Spatial variations in sedimentary N-transformation rates in 1 the North Sea (German Bight)

In this study, we investigate the role of sedimentary N cycling in the Southern North Sea. We present a budget of 13 ammonification, nitrification and sedimentary NO 3 - consumption / denitrification in contrasting sediment types 14 of the German Bight (Southern North Sea), including novel net ammonification rates. We incubated sediment 15 cores from four representative locations in the German Bight (permeable, semi-permeable and impermeable 16 sediments) with labeled nitrate and ammonium to calculate benthic fluxes of nitrate and ammonium and gross 17 rates of ammonification and nitrification. Ammonium fluxes generally suggest oxic degradation of organic 18 matter, but elevated fluxes at one sampling site point towards the importance of bio-irrigation or short-term 19 accumulation of organic matter. Sedimentary fluxes of dissolved inorganic nitrogen are an important source for 20 primary producers in the water column, supporting ~7 to 59 % of the average annual primary production, 21 depending on water depth. We find that ammonification and oxygen penetration depth are the main drivers of sedimentary nitrification, but 23 this nitrification is closely linked to denitrification. One third of freshly produced nitrate in impermeable sediment 24 and two-thirds in permeable sediment were reduced to N 2 . The semi-permeable and permeable sediments are 25 responsible for ~68 % of the total benthic N 2 production rates, which, based solely on our data, amounts to ~1030 N


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The continental shelves and coastal margins make up for <9 % of the total area of ocean surface, but are responsible 31 for vast majority of the biogeochemical cycling both in the water column and in the sediments (Jorgensen, 1983).

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For instance, 30 % of global marine primary production occurs in coastal, estuarine and shelf systems (LOICZ, 33 1995), and nutrient regulation in shelf sediments is a particularly valuable ecosystem service (Costanza et al.,

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Nitrogen availability increases primary production on a variety of spatial and temporal scales. At present, major 44 nitrogen sources for the Southern North Sea are agricultural and urban waste water, and to a lesser extent, a variety 45 of reactive N emission (e.g., nitrogen oxides from burning fossil) (Emeis et al., 2015).

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Internal N cycling in sediments (e.g., assimilation, ammonification and nitrification) change the distribution and 47 speciation of fixed N, but not the overall amount of N available for primary production (Casciotti, 2016). Reduction 48 of reactive nitrogen through denitrification and anammox in anoxic conditions back to unreactive N2, however, 49 does remove N from the biogeochemical cycle (Neumann et al., 2017).

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Because these eliminating processes are confined to suboxic and anoxic conditions, they only occur in sediments 51 in the generally oxygenated North Sea. Due to its putative relevance as an ecosystem service, denitrification has 52 been subject to many studies, but ammonification as a source of N to primary production so far received much less 53 attention. This is in part due to the complexity created by coupled ammonification-nitrification in which different 54 N processes, such as assimilation and denitrification, interact and affect the NH4 + and NO3concentrations in pore 55 waters. To our knowledge, no ammonification rates in the North Sea have been quantified, whereas nitrification 56 rates in permeable sediments were found to be in the same order of magnitude as denitrification rates (<0.1 to ~3.0 57 mmol m -2 d -1 , Tab. 1) (Marchant et al., 2016). N loss in the German Bight has been studied by several authors (e.g. 58 Deek et al., 2013) showing high spatial, temporal and seasonal variability.

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The main N loss process in the North Sea is denitrification, whereas and anammox plays a minor role (Bale et al., 60 2014; Marchant et al., 2016). The main drivers of denitrification are organic matter content and permeability of 61 the sediment (Neumann, 2012), and recent studies suggest that permeable sediments account for about 90 62 % of the total benthic NO3consumption in the German Bight (Neumann et al., 2017).

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Stable isotope techniques offer several approaches to quantify N turnover processes, and 15 N tracer studies have 73 been widely used to determine N transformation rates (e.g. nitrification and denitrification) (Brase et al., 2018; 74 Sanders et al., 2018). The isotope dilution method can be used to distinguish between net and gross rates and so 75 help to unravel several N-processes such as ammonification and assimilation or nitrification and denitrification.

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15 N dilution - (Koike and Hattori, 1978;Nishio et al., 2001) can be used to estimate gross N transformation rates 77 by measuring the isotopic dilution of the substrate and product pools, respectively (e.g. Burger and Jackson, 2003).

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In this study, we used the isotope dilution method with labeled NH4 + and NO3in separate sediment cores to 79 measure gross ammonification and gross nitrification. The net rates are determined by the sediment nutrient fluxes.

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To measure denitrification we determined the produced N2 independently of the labelling in the core. Sediment 81 core incubation experiment setup can never reproduce the identical conditions related to the advective processes 82 in permeable sediments. Nevertheless this method has advantages over just balancing sediment-water exchanges:

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(1) The appearance of 15 N in the NH4 + pool during the incubation allows an estimate of ammonification rates, (2) 84 the isotopic dilution of NO3tracks nitrification rates,

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This study is conducted within the project "North Sea Observation and Assessment of Habitats" (NOAH). One 86 important aspect of the project is to investigate the biogeochemical status and functions of the sea floor, especially 87 nitrogen cycling, to gauge the eutrophication mitigation potential in light of continuing high human pressures 88 (https://www.noah-project.de).

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In this paper, we investigate internal N rates of ammonification, nitrification and denitrification at four stations 90 across sediment types (clay/silt, fine sand, coarse sand) in the German Bight (North Sea) during late summer 91 (August/September) 2016. To assess the internal sediment N processes and the rates of reactive N release to the 92 water column, we incubated sediment cores amended with 15 NH4 + and 15 NO3 -. We quantify the benthic gross and 93 net nitrification and ammonification rates and evaluate the environmental controls underlying spatial variabilities.

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We further discuss the role of ammonification as a source of reactive nitrogen for primary producers, of 95 nitrification and of denitrification in the Southern North Sea.

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The sampling sites are part of the NOAH (North Sea Assessment of Habitats) assessment scheme (Fig. 1). Samples 103 were taken from 4 site (NOAH A, C, D and E) with different water depth and sediment characteristics (Table 2).

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The sites represent typical sediment types based on statistics of granulometric properties, organic matter content,

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Samples were taken every 6 hours. Upon sampling, incubation water was filtered with a syringe filter (cellulose 126 acetate, Sartorius, 0.45 µm pore size) and frozen in exetainers (11.8 ml, Labco, High Wycombe, UK) at -20 °C for 127 later analyses of nutrients and stable isotope signatures (δ 15 NH4 + , δ 15 NO3 -). Additional samples for the analyses of 128 dissolved nitrogen (N2) were taken without filtration, and were preserved in exetainers (5.9 ml, Labco, High    145 analyzed for total carbon and total nitrogen contents with an elemental analyzer (Carlo Erba NA 1500) The total 146 organic carbon content was analyzed after removal of inorganic carbon using 1 mol L -1 hydrochloric acid. The 147 standard deviation of sediment samples was better than 0.6 % for Corg and 0.08 % for N determination.

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Permeabilty and porosity of the sediments were conducted with sediments from the cruise He-471, the methods 149 were described in detail elsewhere (Neumann, 2016).

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Water samples from core incubations were analyzed in duplicate for concentration of NH4 + , NO2and NO3using where d(C) is the oxygen, nutrient or the nitrogen (N2) concentration at the start and at the end of the experiment,

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V is the volume of the overlying water, d(t) is the incubation time and A is the surface area of the sediment..

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Positive fluxes (outflow concentrations above inflow concentrations) imply net production in the sediment.

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We measured gross ammonification rates with the isotope dilution method using 15 NH4 + as tracer, and measured 194 net ammonium fluxes with the flux method. The highest net ammonium flux and gross ammonification rates were 195 measured in the impermeable, organic-rich sediment at station NOAH-C (6.6 ±1.4 mmol N m -2 d -1 and 9.5 mmol 196 N m 2 d -1 for net flux and gross ammonification, respectively).

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Nitrification 203 Likewise to ammonification, we measured gross nitrification rates by means of the stable isotope dilution method 204 with 15 NO3as tracer, and net nitrate fluxes employing the flux method. Net fluxes and gross nitrification rates 205 varied significantly between stations. Net nitrate fluxes were highest at station NOAH-C and at station NOAH-E 206 with 1.1 ± 0.5 mmol N m -2 d -1 and 1.2 ±0.5 mmol N m -2 d -1 , respectively (Fig. 3, Fig. 5). Gross nitrification rates 207 were highest at NOAH-C (2.1 ± 0.1 mmol N m -2 d -1 ). The lowest rates of net nitrate flux (0.3 ± 0.3 mmol N m -2 d -208 1 ) and gross nitrification (1.2 ± 0.0 mmol m -2 d -1 ) were observed in the permeable sediment at station NOAH-A.

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Net and gross nitrification rates are closely correlated (r²=0.87; Fig. 3) with net nitrate fluxes being systematically 210 lower than gross nitrification rates.

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Unlike to ammonification and nitrification, we were not able to make use of the stable isotope tracers to evaluate 213 N2 production rates with an stable isotope technique because the requirements for the Isotope Pairing method 214 (Rysgaard-Petersen et al., 1996) were not met. Our N2 production estimates are thus limited to the flux method.

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A principal goal of this study was to assess the role of ammonification in the nitrogen cycle of the German Bight.

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Ammonification releases NH4 + during the decomposition of organic matter and resupplies the water-column 231 inventory of reactive nitrogen. The quantification of ammonification rates is challenging, because ammonium is 232 readily assimilated by primary producers or is rapidly nitrified, causing low ammonium concentrations and 233 necessitating to use the isotope dilution method.

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This study represents direct measured gross ammonification rates across typical sediment types of the North Sea,

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Nitrification produces NO3 -, which represents the largest DIN pool in the water column of the North Sea and is the 275 substrate for denitrification, and thus the link to an ultimate removal of fixed nitrogen from the water column.

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We observed gross nitrification rates at all four stations ranging from 1.2 ± 0.0 mmol N m -2 d -1 at the sandy station 277 NOAH-A, to 1.3 mmol N m -2 d -1 in the moderately permeable sediment at NOAH-D and to 2.1 ± 0.1 mmol N m -2 278 d -1 in the impermeable sediment at station NOAH-C (Fig. 3, Fig. 5). Gross nitrification at the impermeable

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Lowest gross nitrification rates and nitrate fluxes are found at the permeable station NOAH-A, but apart from this,

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This interplay of factors is mirrored in a clear and statistically significant (a=0.05) correlation of gross nitrification 297 and gross ammonification rates (r 2 = 0.92). Overall, the gross NO3production (1.2 to 2.1 mmol m -2 d -1 ) was small 298 relative to ammonification rates (1.9 to 9.5 mmol N m -2 d -1 ). We find that nitrification is governed by a complex

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Denitrification, the reduction of NO3to gaseous N2, reduces the pool of bioavailable N, and is therefore very 306 relevant in eutrophic coastal areas such as the southern North Sea. In our study, the measured denitrification rates 307 ranged from 1.3 to 1.9 mmol N m -2 d -1 (Fig. 5). This estimate is on the higher end of previous measurements from

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In our study, we find that this coupled nitrification-denitrification determines the total N flux. Denitrification 324 essentially removes, within the given uncertainties (Fig. 5)

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(18,800 km -2 ) are permeable sediments. They estimated that permeable sediment were the most efficient NO3sink 341 accounting for up to 90 % of the total benthic NO3consumption. In our assessment, which better represents the 342 role of nitrification, we arrive at a somewhat lower contribution of ~68 % of total denitrification occurring in 343 moderately permeable and permeable sediments. Based solely on our data, we estimate a total nitrogen removal 344 of ~1030 t N d -1 in our study area, which corresponds to an average N2 flux of approximately 1.5 mmol N m -2 d -1 .

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This daily N2 production during late summer equals the total N discharge (~1.000 t N d -1 ) by the main rivers Maas,

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Rhine, North-Sea Canal, Ems, Weser and Elbe (Pätsch and Lenhart, 2004), and, as such, underscores the role of 347 coastal sediments to counteract the eutrophication in the North Sea.

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Our assessment, however, does reflect the impact of only diffusive transport and faunal activity while not

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to arrive at an improved estimate of sediment denitrification, including nitrification as a source, but also accounting 352 for the increasing importance of advection in permeable sediments.

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In the following, we aim to put our estimates of N-transition rates into perspective by setting an upper limit of N 354 turnover based on primary production since N cycling is linked to organic carbon availability, which is ultimately

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Since benthic N recycling substantially restocks the pelagic N inventory, we further assessed the contribution of

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We evaluated a range of sedimentary nitrogen turnover pathways and found that ammonification in sediments is 407 an important N-source for primary production in the water column of the southeastern North Sea during summer.

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Depending on water depth, 7.1 to 58.7 % of the estimated water column primary production is fueled by