Nitrate Pool Research Articles

Lateral redox variability in ca. 1.9 Ga marine environments indicated by organic carbon and nitrogen isotope compositions.

The stepwise oxygenation of Earth's surficial environment is thought to have shaped the evolutionary history of life. Microfossil records and molecular clocks suggest eukaryotes appeared during the Paleoproterozoic, perhaps shortly after the Great Oxidation Episode at ca. 2.43 Ga. The mildly oxygenated atmosphere and surface oceans likely contributed to the early evolution of eukaryotes. However, the principal trigger for the eukaryote appearance and a potential factor for their delayed expansion (i.e., intermediate ocean redox conditions until the Neoproterozoic) remain poorly understood, largely owing to a lack of constraints on marine and terrestrial nutrient cycling. Here, we analyzed redox-sensitive element contents and organic carbon and nitrogen isotope compositions of relatively low metamorphic-grade (greenschist facies) black shales preserved in the Flin Flon Belt of central Canada to examine open-marine redox conditions and biological activity around the ca. 1.9 Ga Flin Flon oceanic island arc. The black shale samples were collected from the Reed Lake area in the eastern part of the Flin Flon Belt, and the depositional site was likely distal from the Archean cratons. The black shales have low Al/Ti ratios and are slightly depleted in light rare-earth elements relative to the post-Archean average shale, which is consistent with a limited contribution from felsic igneous rocks in Archean upper continental crust. Redox conditions have likely varied between suboxic and euxinic at the depositional site of the studied section, as suggested by variable U/Al and Mo/Al ratios. Organic carbon and nitrogen isotope compositions of the black shales are approximately -23‰ and +13.7‰, respectively, and these values are systematically higher than those of broadly coeval continental margin deposits (approximately -30‰ for δ13Corg and +5‰ for δ15Nbulk). These elevated values are indicative of high productivity that led to enhanced denitrification (i.e., a high denitrification rate relative to nitrogen influx at the depositional site). Similar geochemical patterns have also been observed in the modern Peruvian oxygen minimum zone where dissolved nitrogen compounds are actively lost from the reservoir via denitrification and anammox, but the large nitrate reservoir of the deep ocean prevents exhaustion of the surface nitrate pool. Nitrogen must have been widely bioavailable in the ca. 1.9 Ga oceans, and its supply to upwelling zones must have supported habitable environments for eukaryotes, even in the middle of oceans around island arcs.

Key determinants of soil labile nitrogen changes under climate change in the Arctic: A meta-analysis of the responses of soil labile nitrogen pools to experimental warming and snow addition

The Arctic terrestrial ecosystems are undergoing rapid climate change, causing shifts in the dynamics of soil nitrogen (N), a pivotal but relatively underexplored component. To understand the impacts of climate change on soil labile N pools, we performed meta- and decision-tree analyses of 391 observations from 38 peer-reviewed publications across the Arctic, focusing on experimental warming and snow addition. Soil dissolved organic nitrogen (DON), ammonium (NH4+), and nitrate (NO3-) pools under experimental warming exhibited overall standard mean differences (SMDs) ranging from −0.08 to 0.02, with no significance (P > 0.05); however, specific conditions led to significant changes. The key determinants of soil labile N responses to warming were experimental duration and mean annual summer temperature for DON; annual precipitation, soil moisture, and sampling timing for NH4+; and soil layer for NO3-. Snow addition significantly increased all labile N pools (overall SMD = 0.23–0.36; P < 0.05), influenced by factors such as sampling timing and vegetation type for DON; experimental duration and soil moisture for NH4+; and soil pH for NO3-. By consolidating and reprocessing datasets, we not only showed the overall responses of soil labile N pools to climate manipulation experiments in Arctic tundra ecosystems but also identified key determinants for changes in soil N pools among environmental and experimental variables. Our findings demonstrate that warming and snow-cover changes significantly affect soil labile N pools, highlighting how the unique environmental characteristics of different sites influence terrestrial N cycling and underscoring the complexity of Arctic N dynamics under climate change.

Hydrological and Biogeochemical Controls of the Seasonality of Particulate and Dissolved Nitrogen Exports During Rainfall Events From a Forested Watershed in Monsoon Asia, Japan

AbstractFew studies have investigated the effects of hydrological conditions such as rainfall magnitude and soil nitrogen dynamics on nitrogen export from forests during rainfall events. In this study, the seasonal export of particulate and dissolved nitrogen during 24 rainfall events (range: 3.0–417 mm) in a temperate monsoon forest watershed in Japan was measured along with the seasonality of the soil nitrate () pool size and the nitrification rate. The increase in nitrogen export with increasing streamflow was larger for particulate nitrogen than for and dissolved organic nitrogen, but was always the most abundant nitrogen component exported, even during extreme rainfall. For all rainfall events in summer, measured exports were higher than the averaged exports, regardless of rainfall magnitude or preceding rainfall. Nitrification activity in the soil was high in summer because of the high temperature and wet conditions of the soil. Nitrification also occurred in the upper slope in summer, whereas during other seasons it is hindered by dry conditions. These results indicate that nitrogen dynamics in the soil also affect export during rainfall. Comparisons with previous studies suggested that the effects of soil nitrogen dynamics on concentrations in stream water may be stronger during rainfall than during base flow conditions. They also showed that regions with wet summers due to rainfall are more sensitive to export, compared with other regions, and that increased summer rainfall can have a particularly large impact on nitrogen export from forest watersheds.

Simulation and optimization of nutrient uptake and biomass formation using a multi-parameter Monod-type model of tobacco BY-2 cell suspension cultures in a stirred-tank bioreactor.

Tobacco (Nicotiana tabacum) cv Bright Yellow-2 (BY-2) cell suspension cultures enable the rapid production of complex protein-based biopharmaceuticals but currently achieve low volumetric productivity due to slow biomass formation. The biomass yield can be improved with tailored media, which can be designed either by laborious trial-and-error experiments or systematic, rational design using mechanistic models, linking nutrient consumption and biomass formation. Here we developed an iterative experiment-modeling-optimization workflow to gradually refine such a model and its predictions, based on collected data concerning BY-2 cell macronutrient consumption (sucrose, ammonium, nitrate and phosphate) and biomass formation. The biomass formation was well predicted by an unstructured segregated mechanistic Monod-type model as long as the nutrient concentrations did not approach zero (we omitted phosphate, which was completely depleted). Multi-criteria optimization for sucrose and biomass formation indicated the best tradeoff (in a Paretian sense) between maximum biomass yield and minimum process time by reducing the initial sucrose concentration, whereas the inoculation biomass could be increased to maximize the biomass yield or minimize the process time, which we confirmed in calibration experiments. The model became inaccurate at biomass densities > 8 g L-1 dry mass when sucrose was almost depleted. We compensated for this limitation by including glucose and fructose as sucrose hydrolysis products in the model. The remaining offset between the simulation and experimental data might be resolved by including intracellular pools of sucrose, ammonium, nitrate and phosphate. Overall, we demonstrated that iterative models can be used to systematically optimize conditions for bioreactor-based processes.

Leveraging microbiome rediversification for the ecological rescue of soil function

BackgroundGlobal biodiversity losses threaten ecosystem services and can impact important functional insurance in a changing world. Microbial diversity and function can become depleted in agricultural systems and attempts to rediversify agricultural soils rely on either targeted microbial introductions or retaining natural lands as biodiversity reservoirs. As many soil functions are provided by a combination of microbial taxa, rather than outsized impacts by single taxa, such functions may benefit more from diverse microbiome additions than additions of individual commercial strains. In this study, we measured the impact of soil microbial diversity loss and rediversification (i.e. rescue) on nitrification by quantifying ammonium and nitrate pools. We manipulated microbial assemblages in two distinct soil types, an agricultural and a forest soil, with a dilution-to-extinction approach and performed a microbiome rediversification experiment by re-introducing microorganisms lost from the dilution. A microbiome water control was included to act as a reference point. We assessed disruption and potential restoration of (1) nitrification, (2) bacterial and fungal composition through 16S rRNA gene and fungal ITS amplicon sequencing and (3) functional genes through shotgun metagenomic sequencing on a subset of samples.ResultsDisruption of nitrification corresponded with diversity loss, but nitrification was successfully rescued in the rediversification experiment when high diversity inocula were introduced. Bacterial composition clustered into groups based on high and low diversity inocula. Metagenomic data showed that genes responsible for the conversion of nitrite to nitrate and taxa associated with nitrogen metabolism were absent in the low diversity inocula microcosms but were rescued with high diversity introductions.ConclusionsIn contrast to some previous work, our data suggest that soil functions can be rescued by diverse microbiome additions, but that the concentration of the microbial inoculum is important. By understanding how microbial rediversification impacts soil microbiome performance, we can further our toolkit for microbial management in human-controlled systems in order to restore depleted microbial functions.

Iron-mediated anaerobic ammonium oxidation recorded in the early Archean ferruginous ocean.

The nitrogen isotopic composition of organic matter is controlled by metabolic activity and redox speciation and has therefore largely been used to uncover the early evolution of life and ocean oxygenation. Specifically, positive δ15 N values found in well-preserved sedimentary rocks are often interpreted as reflecting the stability of a nitrate pool sustained by water column partial oxygenation. This study adds much-needed data to the sparse Paleoarchean record, providing carbon and nitrogen concentrations and isotopic compositions for more than fifty samples from the 3.4 Ga Buck Reef Chert sedimentary deposit (BRC, Barberton Greenstone Belt). In the overall anoxic and ferruginous conditions of the BRC depositional environment, these samples yield positive δ15 N values up to +6.1‰. We argue that without a stable pool of nitrates, these values are best explained by non-quantitative oxidation of ammonium via the Feammox pathway, a metabolic co-cycling between iron and nitrogen through the oxidation of ammonium in the presence of iron oxides. Our data contribute to the understanding of how the nitrogen cycle operated under reducing, anoxic, and ferruginous conditions, which are relevant to most of the Archean. Most importantly, they invite to carefully consider the meaning of positive δ15 N signatures in Archean sediments.

Revisiting process-based simulations of soil nitrite dynamics: Tighter cycling between nitrite and nitrate than considered previously

Nitrite is an important precursor of many environmentally hazardous compounds (e.g., nitrate, nitrous oxide, and nitrous acid). However, its dynamics in the soil environment are not yet fully understood. The NtraceNitrite tool has been successful in analyzing 15N tracing data. Here, based on a 15N tracing experiment (under aerobic condition) where either the nitrite, the nitrate, or the ammonium pool was labelled, we developed an extended model (NO2Trace), which was featured by the addition of coupled nitrate reduction and nitrite re-oxidation and the separation of the nitrate pool in two sub-pools. With 5 additional parameters optimized, NO2Trace was able to achieve a superior fit to the data, as compared to the NtraceNitrite tool. The additional features might offer a suitable explanation for the isotopic composition of nitrate produced via nitrification in terrestrial ecosystems. Our results carry two important implications: (i) a key assumption of the classical isotope pool dilution technique (i.e., no reflux of tracer) for estimating gross nitrate fluxes is violated, leading to considerable underestimations (22–99% in the datasets tested); (ii) re-oxidation can dominate the consumption (∼75%) of nitrite derived from nitrate reduction, indicating the potential of this process as a target for nitrogen retention mechanism against gaseous nitrogen losses (through nitrite reduction). The additional features of the extended model show a tighter cycle between soil nitrite and nitrate than considered previously and provide a more comprehensive description of soil nitrite transformations. This study also highlights that more work is needed to develop methods capable of separating process- and pathways-specific nitrate and nitrite pools.

Nitrate isotope investigations reveal future impacts of climate change on nitrogen inputs and cycling in Arctic fjords: Kongsfjorden and Rijpfjorden (Svalbard)

Abstract. Ongoing climate change in the Arctic has caused tidewater glaciers to retreat while increasing the discharge of freshwater and terrestrial material into fjords. This can affect both nutrient inputs and cycling within the fjord systems. In particular, tidewater glaciers and the presence of associated subglacial meltwater plumes can have a large impact on fjord circulation and biogeochemistry. In this study, we assess the influence of tidewater glaciers on nitrogen inputs and cycling in two fjords in Svalbard during the summer using stable isotopic analyses of dissolved nitrate (δ15N and δ18O) in combination with nutrient and hydrographic data. Kongsfjorden receives inputs from tidewater glaciers, whereas Rijpfjorden mainly receives surface inputs from land-terminating glaciers. Results showed that both fjords are enriched in nutrients from terrestrial inputs. Nutrient ratios indicate excess Si and P relative to N. In both fjords, terrestrial nitrate from snowpack and glacier melting are identified as the dominant sources based on high δ18O-NO3- and low δ15N-NO3- of dissolved nitrate. In Kongsfjorden, mixed-layer nitrate is completely consumed within the fjord system, which we attribute to vigorous circulation at the glacial front influenced by the subglacial plume and longer residence time in the fjord. This is in contrast to Rijpfjorden where nutrients are only partially consumed perhaps due to surface river discharge and light limitation. In Kongsfjorden, we estimate terrestrial and marine N contributions to the nitrate pool from nitrogen isotopic values (δ15N-NO3-), and this suggests that nearly half the nitrate in the subglacial plume (50 ± 3 %) and the water column (44 ± 3 %) originates from terrestrial sources. We show that terrestrial N contributes significantly to the regenerated N pool (63 %–88 %) within this fjord suggesting its importance in sustaining productivity here. Given this importance of terrestrial nutrient sources within the fjords, increase in these inputs due to climate change can enhance the fjord nutrient inventory, productivity and nutrient export offshore. Specifically, increasing Atlantification and warmer Atlantic Water will encourage tidewater glacier retreat and in turn increase surface discharge. In fjords akin to Rijpfjorden this is expected to foster more light limitation and less dynamic circulation, ultimately aiding the export of nutrients offshore contributing to coastal productivity. Climate change scenarios postulated for fjords such as Kongsfjorden include more terrestrial N-fuelled productivity and N cycling within the fjord, less vigorous circulation due to the retreat of tidewater glaciers, and the expansion of oxygen-depleted deep waters isolated by the sill.

Nitrate sink function of riparian zones induced by river stage fluctuations

River stage fluctuation (RSF) induced by tides, dam releases, or storms may lead to enhanced nitrogen cycling (N cycling) in riparian zones (RZ). We conducted a laboratory water table manipulation experiment and applied a multiphase flow and transport model (TOUGHREACT) to investigate the role of RSF in N cycling in the RZ. Coupled nitrification and denitrification occur in the water table fluctuation zone under alternating aerobic and anaerobic conditions. Nitrate removal was enhanced in the saturated and unsaturated zones of the RZ. The net nitrate reduction rate in groundwater increased with the increasing number of RSF, as a result, the cumulative water influx. The RZ nitrate sink function became stronger with increasing RSF amplitude and soil organic matter (SOM) content, and weaker with increasing [NH4+]/[NO3−] ratio and mineralization rate. RSF generally creates a hot moment for net nitrate removal in the RZ. However, a hot moment for the nitrate source function might occur if the [NH4+]/[NO3−] ratio of groundwater is much greater than 1 and/or if a large pool of nitrate accumulates in the topsoil over a prolonged dry period. The absence of oxygen diffusion in the model would overestimate the nitrate removal capacity of the RZ.

Microbial Denitrification: Active Site and Reaction Path Models Predict New Isotopic Fingerprints.

The study of isotopic fingerprints in nitrate (δ15N, δ18O, Δ17O) has enabled pivotal insights into the global nitrogen cycle and revealed new knowledge gaps. Measuring populations of isotopic homologs of intact NO3 - ions (isotopologues) shows promise to advance the understanding of nitrogen cycling processes; however, we need new theory and predictions to guide laboratory experiments and field studies. We investigated the hypothesis that the isotopic composition of the residual nitrate pool is controlled by the N-O bond-breaking step in Nar dissimilatory nitrate reductase using molecular models of the enzyme active sites and associated kinetic isotope effects (KIEs). We integrated the molecular model results into reaction path models representing the reduction of nitrate under either closed-system or steady-state conditions. The predicted intrinsic KIE (15ε and 18ε) of the Nar active site matches observed fractionations in both culture and environmental studies. This is what would be expected if the isotopic composition of marine nitrate were controlled by dissimilatory nitrate reduction by Nar. For a closed system, the molecular models predict a pronounced negative 15N-18O clumping anomaly in residual nitrate. This signal could encode information about the amount of nitrate consumed in a closed system and thus constrain initial nitrate concentration and its isotopic composition. Similar clumped isotope anomalies can potentially be used to distinguish whether a system is open or closed to new nitrate addition. These mechanistic predictions can be tested and refined in combination with emerging ESI-Orbitrap measurements.

A stable and moderate nitrate pool in largely anoxic Mesoproterozoic oceans and implications for eukaryote evolution

The Mesoproterozoic (1.6–1.0 Ga) is a particularly important interval in Earth history that witnessed the initial radiation of eukaryotic life. Ocean anoxia and nutrient limitation has been invoked to explain the delayed expansion of more complex, multicellular eukaryotes during the Mesoproterozoic. However, pulsed oxygenation events and elevated nitrate availability have been identified in shallow marine settings. In deeper waters, however, nitrate availability and potential links to eukaryotic evolution remain poorly constrained. Here, we report an integrated geochemical study based on multiple proxies conducted on two new drill-core sections from the ∼1460 Ma Hongshuizhuang Formation and the ∼1380 Ma Xiamaling Formation, North China. Iron speciation and redox-sensitive trace element systematics suggest dominantly ferruginous to weakly euxinic water column conditions during deposition of both units. Our data are consistent with previous inferences of persistently anoxic deep water conditions in Mesoproterozoic oceans. Exclusively positive δ15N (+3.1 ± 0.8 ‰) values reveal the operation of partial denitrification in the water column, implying a relatively stable and moderate nitrate pool in offshore surface waters. Furthermore, a compilation of the Mesoproterozoic sedimentary δ15N record suggests a global nearshore to offshore oceanic nitrate gradient, with minimal but still bioavailable nitrate in offshore environments. The overall size of the nitrate pool was therefore apparently smaller than that of eukaryote-dominated modern oceans. Coupling Mesoproterozoic sedimentary δ15N data with fossil and biomarker records, we propose that this moderate level of nitrate may have been able to sustain a relatively low abundance of eukaryotic primary producers in the marine ecosystem, but was still not sufficient to trigger eukaryotic diversification and the rise of eukaryotes to ecological dominance.

In-vivo protein nitration facilitates Vibrio cholerae cell survival under anaerobic, nutrient deprived conditions

Protein tyrosine nitration (PTN), a highly selective post translational modification, occurs in both prokaryotic and eukaryotic cells under nitrosative stress. However, its physiological function is not yet clear. Like many gut pathogens, Vibrio cholerae also faces nitrosative stress, which makes its proteome more vulnerable to PTN. Here, we report for the first time in-vivo PTN in V. cholerae by immunoblotting and LC-ESI-MS/MS proteomic analysis. Our results indicated that in-vivo PTN in V. cholerae was culture media independent. Surprisingly, in-vivo PTN was reduced in V. cholerae proteome under anaerobic or hypoxic condition in a nutrient deprived state. Interestingly, intracellular nitrate content was more than the nitrite content in V. cholerae under anaerobic conditions. Additionally, biochemical measurement of GSH/GSSG ratio, activities of catalase and SOD, ROS and RNS imaging by confocal microscopy confirmed a relative intracellular oxidizing environment in V. cholerae under anaerobic conditions. This altered redox environment favors the oxidation of nitrite which may be generated from protein denitration enriching the intracellular nitrate pool. The cell survival of V. cholerae can finally be facilitated by nitrate reductase (NapA) utilizing that nitrate pool. Our cell viability study using wild type and ΔnapA strain of V. cholerae also supported the role of NapA mediated cell survival under nutrient deprived anaerobic conditions. In spite of having nitrate reductase (NapA), V. cholerae lacks any nitrite reductase (NiR). Hence, in-vivo nitration may provide an avenue for toxic nitrite storage and also may help in nitrosative stress tolerance mechanism preventing further unnecessary protein nitration in V. cholerae proteome.

Ammonium and Sulfate Assimilation Is Widespread in Benthic Foraminifera

Nitrogen and sulfur are key elements in the biogeochemical cycles of marine ecosystems to which benthic foraminifera contribute significantly. Yet, cell-specific assimilation of ammonium, nitrate and sulfate by these protists is poorly characterized and understood across their wide range of species-specific trophic strategies. For example, detailed knowledge about ammonium and sulfate assimilation pathways is lacking and although some benthic foraminifera are known to maintain intracellular pools of nitrate and/or to denitrify, the potential use of nitrate-derived nitrogen for anabolic processes has not been systematically studied. In the present study, NanoSIMS isotopic imaging correlated with transmission electron microscopy was used to trace the incorporation of isotopically labeled inorganic nitrogen (ammonium or nitrate) and sulfate into the biomass of twelve benthic foraminiferal species from different marine environments. On timescales of twenty hours, no detectable 15N-enrichments from nitrate assimilation were observed in species known to perform denitrification, indicating that, while denitrifying foraminifera store intra-cellular nitrate, they do not use nitrate-derived nitrogen to build their biomass. Assimilation of both ammonium and sulfate, with corresponding 15N and 34S-enrichments, were observed in all species investigated (with some individual exceptions for sulfate). Assimilation of ammonium and sulfate thus can be considered widespread among benthic foraminifera. These metabolic capacities may help to underpin the ability of benthic foraminifera to colonize highly diverse marine habitats.

Intracellular nitrate storage by diatoms can be an important nitrogen pool in freshwater and marine ecosystems

Identifying and quantifying nitrogen pools is essential for understanding the nitrogen cycle in aquatic ecosystems. The ubiquitous diatoms represent an overlooked nitrate pool as they can accumulate nitrate intracellularly and utilize it for nitrogen assimilation, dissipation of excess photosynthetic energy, and Dissimilatory Nitrate Reduction to Ammonium (DNRA). Here, we document the global co-occurrence of diatoms and intracellular nitrate in phototrophic microbial communities in freshwater (n = 69), coastal (n = 44), and open marine (n = 4) habitats. Diatom abundance and total intracellular nitrate contents in water columns, sediments, microbial mats, and epilithic biofilms were highly significantly correlated. In contrast, diatom community composition had only a marginal influence on total intracellular nitrate contents. Nitrate concentrations inside diatom cells exceeded ambient nitrate concentrations ∼100–4000-fold. The collective intracellular nitrate pool of the diatom community accounted for <1% of total nitrate in pelagic habitats and 65–95% in benthic habitats. Accordingly, nitrate-storing diatoms are emerging as significant contributors to benthic nitrogen cycling, in particular through Dissimilatory Nitrate Reduction to Ammonium activity under anoxic conditions.

The influence of African elephants on litter and soil nitrogen attributes in mopane woodland in Hwange National Park, northwest Zimbabwe

Little is known about modification of nutrient fluxes through elephant-induced woodland change. We assessed litter quality, soil N attributes, and N transformations in five 20 × 20 m plots each established on sites of low, medium, and high elephant utilization categorized using dung-count surveys in three mopane woodland patches in Hwange National Park, Zimbabwe. Litter standing crop significantly declined but litter N, lignin, lignin:N, condensed tannins, and total phenols increased with increasing elephant utilization, peaking at intermediate levels of elephant utilization. Medium elephant utilization sites had more than twice the nitrate pools in low and high elephant utilization sites (p < 0.001). Ammonium concentration at low elephant utilization was lower by 65% and 92% than at high and medium elephant utilization, respectively. Soil moisture, infiltration rates, N mineralization, and nitrification decreased with increasing elephant utilization. Nitrogen mineralization and nitrification were up to four times greater in low than in medium and high elephant utilization sites. Ammonium, N mineralization, and nitrification were positively correlated to litter N but negatively correlated to condensed tannins. These results indicate that elephant-induced woodland change plays an important role in carbon and nutrient fluxes potentially increasing resources heterogeneity and reinforcing the patch dynamics of savanna.

Comparative Study of Nitrogen Cycling Between a Bay With Riverine Input and a Bay Without Riverine Input, Inferred From Stable Isotopes

The eutrophication degree in the coastal bay has been increasing significantly for the past years, due to the increasing nutrient discharge. However, the factors controlling sources and nitrogen (N) cycling in the different types of bays are still poorly understood. In this comparative study, nitrate dual isotopes (δ15N−NO3− and δ18O−NO3−) and ammonium nitrogen isotope (δ15N−NH4+), were used to determine the origin of nitrate in Qinzhou Bay (with riverine input) and Tieshangang Bay (without riverine input) in Beibu Gulf and to study biogeochemical processes associated with nitrogen cycling. The nutrient concentrations generally showed a decreased trend from the upper bay to the outer bay. The isotopic values in Tieshangang Bay were generally higher than that in the Qinzhou Bay, suggesting that there are differences in nitrate sources and transformation processes between the two bays. The dominant sources from manure and sewage (41%) and soil N (30%) from runoff input were responsible for the high nitrate observed in the upper Qinzhou Bay. Though manure and sewage (59%) were also dominant nitrate sources in the upper Tieshangang Bay, a decrease in source from soil N (20%) occurred due to less runoff input. Nutrients were retained in the upper Tieshangang Bay due to weak hydrodynamic conditions, which caused higher NH4+ concentrations in the upper bay. Significant nitrate loss occurred in the outer Qinzhou Bay, which was related to the intense hydrodynamic condition. Moreover, phytoplankton assimilation mainly utilized NH4+ due to sufficient NH4+ in the outer Qinzhou Bay. In contrast, nitrate loss was also found in the outer Tieshangang Bay, which is mainly related to the phytoplankton assimilation due to the weak hydrodynamic condition. In addition, the greater enrichment of δ18O−NO3− than δ15N−NO3− during both bays suggests that atmospheric deposition also contributes to the nitrate pool in the water and the impact of atmospheric deposition on the whole Beibu Gulf is relatively consistent. By this comparative study, we found that different nitrate biogeochemical processes occurred in these two types of bays, which were mainly determined by the difference of human activities and hydrodynamic conditions.

A nitrate budget of the Bohai Sea based on an isotope mass balance model

Abstract. The Bohai Sea (BHS) is a semi-closed marginal sea impacted by one of the most populated areas of China. The supply of nutrients, markedly that of reactive nitrogen, via fluvial and atmospheric transport has strongly increased in parallel with the growing population. Therefore, it is crucial to quantify the reactive nitrogen input to the BHS and understand the processes and determine the quantities of nitrogen eliminated in and exported from the BHS. The nitrogen budget and in particular the internal sources and sinks of nitrate were constrained by using a mass-based and dual stable isotope approach based on δ15N and δ18O of nitrate (NO3-). Samples of water, suspended matter, and sediments were taken in the BHS in spring (March and April) and summer (July and August) 2018. The Yellow River (YR) was sampled in May and July to November, and Daliao River, Hai River, Luan River, and Xiaoqing River were sampled in November of 2018. In addition to nutrient, particulate organic carbon, and nitrogen concentrations, the dual isotopes of nitrate (δ15N and δ18O), δ15N of suspended matters, and sediments were determined. Based on the available mass fluxes and isotope data an updated nitrogen budget is proposed. Compared to previous estimates, it is more complete and includes the impact of interior cycling (nitrification) on the nitrate pool. The main nitrate sources are rivers contributing 19.2 %–25.6 % and the combined terrestrial runoff (including submarine fresh groundwater discharge of nitrate) accounting for 27.8 %–37.1 % of the nitrate input to the BHS, while atmospheric input contributes 6.9 %–22.2 % to total nitrate. An unusually active interior nitrogen cycling contributes 40.7 %–65.3 % to total nitrate via nitrification. Nitrogen is mainly trapped in the BHS and mainly removed by sedimentation (70.4 %–77.8 %), and only very little is exported to the Yellow Sea (YS) (only 1.8 %–2.4 %). At present denitrification is active in the sediments and removes 20.4 %–27.2 % of nitrate from the pool. However, a further eutrophication of the BHS could induce water column hypoxia and denitrification, as is increasingly observed in other marginal seas and seasonally off river mouths.

Water Mass Structure and Mixing Fractions in the Deepwater Region of the Gulf of Mexico

AbstractHere, we estimate water mass fractions in the deepwater region of the southern Gulf of Mexico from samples collected during June 2016. Past water mass characterizations have relied mainly on temperature, salinity, and dissolved oxygen concentrations, although the degree of mixing and the effects on the distribution of nutrients and other dissolved constituents remain uncertain. Based on nine water types defined in the eastern Caribbean Sea and North Atlantic and excluding surface waters, five dominant water masses with contributions between 50% and 93% in their cores were identified. North Atlantic Subtropical Underwater contributed 42% ± 9% between 49 and 105 m. Modal water with origins in the northern subtropical gyre (Eighteen Degree Water) were ubiquitous in the gulf, with average contributions of 62% ± 25% between 110 and 153 m and 51% ± 12% between 248 and 306 m, while the core of Tropical Atlantic Central Water (TACW; 70% ± 5%) dominated between 355 and 510 m. Water at the nitrate maximum consisted of TACW (48% ± 13%) and Antarctic Intermediate Water (AAIW; 32 ± 14%), with the local remineralization of organic matter adding ∼10% to the nitrate pool. Below ∼800 m, water transitioned from AAIW to a domain of two components of Labrador Seawater. Mesoscale eddies controlled the observed depth variability of the water masses from the surface down to ∼1,000 m. This study provides a quantitative approximation to evaluate the addition of nutrients to the different water masses by biogeochemical processes driven by an epipelagic ecosystem generally considered to be oligotrophic.

Anion Exchange Capacity Explains Deep Soil Nitrate Accumulation in Brazilian Amazon Croplands

Deep tropical soils with net anion exchange capacity can adsorb nitrate and might delay the eutrophication of surface waters that is often associated with many temperate croplands. We investigated anion exchange capacity and soil nitrate pools in deep soils in the Southern Brazilian Amazon, where conversion of tropical forest and Cerrado to intensive fertilized soybean and soybean-maize cropping expanded rapidly in the 2000s. We found that mean soil nitrate pools in the top 8 m increased from 143 kg N ha−1 in forest to 1,052 in soybean and 1,161 kg N ha−1 in soybean-maize croplands. This nitrate accumulation in croplands aligned with the estimated N surpluses in the croplands. Soil anion exchange capacity explained the magnitude of nitrate accumulation. High nitrate retention in soils was consistent with current low levels of streamwater nitrate exported from croplands. Soil exchange sites were far from saturation, which suggests that nitrate accumulation can continue for longer under current cropping practices, although mechanisms such as competition with other anions and preferential water flowpaths that bypass exchange sites could reduce the time to saturation.

Isotopic and biogenic silica evidence for eutrophic conditions in South China during the Frasnian-Famennian biotic crisis

The Late Devonian Frasnian-Famennian (F F) transition at ca. 367 Ma was a key event in Phanerozoic Eon involving a massive biotic crisis, which coincided with a positive carbon isotope anomaly and large climate changes associated with eustatic fluctuations. The details underpinning the initiation of the mass extinction, however, remain problematic. To better understand this scientific issue, we conducted a time series of biogeochemical studies that included C N isotopes and biogenic silica (BSi) from the F F transition in the Yangdi section, South China. The carbon isotope data accurately constrain the Upper Kellwasser Horizon (UKH) and Lower Kellwasser Horizon (LKH) in this section. A short-term decrease in carbon isotope fractionation in the F F transition indicates a decreased p CO 2 level in the LKU and UKH, while the increased fractionation in the uppermost UKH was caused by the rapidly negative δ 13 C org excursion, likely as the result of enhanced inputs from chemoautotrophs. On the other hand, the nitrogen isotope data suggest that the surface water had limited availability of dissolved N before and after the F F boundary due to enhanced benthic denitrification; these conditions led to a supply of external nitrogen that was likely driven by persistent diazotrophy. Increased sedimentary δ 15 N (up to +7.3‰) during the UKH suggests a rapidly amplified nitrate pool associated with water column denitrification, implying intensification of marine productivity and expansion of dysoxic-anoxic conditions linked to intensified oceanic circulation. In addition, BSi in sediments has the potential to evaluate historic changes in primary productivity of oceans and the maxima of BSi mass values (up to 1.96%) are clearly documented in the upper part of the UKH. Insofar as the elevated abundance of biolimited nitrogen coincided with strongly increased marine phytoplankton productivity at the F F boundary, we speculate that the nutrient-driven eutrophic conditions in surface waters associated with attendant redox changes may have been the main killing mechanism in the mass extinction. • The cooling event in the UKH coincided with declined △ 13 C and elevated δ 15 N. • We attribute the low δ 15 N values to enhanced microbial nitrogen fixation. • Sedimentary δ 15 N increases are indicative of an amplified N pool. • Siliceous organism blooms are consistent with positive δ 13 C carb and δ 15 N shifts. • The F F crisis was likely the result of eutrophication in shallow water.