Colloquium: Scott Wankel

Scott Wankel

Assistant Scientist, Marine Chemistry & Geochemistry, Woods Hole Oceanographic Institution

Abstract - As an essential nutrient, nitrogen plays a key role in regulating primary productivity in most ecosystems across the global surface. Recently, the biogeochemical landscape of the nitrogen cycle has become even more complex, with new players and processes demanding an even more sophisticated approach for understanding this complexity. Naturally occurring stable isotopes provide useful records of the integrated influence of this menagerie of cycling processes. By using multi-isotope approaches (e.g., ¹⁵N, ¹⁸O and ¹⁷O), my work aims to reveal the nature and relative magnitude of these processes in order to better understand their roles in elemental cycles and to predict their response to environmental change.

Here, I will present work from both field and experimental studies highlighting the utility of coupled isotope approaches for shedding light on previously unrecognized N cycling mechanisms in a variety of environments. First, as an example from a coastal marine environment, a nitrate N and O isotopes reveal the importance of a previously unrecognized component of the surface ocean nitrogen cycle – nitrification in the euphotic zone – with important implications for estimates of carbon export from the global surface ocean. Next, through focused lab-based studies, we are exploring fundamental interactions between the nitrogen and iron cycles and the potentially overlooked role that iron may play in the fate of nitrogen, particularly in the organic lean subsurface biosphere. In particular, under iron-reducing conditions, Fe(II) represents a strong reductant of both nitrate and nitrite, a redox potential that may or may not be exploited by microbial metabolism. Indeed, our experimental multi-isotope data may help explain a currently unresolved conundrum between decades of groundwater field data and more recent microbial culture experiments – with implications for the importance of interactions among N, C and Fe cycling in groundwaters and perhaps even the subsurface biosphere of marine sediments.