Fate of microphytobenthos nitrogen in subtropical subtidal sediments: A ¹⁵ N pulse-chase study

Abstract

Microphytobenthos (MPB) are an important nitrogen (N) sink in coastal systems, but little is known about the fate of this N after it has been assimilated. We used an in situ 15N pulse‐chase experiment in subtidal sands to follow the assimilation, trophic transfer, transformation, and flux pathways of MPB‐N over 33 d. Throughout the study MPB dominated 15N uptake, on average representing only 18.1% of the biomass but 63.9% of the 15N within 0–2 cm sediment. Following assimilation, 15N was rapidly transferred to deeper sediment, with 32.1% below 2 cm and 16.5% below 5 cm after 60 h. In contrast to MPB, bacteria represented 39.5% of sediment biomass but accounted for only up to 27.3% of assimilated 15N. Foraminifera accumulated and stored 15N more than bacteria; their contribution to the 15N remaining in 0–2 cm sediment at the end of the study was more than double their biomass contribution. Thirty‐three days after the 15N was assimilated by MPB 27% remained in the sediment, 16.5% had been effluxed as NO3−, 20.8% had been effluxed as NH4+, 20.7% had been effluxed as N2 and 15.1% was missing. Most (12.6%) of 15N label that was missing at the end of the study was probably lost as dissolved organic N (DON) fluxes. Of the 15N remaining in 0–2 cm sediment, 80.4% was in MPB, 2.7% in bacteria, 1% in foraminifera and the remaining 15.9% was uncharacterized. Overall there was little benthic trophic transfer with most of the MPB‐assimilated N remineralized over 33 d.

Production of N2 and N2O from nitrate ingested by sheep

“…Analyses of 15 N abundance in N 2 O and N 2 gases were performed at the Centre for Coastal Biogeochemistry Research, Southern Cross University, Lismore NSW. Gas samples in 12‐ml tubes were analysed for the isotopic composition of N 2 on a Thermo Trace GC Ultra (Thermo Fisher, Waltham, MA, USA) interfaced to a Thermo Delta V Plus (Thermo Fisher) IRMS (Eyre, Oakes, & Middelburg, ). After helium sparging and cryogenic trapping, the concentrations and abundances of 15 N in N 2 O in 500‐ml bottles were analysed by a Thermo Fisher GasBench II (Thermo Fisher) interfaced to a Thermo Delta V Plus (Thermo Fisher) isotope ratio mass spectrometer (IRMS; Erler et al., ).…”

Short-term fate of intertidal microphytobenthos carbon under enhanced nutrient availability: a <sup>13</sup>C pulse-chase experiment

“…When quantified, the uncharacterized C pool typically has a high C : N ratio (10 to 60; Cook et al, 2009;Eyre et al, 2016a), indicating that nitrogen availability may have a role in regulating its content and accumulation. Given that nitrogen limitation has been observed to suppress processing pathways of otherwise labile OM in soils (Jian et al, 2016;Schimel and Bennett, 2004), a similar mechanism may be possible in estuarine sediments.…”

The short-term combined effects of temperature and organic matter enrichment on permeable coral reef carbonate sediment metabolism and dissolution

“…These subsamples were filtered on pre-combusted 25 mm GF/F filters, dried at 60 • C, fumed with 12 M HCl to dissolve any particulate carbonates on the filter, and wrapped in pre-combusted tin capsules. These capsules were analysed for carbon (C) and nitrogen (N) using an elemental analyser (Thermo Flash ES) coupled to an isotope ratio mass spectrometer (Thermo Delta V PLUS) via a Thermo Conflo V (see Eyre et al, 2016, for details).…”