Influence of labile dissolved organic matter on nitrate reduction in a seepage face

Abstract

Seepage faces, the outer rim of subterranean estuaries, are an important reaction node for SGD-borne nitrate (NO) on a global scale. Labile dissolved organic matter (DOM) has been suggested to be a key factor constraining the NO removal rate in aquifer systems. To determine whether and to what extent the availability of labile DOM affects benthic NO reduction in seepage faces, a series of flow-through reactor (FTR) experiments with sandy sediment collected from a seepage face was conducted under oxic conditions. Experimental results revealed that the addition of labile DOM (glucose) to porewater did not trigger a significant enhancement in NO reduction rate. In contrast, the aerobic respiration was boosted from ca. 50 to 90 μmol dm sediment h by glucose amendments, accounting for approximately 70% consumption of the labile DOM pool. This rapid consumption may increase the NO reducing capability within the sediment, but only indirectly. Together with fluorescent DOM (FDOM) analyses, it can be inferred that NO reducers tend to choose sediment organic matter the prime electron donor under the experimental conditions. As a result, enrichment of DOM in seepage faces, depending on composition, might only stimulate aerobic respiration and nitrification, thus promoting the increase of ensuing NO fluxes to adjacent coastal waters.

Response of Nitrate Processing to Bio-labile Dissolved Organic Matter Supply Under Variable Oxygen Conditions in a Sandy Beach Seepage Face

“…The rapid DO consumption in permeable sediments after glucose amendment was also comparable to the results obtained by Rusch et al (2006) with sediments from the Atlantic Bight (35-57% enhancement). NO 3 − removal rates obtained in the present study are similar to those calculated from field survey data in the same STE (13.6 nmol cm −3 h −1 , unpublished data) and those measured with FTR in the Ria Formosa Lagoon (0.7-6.8 nmol cm −3 h −1 ; Jiang et al, 2018).…”

“…Amendments with NO 3 − increased DOC production rates and added small-molecular DOM into the porewater during both seasons at saturated DO conditions (Figures 3B-D). A similar DOC production pattern during net NO 3 − reduction under aerobic conditions was found by Jiang et al (2018) in intertidal permeable sediments of the Ria Formosa Lagoon, Portugal. On the other hand, Ibánhez et al (2021b) found that porewater nitrate enrichment accelerates SOM mineralization by overcoming N limitation of the local microbial metabolism, thus shortening SOM turnover times.…”

“…Sediment samples from both sampled seasons were also used in a series of adsorption experiments to quantitively determine the influence of adsorption on glucose and NO 3 − concentrations under aerobic conditions. In particular, the sediment used in the adsorption was autoclaved for 30 min at 121 • C to eliminate the influence of sediment prokaryotes (Jiang et al, 2018). After gently washing with filtered seawater, the autoclaved sediments (10 g) were mixed with aged seawater (40 mL) spiked with NO 3 − (final concentration: 80 µM) and glucose (final DOC concentration: 480 µM), respectively.…”

“…However, permeable sediments are frequently enriched in DO by porewater exchange with DO saturated seawater and DO diffusion from the atmosphere (Berg et al, 2013;Ibánhez and Rocha, 2016). The bio-labile DOM might be consumed in aerobic mineralization prior to be available for denitrification (Jiang et al, 2018), suggesting a possible spatial isolation between bio-labile DOM enrichment and denitrifiers (e.g., spatial mismatch in coastal aquifers; Siemens et al, 2003). In addition, DO availability might directly hamper the denitrifying bacterial activity, leading to depression of denitrification process rates under oxic conditions (Evard et al, 2012).…”

Reactive Solute Transport Through Two Contrasting Subterranean Estuary Exit Sites in the Ría de Vigo (NW Iberian Peninsula)

“…Solute flux estimations associated to SGD need to take into account the high biogeochemical reactivity commonly found at the exit sites that can significantly modify the composition of the circulating waters prior to discharge (Slomp and Van Cappellen, 2004;Spiteri et al, 2008;Santos et al, 2008Santos et al, , 2009aRocha et al, 2009;Ibánhez et al, 2011Ibánhez et al, , 2013Ibánhez and Rocha, 2014Jiang et al, 2018Jiang et al, , 2020. This zone is called subterranean estuary (STE), by analogy with surface estuaries (Moore, 1999;Burnett et al, 2003).…”