Nitrate and Carbon Availability as Drivers of Isotopic Fractionation of Nitrate in Estuarine Sediments

Abstract

This study investigates isotopic fractionation associated with nitrate reduction in estuary sediments, focusing on the influence of nitrate and organic carbon availability. Flow-through reactor experiments using sediments from the Trieux, Seine, and Scheldt estuaries revealed that the isotopic enrichment factor (ε) varied widely, showing an inverse relationship with nitrate reduction rates only under nonlimiting nitrate conditions. Under these conditions, increasing reduction rates diminished enzyme selectivity, leading to lower isotopic discrimination, while at lower rates, isotopic enrichment increased due to substrate selectivity. The addition of carbon further enhanced reduction rates, confirming the carbon limitation as a key driver of isotopic fractionation patterns. These results highlight the importance of substrate availability in shaping isotopic signatures in natural sediments beyond reduction rates alone. Our findings provide critical insights for interpreting nitrate isotopic data in field studies, emphasizing the need to consider local substrate dynamics for accurate assessments of nitrogen cycling in coastal ecosystems.