The Project

Nitrogen (N) is an essential nutrient for all living beings (microorganisms, plants, and animals) and the most abundant element in Earth’s atmosphere where it mainly occurs as dinitrogen gas (N₂). Although there is a lot of N₂ around us (∼78% of the air we breathe), plants or animals cannot directly use this chemically unreactive form without first been transformed or “fixed” into reactive nitrogen (Nr) containing compounds such as ammonia (NH₃) or N oxides. N₂ can be naturally converted into N containing compounds through lightning strikes or by specialized Ν-fixing microorganisms either free-living in the soil or attached to the roots of certain plants like beans or clover. However, the supply of naturally fixed N in most natural ecosystems and in farmlands is low, often limiting ecosystem productivity. The early 1900’s invention of the Haber-Bosch process for the production of synthetic ammonium-based fertilizers has doubled Nr input into the World’s soils and enabled us to nutritionally sustain the global population increase of the 20th century. While the application of synthetic fertilizers is currently indirectly feeding 50% of humanity, it has also, accelerated the global N cycle. Critically, the low N fertilizer use efficiency (NUE) in agriculture, a measure of N retained in food per amount of N fertilizer applied, leads to massive N fertilizer losses (50–70%) to the environment, as NH₃, nitrate (NO₃^–), and nitrous oxide (N₂O), a greenhouse gas (GHG) with a warming potential ∼300 times greater than carbon dioxide. These losses are connected to pernicious environmental issues that we currently face such as biodiversity loss, eutrophication of aquatic ecosystems, ozone and air quality degradation, and GHG-driven climate change. Beyond its environmental impact, Nr pollution costs to EU up to €320 billion per year, double the value that N fertilizers are estimated to add to European farm income.

Nr losses from fertilized soils are directly linked to the microbially mediated process of nitrification, which generates the soil-mobile anion NO₃^– from relatively immobile ammonium (NH₄⁺) pools, causing significant losses of Nr from agricultural systems via NO₃^– leaching, and providing substrates to specialized soil microorganisms (both nitrifying and denitrifying microorganisms) to produce N₂O. In N-limited soils, nitrification rates are typically low, but in fertilized soils, about 90% of N flows through the nitrification pathway. These make the nitrification process and the associated microbial players an appealing inhibition target to minimize N loss from agricultural soils.

Three types of microorganisms, including ammonia-oxidizing bacteria and archaea, and the recently discovered complete ammonia-oxidizing (comammox) bacteria can perform the first step of nitrification, the oxidation of NH₃ to NO₂^–. All three groups of ammonia oxidizers (AOM) utilize NH₃ as their main energy source, through a reaction catalysed by ammonia monooxygenase (AMOs) enzymes. Apart from being necessary for the activity of all aerobic AOM, AMOs have a broad substrate range and mediate the initial and often rate- limiting step of nitrification. Thus, AMOs are generally considered as an ideal target for the suppression of the overall nitrification process.

Today, the direct inhibition of soil AOM using synthetic nitrification inhibitors (SNIs) along with N fertilizers, is a well-established strategy for improving NUE. However, while the underlying inhibition mechanisms of all currently used in agriculture NIs have been associated with the inactivation of AMO, their efficacy in regulating soil N transformations is highly variable across soils and often suboptimal due to the variable sensitivity of soil AOM to different NIs, the dependency of NIs performance on the composition of the metabolically active AOM community in soil and the lack of knowledge on the actual biochemical mode of action of NIs which prohibit any prediction on their activity against the different groups of AOM in soil.

In addition to SNIs, functionally similar plant derived compounds that inhibit nitrification, called biological nitrification inhibitors (BNIs), have recently received increased attention as safer alternatives to SNIs. However, the potential of BNIs to inhibiting nitrification is still unknown. Previous research has shown that BNIs in crop root exudates can suppress up to 90% of nitrification activity in soil and increase soil N retention, providing plants with a competitive advantage for N fertilizer. However, before BNIs can be applied in agricultural practices it is necessary to acquire a deeper understanding of their effect on N cycling, as well as on the overall soil microbiome integrity, plant health and nutrition.