Nitrogen cycling in sediments on the NW African margin inferred from N and O isotopes in benthic chambers

Abstract

Benthic nitrogen cycling in the Mauritanian upwelling region (NW Africa) was studied in June 2014 from the shelf to the upper slope where minimum bottom water O2 concentrations of 25 µM were recorded. Benthic incubation chambers were deployed at 9 stations to measure fluxes of O2, dissolved inorganic carbon (DIC) and nutrients (NO3-, NO2-, NH4+, PO43-, H4SiO4) along with the N and O isotopic composition of nitrate (δ15N-NO3- and δ18O-NO3-) and ammonium (δ15N-NH4+). O2 and DIC fluxes were similar to those measured during a previous campaign in 2011 whereas NH4+ and PO43- fluxes on the shelf were 2 – 3 times higher and possibly linked to a long-term decline in bottom water O2 concentrations. The mean isotopic fractionation of NO3- uptake on the margin, inferred from the loss of NO3- inside the chambers, was 1.5 ± 0.4 ‰ for 15/14N (15ϵapp) and 2.0 ± 0.5 ‰ for 18/16O (18ϵapp). The mean 18ϵapp:15ϵapp ratio on the shelf (< 100 m) was 2.1 ± 0.3, and higher than the value of 1 expected for microbial NO3-reduction. The 15ϵapp are similar to previously reported isotope effects for NO3- respiration in marine sediments but lower than determined in 2011 at a same site on the shelf. The sediments were also a source of 15N-enriched NH4+ (9.0 ± 0.7 ‰). A numerical model tuned to the benthic flux data and that specifically accounts for the efflux of 15N-enriched NH4+ from the seafloor, predicted a net benthic isotope effect of N loss (15ϵsed) of 3.6 ‰; far above the more widely considered value of ~0‰. This result is further evidence that the assumption of a universally low or negligible benthic N isotope effect is not applicable to oxygen-deficient settings. The model further suggests that 18ϵapp:15ϵapp trajectories > 1 in the benthic chambers are most likely due to aerobic ammonium oxidation and nitrite oxidation in surface sediments rather than anammox, in agreement with published observations in the water column of oxygen deficient regions.

Physiological Influence of Fe and Cu Availability on Nitrogen Isotope Fractionation during Ammonia Oxidation

“…Depending on the physiological conditions limiting these lower rates, environmental 15 ε AO values may be even lower than values estimated from culture studies. Isotope mass balance models of modern and ancient Earth often aim to constrain rates of different N cycling processes from isotopic measurements of N species and a system of equations underpinned by a range or average 15 ε. − Of these aforementioned studies, a range from −14 to −38‰ is commonly assigned to 15 ε AO on the basis of community diversity, which is comparable to field-based estimates that report similarly wide ranges, − yet notably more difficult to constrain given co-occurring processes producing NH 4 + and NO 3 – . Our results further complicate this by demonstrating that 15 ε AO can also vary based on the rate and temperature.…”

“…111−113 For example, sediment−water interfaces are characterized by sharp redox gradients and transport processes that result in only partial consumption and thus release of highly fractionated pools. 109,114,115 Consequently, the isotopic flux of NH 4 + to the overlying water column (and the resulting NO 3 − or biomass formed) will directly relate to the magnitude of 15 ε AO . Alternatively, AO may compete with assimilative pathways, resulting in net isotope fluxes that directly relate to both pathways despite persistently low levels of NH 4 + .…”

“…While low environmental levels of NH 4 + in the widely oxygenated waters of most aquatic environments point to the quantitative consumption of NH 4 + as quickly as it is generated (and thus no expression of isotope fractionation), some environments exhibit pervasive overlap of reductive and oxidative processes. − For example, sediment–water interfaces are characterized by sharp redox gradients and transport processes that result in only partial consumption and thus release of highly fractionated pools. ,, Consequently, the isotopic flux of NH 4 + to the overlying water column (and the resulting NO 3 – or biomass formed) will directly relate to the magnitude of 15 ε AO . Alternatively, AO may compete with assimilative pathways, resulting in net isotope fluxes that directly relate to both pathways despite persistently low levels of NH 4 + . , Further still, fractionation is arguably relevant at depths where NH 4 + accumulates, and incidentally, where nitrification rates are most dynamic. , Modulation of 15 ε AO will also impact reaction byproducts, including the greenhouse gas N 2 O, the fluxes of which are widely constrained by stable isotope analyses. , Depending on the pathway of N 2 O formation, plasticity in 15 ε AO may propagate into the δ 15 N of produced N 2 O, thereby impacting the isotope mass balances of N 2 O budgets.…”

The impact of metals and other stress factors on microbial ammonia oxidation physiology and isotope effects

“…Isotope mass balance models of modern and ancient Earth often aim to constrain rates of different N cycling processes from isotopic measurements of N species and a system of equations underpinned by a range or average 15 ε. [100][101][102][103][104][105][106] Of these aforementioned studies, a range of -14 to -38‰ is commonly assigned to 15 εAO on the basis of community diversity, 32 which is comparable to field-based estimates that report similarly wide ranges, [107][108][109][110] yet notably more difficult to constrain given cooccurring processes producing NH4 + and NO3 -. Our results further complicate this by demonstrating 15 εAO can also vary based on rate and temperature.…”

“…[111][112][113] For example, sediment-water interfaces are characterized by sharp redox gradients and transport processes that result in only partial consumption, and thus release of highly fractionated pools. 109,114,115 Consequently, the isotopic flux of NH4 + to the overlying water column (and the resulting NO3or biomass formed) will directly relate to the magnitude of 15 εAO.…”