The nitrogen cycle deserves our full attention because nitrogen makes up key molecules of life, such as DNA or ATP for instance. Ironically, 99% of the nitrogen available on Earth eludes 99% of living beings because it is present in the form of N2, which is not easily metabolizable. Bioavailable nitrogen, or reactive nitrogen, is thus a limiting factor for primary production on Earth, especially beneath the ocean's surface, where its availability largely determines the oceans' capacity to offset the increase in atmospheric CO2 and, consequently, global warming. Paradoxically, modern industrial processes enable us to overcome this nitrogen limitation by converting N2 into bioavailable nitrogen, primarily in the form of fertilizers. Unfortunately, the overconsumption of industrial fertilizers leads to uncontrolled nitrogen discharge, which has a detrimental effect on coastal ecosystems. Yet, the natural processes that control the bioavailability of nitrogen in the ocean are poorly understood, as are the consequences of anthropogenic nitrogen inputs on marine ecosystems. 

 

This research field is severely limited by the fact that 1) our current understanding of the nitrogen cycle variability is based on (short) nutrient concentration time series and 2) we lack the mechanistic understanding of N transfer across natural/anthropized-land/ocean interfaces. Recent methodological improvements in geochemistry allow to measure the N and O isotopes of the molecule of nitrate, unravelling the key processes governing the N-cycle. Moreover, it is now possible to measure the N isotopes trapped within the aragonitic or calcitic matrix of marine calcifiers, such as corals or foraminifera, opening an entire new field of research about the past N-cycle variability over time scale ranging from decades to million years. This seminar will briefly review the systematics of N isotopes, present several applied case studies and propose future research plans.