The Microorganisms Behind Nitrogen Cycling: A Tale of Tiny Titans

Nitrogen is a vital element for all living organisms, forming an essential part of DNA, proteins and other cellular structures. Dinitrogen gas is abundant and freely accessible in the atmosphere, yet most organisms cannot use it in this form and rely on ‘fixed’ nitrogen molecules such as ammonium or nitrate. In nature, dinitrogen is fixed by diverse species of bacteria and archaea into ammonium, which is then assimilated into the biomass of organisms. When they die, the ammonium is released into the environment where it (re-)enters the nitrogen cycle. It can then be transformed into nitrate and eventually back into dinitrogen gas or ammonium through a series of chemical reactions mediated by microorganisms. These microbial processes thus control the flow of nitrogen in ecosystems—how it enters, is retained, or is lost.

In recent decades, human activities have drastically changed the nitrogen cycle, especially through the industrial production of synthetic fertilizers via the Haber-Bosch process, which converts dinitrogen and hydrogen into fertilizer. The widespread use of ammonium nitrate fertilizers in agriculture has doubled the amount of fixed nitrogen entering the biosphere, leading to significant increases in crop yields. However, agricultural systems tend to have low nitrogen-use efficiency, with crops absorbing less than half of the nitrogen applied. As a result, much of the nitrogen undergoes nitrification and denitrification, two microbial processes that lead to significant nitrogen losses. These losses occur both as nitrogen gases, including the potent greenhouse gas nitrous oxide, and as nitrate leaching into water bodies. Consequently, agriculture has become the largest source of global nitrogen pollution, contributing to the degradation of aquatic ecosystems and posing a threat to safe drinking water. It is also responsible for about half of the human-caused emissions of nitrous oxide. Therefore, it is crucial to develop strategies to reduce our reliance on synthetic nitrogen fertilizers and minimize nitrogen losses to meet food demands sustainably.

My research aims to boost nitrogen-use efficiency in agricultural soils by engineering soil microbial communities. This requires a deep understanding of the microorganisms involved in nitrogen cycling and their interactions with crops. In my lecture, I will share the latest insights into the ecology of these organisms, including findings from our recent study on ammonifiers—an important but often overlooked group that plays a crucial role in retaining nitrogen in soils. I will also discuss my ongoing projects that explore (1) which environmental conditions and farming practices favour microbial groups promoting nitrogen retention (such as ammonifiers) versus those contributing to nitrogen loss (such as nitrifiers and denitrifiers) and (2) the ecological diversity within these microbial groups. A particular emphasis will be placed on root-microbe interactions, as plant roots affect soil conditions and resource availability for microorganisms. Finally, I will outline a new direction in my research that examines the evolution of nitrogen-cycling microorganisms across Earth’s long history. Taken together, this work will uncover fundamental aspects of the ecology and evolution of nitrogen-cycling microorganisms, with significant implications for environmental stewardship and sustainable agriculture.