The nitrogen isotope effect of benthic remineralization-nitrification-denitrification coupling in an estuarine environment

Abstract

The nitrogen (N) stable isotopic composition of pore water nitrate and total dissolved N (TDN) was measured in sediments of the St. Lawrence Estuary and the Gulf of St. Lawrence. The study area is characterized by gradients in organic matter reactivity, bottom water oxygen concentrations, as well as benthic respiration rates. N isotope effects on the water column associated with the benthic exchange of nitrate (ε<sub>app</sub>) and TDN (ε<sub>sed</sub>) during benthic nitrification-denitrification coupling were investigated. The sediments were a major sink for nitrate and a source of reduced dissolved N (RDN = DON + NH<sub>4</sub><sup>+</sup>). We observed that both the pore water nitrate and RDN pools were enriched in <sup>15</sup>N relative to the water column, with increasing δ<sup>15</sup>N downcore in the sediments. As in other marine environments, the biological nitrate isotope fractionation of net fixed N loss was barely expressed at the scale of sediment-water exchange, with ϵ<sub>app</sub> values <3‰. The strongest under-expression (i.e. lowest ε<sub>app</sub>) of the biological N isotope fractionation was observed at the most oxygenated sites with the least reactive organic matter, indicating that, through their control on the depth of the denitrification zone, bottom water oxygen concentrations and the organic matter reactivity can modulate ε<sub>app</sub>. For the first time, actual measurements of δ<sup>15</sup>N of pore water RDN were included in the calculations of ε<sub>sed</sub>. We argue that large fractions of the sea-floor-derived DON are reactive and, hence, involved in the development of the δ<sup>15</sup>N of dissolved inorganic N (DIN) in the water column. In the St. Lawrence sediments, the combined benthic N transformations yield a flux of <sup>15</sup>N-enriched RDN that can significantly elevate ε<sub>sed</sub> above ε<sub>app</sub>. Calculated ε<sub>sed</sub> values were within the range of 4.6 ± 2‰ and were related to organic matter reactivity and oxygen penetration depth in the sediments. ϵ<sub>sed</sub> reflects the δ<sup>15</sup>N of the N<sub>2</sub> lost from marine sediments and thus best describes the isotopic impact of fixed N loss from sediments on the oceanic fixed N pool. Our mean value for ε<sub>sed</sub> is larger than assumed by earlier work, questioning current ideas with regards to the state of balance of the modern N budget

Isotopic constraints on the pre-industrial oceanic nitrogen budget

“…Our experiments show that increasing this factor from 0 to 4 ‰ requires almost triple the BD rate needed to maintain the global mean δ 15 NO 3 at observed levels. Recent estimates suggest that the net fractionation factor may be even higher (4-8 ‰) due to fractionation within the nitrification-denitrification loop in the sediments (Granger et al, 2011;Alkhatib et al, 2012). If these high-end estimates are validated on a global scale, this could require larger BD : WCD ratios than our largest value simulated here (> 3.5).…”

“…Recent studies (Lehmann et al, 2007;Granger et al, 2011;Alkhatib et al, 2012) have suggested a higher net fractionation factor associated with BD (ε BD = 4-8 ‰) compared to previous estimates (ε BD = 1-3 ‰) (Brandes and Devol, 2002;Lehmann et al, 2004). They suggest BD should have a much higher net fractionation factor due to the measured high δ 15 NH + 4 that is presumably released from the sediments where BD occurs.…”

“…The small increase in bottom water δ 15 NO − 3 overlying BD zones suggests the fractionation factor should be in the range 0-3 ‰ (Brandes and Devol, 2002;Lehmann et al, 2004). However, more recent studies have suggested a much higher net fractionation (Lehmann et al, 2007;Granger et al, 2011;Alkhatib et al, 2012). They suggest the 15 N-enriched ammonium measured in the overlying bottom water, presumably released from the sediments, was due to fractionation during the nitrificationdenitrification loop in the sediments.…”

“…Benthic denitrification (BD) is observed to have a much lower net fractionation factor (ε BD = 0-3 ‰), which is attributed to near complete NO − 3 utilization in pore-water sediments (Brandes and Devol, 2002;Lehmann et al, 2004). However, recent studies (Lehmann et al, 2007;Granger et al, 2011;Alkhatib et al, 2012) investigating fractionation involved with the nitrification-denitrification loop in the sediments suggest a larger net fractionation factor (ε BD = 4-8 ‰).…”

Nitrogen isotope dynamics and fractionation during sedimentary denitrification in Boknis Eck, Baltic Sea

“…In sediments, the reaction rate is limited by diffusion, and, accordingly, this fractionation effect is not visible in the apparent fractionation during sedimentary denitrification (e.g., Brandes and Devol, 2002). Recent studies (Lehmann et al, 2007;Alkhatib et al, 2012) propose that the intrinsic isotope effect of denitrification in sediments is comparable to that of water column denitrification.…”