Application of the isotope pairing technique in sediments: Use, challenges, and new directions

Abstract

Determining accurate rates of benthic nitrogen (N) removal and retention pathways from diverse environments is critical to our understanding of process distribution and constructing reliable N budgets and models. The whole‐core 15N isotope pairing technique (IPT) is one of the most widely used methods to determine rates of benthic nitrate‐reducing processes and has provided valuable information on processes and factors controlling N removal and retention in aquatic systems. While the whole core IPT has been employed in a range of environments, a number of methodological and environmental factors may lead to the generation of inaccurate data and are important to acknowledge for those applying the method. In this review, we summarize the current state of the whole core IPT and highlight some of the important steps and considerations when employing the technique. We discuss environmental parameters which can pose issues to the application of the IPT and may lead to experimental artifacts, several of which are of particular importance in environments heavily impacted by eutrophication. Finally, we highlight the advances in the use of the whole‐core IPT in combination with other methods, discuss new potential areas of consideration and encourage careful and considered use of the whole‐core IPT. With the recognition of potential issues and proper use, the whole‐core IPT will undoubtedly continue to develop, improve our understanding of benthic N cycling and allow more reliable budgets and predictions to be made.

Behavioural patterns of the soft-shell clam Mya arenaria: implications for benthic oxygen and nitrogen dynamics

“…Anaerobic NH 4 + oxidation (anammox) rates have previously been shown to be negligible at the collection site (data not shown). The approach is thoroughly reviewed by Robertson et al (2019).…”

Controls of H2S, Fe2 +, and Mn2 + on Microbial NO3–-Reducing Processes in Sediments of an Eutrophic Lake

“…Wenk et al (2014) reported a maximum DNRA contribution to NO 3 − reduction of ∼12%, but also suggested that these measurements should be considered conservative, because they neither accounted for the production of 14 NH 4 + from ambient background NO 3 − nor for product NH 4 + potentially retained in the intact sediment cores. The use of intact sediment cores provides a more accurate representation of in situ N-cycling conditions by maintaining the biogeochemical zonation of the sediment relative to slurry methods (Hansen et al, 2000;Robertson et al, 2019). However, relative potential process rates and their specific regulating factors can better be resolved through controlled manipulation experiments (Robertson et al, 2019), such as those performed here (discussed below).…”

“…The use of intact sediment cores provides a more accurate representation of in situ N-cycling conditions by maintaining the biogeochemical zonation of the sediment relative to slurry methods (Hansen et al, 2000;Robertson et al, 2019). However, relative potential process rates and their specific regulating factors can better be resolved through controlled manipulation experiments (Robertson et al, 2019), such as those performed here (discussed below).…”

Oxygen and light determine the pathways of nitrate reduction in a highly saline lake

“…The existence of NO3 − reduction pathways, mainly during S2, is the most plausible explanation according to the isotopic data. However, other major issues for 15 N-IPT studies such as uptake and intracellular storage should not be discarded (Robertson et al, 2019). Significant inputs of NO3 − may also promote blooms of diatoms (frequent in Pétrola Lake), which are physiologically adapted to grow rapidly under nitrate-rich conditions (Bronk et al, 2007).…”

“…The 15 N-IPT was firstly applied on sediment cores to quantify N2 production derived from denitrification (Nielsen, 1992). Since then, many studies have focused on discriminating the relative contribution of N processes using 15 N-IPT, including DNRA, and recently coupled DNRA-anammox (Risgaard-Petersen et al, 2003;Holtappels et al, 2011;Hsu and Kao, 2013;Robertson et al, 2019). Hence, processes such as anammox have been traditionally underestimated.…”