typically justified on the basis of local air quality and ecosystem protection. If they also reduce nitrous oxide production in surrounding soils, the climate benefit adds to the case.

The study was conducted at a single pig farm in one region of China, and the researchers note that soil type, climate, and management practices all influence how much nitrous oxide is generated from deposited ammonia. Extrapolating from one site to global estimates would be premature. But the mechanism is well established in soil science, and the field measurements provide direct evidence that it operates at meaningful scale near real livestock facilities.

As global livestock production continues to expand, the hidden links between one form of pollution and another will matter increasingly for climate accounting. Ammonia does not warm the planet directly. But it feeds the organisms that do.