[B-BG01] Elemental cycling in terrestrial and coastal ecosystems- Carbon and Nitrogen perspectives

[BBG01-02] ¹⁷O-rich nitrate as a tracer for constraining nitrogen transformations in coastal sediments

*Chawalit Charoenpong^1,2, Scott D Wankel², Carolyn Buchwald^3,2, Wiebke Ziebis⁴ (1.Chulalongkorn University, 2.Woods Hole Oceanographic Institution, 3.Dalhousie University, 4.University of Southern California)

Keywords:nitrogen, isotope, 17O, sediments

Coastal sediments are often replete in organic matter and exhibit sharp gradient in redox conditions. In addition, the oxic-anoxic interfaces are often around a few centimeters if not less below the sediment surface. This make this environment poised to harbor both oxidative and reductive nitrogen transformations. As such, the use of a single or a couple of isotope-labeling tracers to determine the rates of these many processes might suffer from the systems being underconstrained. In this work, we demonstrate how naturally-occurring, ¹⁷O-rich nitrate can aid in the studying these complex systems through the ability to follow the transformations of added nitrate (NO₃^-) into other different pools of N-species and closely investigate the triple isotopic compositions (δ¹⁵N, δ¹⁸O, and Δ¹⁷O). While we followed five different pools of N-species namely NO₃^-, NO₂^-, N₂O, NH₄⁺, and total reduced N (NH₄⁺ plus DON), we chose to focus this work on the intermediate nitrite, NO₂^-. For all intact flow-through core incubations done on the sediments collected from Sylt Island, Germany, sediments acted as sources for NO₂^- in all experimental manipulations including sediment type, dissolved oxygen level, and NO₃^- loading. Unlike in the environments that are solely driven by reductive processes where the changes in δ¹⁵N and δ¹⁸O are coupled, the co-occurrence of both oxic and anoxic in the sediments such as ones from this study cause the δ¹⁵N and δ¹⁸O to decouple primarily because O is often subject to more processes than N. By using three isotope systems along with the change in concentrations, we demonstrate how we can use a natural abundance appraoch and rely on a series of mathematical equations to solve for different N transformation rates.