The nitrification inhibitor DMPP preferentially suppresses ammonia-oxidising bacteria and reduces nitric oxide emissions in an archaeal-dominated grassland soil

The soil beneath our feet is a bustling factory of invisible chemistry, constantly transforming the nutrients we add to our fields into forms plants can use. One of the most critical processes happening in this underground world is nitrification, where specific microbes turn ammonium, a common form of fertilizer, into nitrate. While this transformation feeds crops, it also has a side effect: it releases gases into the air. Two of these gases are particularly important. Nitrous oxide is a powerful greenhouse gas that traps heat in the atmosphere, while nitric oxide plays a major role in forming smog and acid rain, affecting air quality and human health. For decades, scientists have known that two different types of microscopic life—bacteria and archaea—drive this process. In many modern agricultural soils, the archaea are far more numerous, leading researchers to assume they are the primary drivers of gas emissions. However, the exact role of the less common bacteria in releasing these specific gases has remained a mystery, leaving farmers and scientists unsure of how best to stop these emissions without hurting crop growth.

To solve this puzzle, a team of researchers turned their attention to a grassland soil in the United Kingdom where the archaea clearly outnumber the bacteria. They set up a controlled experiment to watch what happened when they added urea, a standard nitrogen fertilizer, to the soil. They wanted to see if the dominant archaea or the rare bacteria were responsible for the release of nitric oxide and nitrous oxide. To test this, they introduced two different chemical inhibitors, substances designed to slow down the nitrification process. One of these inhibitors, a compound called DMPP, is known to be particularly effective at targeting the bacteria. By comparing soil treated with fertilizer alone against soil treated with fertilizer plus the inhibitor, the team could observe which microbes were doing the heavy lifting when it came to gas production. They monitored the soil continuously for twenty-eight days, measuring the flow of gases escaping from the surface and tracking how the populations of these microscopic communities changed over time.

The results revealed a surprising twist in the story of soil chemistry. Even though the archaea made up about two-thirds of the ammonia-oxidizing community in the soil, the bacteria were the ones disproportionately responsible for releasing nitric oxide. When the researchers applied the DMPP inhibitor, it successfully suppressed the population of the bacteria while leaving the archaea largely untouched. This selective action led to a dramatic drop in nitric oxide emissions, which fell by as much as seventy-five percent at certain points during the experiment. The fact that stopping the bacteria caused such a sharp decline in gas output, despite their small numbers, suggests that these rare microbes are highly efficient at producing nitric oxide. It appears that the bacteria act as a potent, high-output source of this gas, while the more numerous archaea contribute much less to this specific emission.

The story was different for nitrous oxide. While the DMPP inhibitor did reduce the release of this gas, the effect was much more modest, with reductions ranging from almost nothing to about twenty-seven percent. This difference highlights that the two gases come from different sources and pathways within the soil. Nitric oxide seems to be tightly linked to the activity of the bacteria, so when the bacteria are slowed down, the gas stops. Nitrous oxide, however, is produced by a wider variety of processes, including activities from the archaea and other soil organisms that the inhibitor did not stop. Because the inhibitor only targeted one specific group, the other pathways continued to release nitrous oxide, preventing a total shutdown of emissions. This finding suggests that managing one gas does not automatically solve the problem of the other, as they are driven by different biological mechanisms.

The study also confirmed that the inhibitor DMPP was more effective and longer-lasting than another common inhibitor, dicyandiamide. The soil treated with DMPP held onto more of the fertilizer nitrogen in its ammonium form for a longer period, which is beneficial for crops because it means the plants have more time to absorb the nutrients before they are lost to the air or washed away. This improved retention of nitrogen, combined with the significant reduction in nitric oxide, points to a clear advantage for using this specific inhibitor in farming. The research demonstrates that simply counting how many of each microbe are present in the soil is not enough to predict what gases will be released. Instead, the functional activity of the less common bacteria is the key driver for nitric oxide emissions. This insight offers a new target for farmers and scientists: by focusing on suppressing these specific bacterial pathways, it may be possible to drastically cut nitric oxide pollution without necessarily affecting the other gases, leading to cleaner air and more efficient use of fertilizer.