Influence of Terrigenious Inorganic Nitrogen on Marine Nitrogen Cycle Reconstruction Inferred from Sedimentary Bulk Nitrogen Isotope

Abstract

This study reevaluates the application of sedimentary bulk nitrogen isotopes (δ15NTN) as proxies for reconstructing past marine nitrogen dynamics, traditionally assumed to reflect organic nitrogen (δ15NON) derived from marine productivity. Employing a novel method that utilizes persulfate oxidation to convert organic nitrogen (ON) and ammonium into nitrate, followed by nitrate reduction to nitrous oxide via denitrifying bacteria, we quantified and characterized both ON and inorganic nitrogen (IN) across diverse environments, including soils, rivers, lakes, and marine sediments. We found that IN, primarily sourced from clay-fixed ammonium via riverine and dust inputs, constitutes 30 % to 50 % of total nitrogen (TN) in marine sediments. Our findings reveal that δ15N values of IN are typically 2 ‰ to 5 ‰ lower than those of δ15NON, resulting in a − 0.5 ‰ to −2 ‰ negative shift in marine δ15NTN. This significant isotopic deviation necessitates a reevaluation of nitrogen cycle interpretations inferred from δ15NTN, especially regarding the potential underestimation of nitrate utilization in nitrate-rich regions and the overestimation of nitrogen fixation in oligotrophic oceans.