Probing the cryptic nitrogen cycle

The element nitrogen (N) is key to life - building proteins and the very DNA that tells life what to do. Nitrogen exists largely as di-nitrogen gas (N2) in the atmosphere, with a fraction being present in the organic N of life (humans, animals, plants, microbes etc.). Following death and decay, organic N decomposes to ammonia. The N in ammonia is then cycled back to the atmosphere through a coupling between microbes (microscopic organisms known as bacteria and archaea) that, on the one-hand, use oxygen to convert ammonia into nitrates and, on the other, microbes that respire nitrate in the absence of oxygen back to N2 gas. Oxygen rich, oxic-habitats are all around us, be it agricultural, grassland or forest soils or, indeed, your own back garden. If those soils become water logged, they will lose their oxygen and become anoxic-habitats and the same holds true for muddy sediments at the bottom of seas and lakes - and microbes in those anoxic habitats respire nitrates to N2 gas. This is the N cycle taught at school and although it has been updated in the past 20 years to include novel microbial pathways of producing N2 gas - the coupling between ammonia and N2 gas mediated through nitrates sits at its very heart.

What's new? In 2016, Trimmer and his grouped published a paper showing that the division between the recognised oxic and anoxic parts of the N cycle was blurred, with ammonia being converted to N2 gas in clean, oxygen-rich gravel riverbeds. Subsequently, Trimmer had a PhD student continue to explore the N cycle in oxygen-rich gravel riverbeds. The opportunity now for a new international collaboration arouse fortuitously during the examination that PhD student by the external examiner Prof. Bo Thamdrup (University of Southern Denmark) who identified a mistake in an equation in Liao's thesis. Correcting this seemingly innocuous mistake turned out to have profound implications for our understanding of the N cycle; though not only in oxic riverbeds but in many other habitats that drive the Earth's N cycle. What has changed? Correcting the equation led to a new mathematical framework and placing our data into that new framework showed that the patterns in the N2 gas data - in the PhD thesis - disagreed with those expected for a coupling between distinct oxic and anoxic steps in the N cycle. Where that well-recognised coupling should include nitrates, our new mathematical framework argues for a cryptic-coupling that does not involve nitrates.

Why does this matter? A cryptic-coupling not only changes our view of a fundamental step in the N cycle but - being hidden - a cryptic-coupling undoes 20 years' of research into the different microbial pathways that make N2 gas and our overall understanding of the N cycle is now challenged. Our new framework suggests a new pathway or at least a new type of coupling between known pathways in the N cycle that needs to be characterised before we can understand the cycling of a key bio-element on Earth. Further, unravelling this cryptic-coupling could facilitate the development of more efficient waste-water treatment i.e., by removing the need for separate oxic and anoxic treatment processes. We cannot, however, probe this new cryptic-coupling in the N cycle using current and widely available techniques - as they are simply blind to what it is we need to study. Hence, now in a new international collaboration we will pioneer the development of new tools to probe a cryptic-coupling in the N cycle. We will share complimentary mass-spectrometer facilities, along with contrasting field-sites and novel isotope and molecular techniques to deliver new fundamental and applied knowledge about the all too common, yet still enigmatic cycling of N on Earth.