Nitrate Flux Research Articles

Groundwater dynamics and effect of tile drainage on water flow across the redox interface in a Danish Weichsel till area

Many agricultural soils are tile drained in order to lower water tables and improve crop production, but tile drains have a large impact on water flows and nutrient transport. The objectives of the study were to assess modelling requirements for describing shallow groundwater and tile drainage dynamics and to evaluate how tile drains affect groundwater flow across the redox interface, where nitrate reduction in the subsurface occurs. The study was conducted on a 33-hectare tile drained field within a Weichsel till area in Fensholt catchment, Denmark. The study consisted of 2.5 years high-frequency monitoring and numerical modelling using a variably-saturated 3D hydrological model (FEFLOW) with a detailed representation of the tile drain network. Field data showed extremely rapid increases, up to 1 m per day, in the groundwater heads when the autumn water surplus starts. Drain discharge was found to follow the dynamics in heads. High-frequency monitoring (at least once per day) is therefore necessary in order to capture the dynamics in shallow groundwater and tile discharge. The dynamics showed simultaneous rapid increase in groundwater heads down to a depth of 22 m, illustrating the key importance of deeper geology in understanding the hydrological processes in this area. The temporal dynamics in head and drain discharge were successfully captured by the hydrological model and the steepness of the retention curve was found to be important for this. The calibrated model was successfully tested against discharge in the stream draining a larger area. The model results showed, that tile drains affect the water flow in the area and significantly decrease the downward fluxes across the redox interface, located on average at 3.6 m depth. Tile drains therefore have a large effect on the amount of nitrate reduction in the subsurface and on the nitrate fluxes to receiving surface waters.

Coastal Upwelling Revisited: Ekman, Bakun, and Improved Upwelling Indices for the U.S. West Coast

AbstractCoastal upwelling is responsible for thriving marine ecosystems and fisheries that are disproportionately productive relative to their surface area, particularly in the world's major eastern boundary upwelling systems. Along oceanic eastern boundaries, equatorward wind stress and the Earth's rotation combine to drive a near‐surface layer of water offshore, a process called Ekman transport. Similarly, positive wind stress curl drives divergence in the surface Ekman layer and consequently upwelling from below, a process known as Ekman suction. In both cases, displaced water is replaced by upwelling of relatively nutrient‐rich water from below, which stimulates the growth of microscopic phytoplankton that form the base of the marine food web. Ekman theory is foundational and underlies the calculation of upwelling indices such as the “Bakun Index” that are ubiquitous in eastern boundary upwelling system studies. While generally valuable first‐order descriptions, these indices and their underlying theory provide an incomplete picture of coastal upwelling. Here we review the relevant dynamics and limitations of classical upwelling indices, particularly related to representation of the surface wind stress, the influence of geostrophic currents, and the properties of upwelled water. To address these shortcomings, we present two new upwelling indices for the U.S. West Coast (31–47°N), which are available from 1988 to present. The Coastal Upwelling Transport Index and the Biologically Effective Upwelling Transport Index provide improved estimates of vertical transport and vertical nitrate flux, respectively, by leveraging technological and scientific advances realized since the introduction of the Bakun Index nearly a half century ago.

Effects of Oral Supplementation With Nitrate-Rich Beetroot Juice in Patients With Pulmonary Arterial Hypertension—Results From BEET-PAH, an Exploratory Randomized, Double-Blind, Placebo-Controlled, Crossover Study

BackgroundThe nitrate–nitrite–nitric oxide (NO) pathway may represent a potential therapeutic target in patients with pulmonary arterial hypertension (PAH). We explored the effects of dietary nitrate supplementation, with the use of nitrate-rich beetroot juice (BRJ), in patients with PAH. Methods and ResultsWe prospectively studied 15 patients with PAH in an exploratory randomized, double-blind, placebo-controlled, crossover trial. The patients received nitrate-rich beetroot juice (∼16 mmol nitrate per day) and placebo in 2 treatment periods of 7 days each. The assessments included; exhaled NO and NO flow–independent parameters (alveolar NO and bronchial NO flux), plasma and salivary nitrate and nitrite, biomarkers and metabolites of the NO-system, N-terminal pro–B-type natriuretic peptide, echocardiography, ergospirometry, diffusing capacity of the lung for carbon monoxide, and the 6-minute walk test. Compared with placebo ingestion of BRJ resulted in increases in; fractional exhaled NO at all flow-rates, alveolar NO concentrations and bronchial NO flux, and plasma and salivary levels of nitrate and nitrite. Plasma ornithine levels decreased and indices of relative arginine availability increased after BRJ compared to placebo. A decrease in breathing frequency was observed during ergospirometry after BRJ. A tendency for an improvement in right ventricular function was observed after ingestion of BRJ. In addition a tendency for an increase in the peak power output to peak oxygen consumption ratio (W peak/VO2 peak) was observed, which became significant in patients reaching an increase of plasma nitrite >30% (responders). ConclusionsBRJ administered for 1 week increases pulmonary NO production and the relative arginine bioavailability in patients with PAH, compared with placebo. An increase in the W peak/VO2 peak ratio was observed after BRJ ingestion in plasma nitrite responders. These findings indicate that supplementation with inorganic nitrate increase NO synthase–independent NO production from the nitrate-nitrite-NO pathway.

Water and nitrate exchange between a managed river and peri-urban floodplain aquifer: Quantification and management implications

The management of rivers for navigation, hydropower and flood risk reduction involves the installation of in-channel structures. These influence river levels and can affect groundwater flow within hydraulically-connected riparian floodplain aquifers. A comprehensively monitored, peri-urban, lowland river floodplain in the southern United Kingdom was used to explore these dependencies and to examine the implications for the flux exchange of water and nitrate between the river and the floodplain alluvial aquifer. The study demonstrated that rivers maintained at high levels by management structures, result in raised groundwater levels in the adjacent aquifer and complex groundwater flow patterns. Engineered river management structures were shown to promote flow from river to aquifer through the river bed but the majority of the associated nitrate was removed in the hyporheic zone. High-nitrate groundwater recharge to the alluvial aquifer also occurred through overbank flood flows. Across the floodplain, substantial denitrification occurred due to anaerobic conditions resulting from carbon-rich sediments and the shallow water table, the latter linked to the river management structures. An upper limit on the total annual mass of nitrate removed from river water entering the floodplain aquifer was estimated for the study site (2.9 × 104 kg), which was three orders of magnitude lower than the estimate of annual in-channel nitrate flux (1.8 × 107 kg). However, this capacity of lowland floodplains to reduce groundwater nitrate concentrations has local benefits, for example for private and public water supplies sourced from alluvial aquifers. The insights from the study also have relevance for those considering schemes that include the installation, removal or redesign of river management structures, as the resultant change in groundwater levels may have consequences for floodplain meadows and the nutrient status of the aquatic system.

Simulation of Inorganic Nitrogen Fluxes at the Sediment-water Interface in a Typical Intertidal Zone, Eastern China

Taking 12 typical intertidal zones along the eastern coast of China as the research object, indoor tide simulation experiments were conducted to measure exchange fluxes of nitrate nitrogen (NO3--N) and ammonia nitrogen (NH4+-N) between overlying water and sediments, to investigate their spatial distribution, and to clarify controlling factors such as salinity, temperature, and organic matter. Results showed that the total NO3--N flux was -2.91-3.34 mmol·(m2·h)-1, while the total flux of NH4+-N was -4.36-2.34 mmol·(m2·h)-1. The average flux, at 12℃ and 35℃, was -0.04 mmol·(m2·h)-1, indicating that typical intertidal zone sediment is an effective sink for ammonia nitrogen and nitrate nitrogen. There was a significant difference in the spatial distribution of nitrate and ammonia nitrogen fluxes. At 12℃, the higher the latitude, the greater the ammonia nitrogen flux; results for the 25°-35°N intertidal nitrate flux were as follows:<15°-25°N < 35°-45°N at 25℃ and 35℃, while the flux of ammonia nitrogen was 25°N-35°N > 15°-25°N > 35°-45°N. The fluxes of the three intertidal zones decreased with increase in temperature, which controls the coupled nitrification-denitrification taking place in the upper layer of sediment and at the bottom of overlying water. NO3--N fluxes first increased and then decreased with temperature at 15°-25°N and 35°-45°N, while NO3--N fluxes at 25°-35°N always decreased with temperature. At each latitude, the higher the temperature, the lower the NH4+-N flux. There was no single significant effect of environmental factors on fluxes. Salinity, sediment organic carbon (OC), sediment total nitrogen (TN), concentrations of ammonia nitrogen and nitrate nitrogen in pore water, and bulk density synergistically affected the spatial differentiation of exchanged NO3--N and NH4+-N fluxes.

Nitrogen cycling players and processes in green roof ecosystems

Green roofs typically consist of an engineered soil paired with drought-tolerant plants installed on a rooftop. These emerging ecosystems are installed for the services they provide, which in many cases depend on plant performance. In terrestrial ecosystems, nitrogen (N) is most often the limiting nutrient for plant productivity and its availability is heavily influenced by soil microorganisms. However, little is known about the green roof microbiome or their influence on N availability. We employed 16S rRNA and nifH gene sequencing to assess the presence of nitrifying and nitrogen-fixing prokaryotes and their relationships to evidence of nitrification (runoff fluxes of nitrate and gaseous fluxes of nitrous oxide) and N fixation (presence of plants with symbiotic nitrogen-fixers) on 6 green roofs near Cincinnati, Ohio, USA. Our data indicate that green roofs in this region are a net source of reactive N: the majority of roofs released elevated nitrate levels in runoff relative to precipitation inputs, and also emitted N2O, although at low rates comparable to those in natural ecosystems. We also recovered abundant nifH genes affiliated with Rhizobiales and observed root nodules on leguminous plants on 5 of the 6 studied roofs, together indicating active N fixation. Our findings suggest N loss from green roofs could be alleviated by applying nitrification inhibitors, and that fostering plant-microbe N fixing communities could reduce the need for fertilizer amendments. Our data also highlight the role of microbial N cycling on green roofs and underscore the need for further work to quantify N-processing rates.

Seasonal variability of land-ocean groundwater nutrient fluxes from a tropical karstic region (southern Java, Indonesia)

In tropical karstic regions, knowledge about the timing and quantity of land-ocean groundwater nutrient fluxes is important, as those nutrients may affect sensitive coastal ecosystems such as coral reefs. High permeability of karst aquifers, combined with high discharge during heavy rain events, lead to a close connectivity between groundwater in the hinterland and the coastal zone. Alteration between dry and wet periods can lead to a temporal variability of groundwater discharge volume and its associated nutrient fluxes. We studied the seasonal variability of land-ocean groundwater nutrient fluxes in the tropical karstic region of Gunung Kidul (southern Java, Indonesia) from November 2015 to December 2016. Satellite thermal infrared imagery revealed two major areas of direct submarine and coastal groundwater discharge. Nutrient fluxes were estimated at the largest coastal spring using a discharge dataset from a subsurface river dam and a monthly record of nutrient concentrations sampled from the spring. Nitrate fluxes ranged from 6 × 106–245 × 106 mol/day, dissolved silicon fluxes from 58 × 106–546 × 106 mol/day and phosphate fluxes from 17 × 103–1571 × 103 mol/day. Nutrient fluxes are mostly controlled by discharge and show a high variability through time. Extraordinarily high nitrate and phosphate fluxes were observed after a period of constant and heavy rain. Most likely a nutrient pool in the top soil in the hinterland from untreated sewage or fertilizer is flushed during rain events through the aquifer to the coast. In tropical karstic regions sudden inputs of large amounts of nutrients via groundwater discharge may affect coastal ecosystems such as coral reefs making them especially vulnerable during high discharge events.

Observations of vertical turbulent nitrate flux during summer in the Great Australian Bight

To assist with developing an understanding of the nutrient supply dynamics in the Great Australian Bight (GAB) the vertical turbulent nitrate flux was calculated at stations along two cross-shelf transects using direct measurements of turbulence and nitrate concentrations. Coincident hydrographic profiling conducted at the onset of the summertime upwelling season revealed upwelling onto the shelf in the eastern GAB (eGAB) and downwelling at the shelf slope in the central GAB (cGAB). Under these conditions, examination of the strength of competing vertical temperature and salinity gradients using the Turner angle suggested a high potential for double diffusive convective mixing. However, corresponding estimates of the mixing efficiency derived from microstructure profiling indicated turbulence was the main mixing process driving vertical fluxes across the region. The average upward nitrate flux between the surface mixed layer depth(MLD) and the base of the euphotic layer (92 m depth) was O(10−6−10−5) mmol N m−2 s−1 in the cGAB, with the peak flux observed at the upper-slope station (~ 400 m total water depth). In the eGAB, the magnitude of flux estimates at the shelf-slope and offshore stations were similar to those measured in the cGAB. An enhanced nitrate flux O(10−4) mmol N m−2 s−1 was observed on the shelf in the eGAB (~ 100 m total water depth) as the result of enhanced turbulence and an increased vertical nitrate gradient. Application of Redfield stoichiometry suggests vertical turbulent nitrate fluxes play a significant role in supporting up to 50% of the primary production in the GAB. We hypothesize that mixing processes resulting from interactions between upwelled and downwelled water masses and new water masses formed on the shelf during summer play an important role in maintaining the bands of subsurface chlorophyll a maxima observed below the MLD at the nitricline during periods of strong stratification. The snapshot provided by this study, emphasizes the need to better understand the variability and influence of mixing processes on nutrient supply and productivity across the GAB marine ecosystem.

Non-linear response of summertime marine productivity to increased meltwater discharge around Greenland

Runoff from the Greenland Ice Sheet (GrIS) is thought to enhance marine productivity by adding bioessential iron and silicic acid to coastal waters. However, experimental data suggest nitrate is the main summertime growth-limiting resource in regions affected by meltwater around Greenland. While meltwater contains low nitrate concentrations, subglacial discharge plumes from marine-terminating glaciers entrain large quantities of nitrate from deep seawater. Here, we characterize the nitrate fluxes that arise from entrainment of seawater within these plumes using a subglacial discharge plume model. The upwelled flux from 12 marine-terminating glaciers is estimated to be >1000% of the total nitrate flux from GrIS discharge. This plume upwelling effect is highly sensitive to the glacier grounding line depth. For a majority of Greenland’s marine-terminating glaciers nitrate fluxes will diminish as they retreat. This decline occurs even if discharge volume increases, resulting in a negative impact on nitrate availability and thus summertime marine productivity.

High-frequency NO&amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt;&amp;lt;sup&amp;gt;−&amp;lt;/sup&amp;gt; isotope (&amp;lt;i&amp;gt;δ&amp;lt;/i&amp;gt;&amp;lt;sup&amp;gt;15&amp;lt;/sup&amp;gt;N, &amp;lt;i&amp;gt;δ&amp;lt;/i&amp;gt;&amp;lt;sup&amp;gt;18&amp;lt;/sup&amp;gt;O) patterns in groundwater recharge reveal that short-term changes in land use and precipitation influence nitrate contamination trends

Abstract. Poultry manure is the primary cause of nitrate (NO3-) exceedances in the transboundary Abbotsford–Sumas aquifer (ASA; Canada–USA) based on synoptic surveys two decades apart, but questions remained about seasonal and spatial aspects of agricultural nitrate fluxes to the aquifer to help better focus remediation efforts. We conducted over 700 monthly δ15N and δ18O of nitrate assays, focusing on shallow groundwater (&lt; 5 years old) over a 5-year period to gain new insight on spatio-temporal sources and controls of groundwater nitrate contamination. NO3- concentrations in these wells ranged from 1.3 to 99 mg N L−1 (n=1041) with a mean of 16.2±0.4 mg N L−1. The high-frequency 15N and 18O isotope data allowed us to identify three distinctive NO3- source patterns: (i) primarily from synthetic fertilizer, (ii) dynamic changes in nitrate due to changes in land use, and (iii) from a mix of poultry manure and fertilizer. A key finding was that the source(s) of nitrate in recharge could be quickly influenced by short-term near-field management practices and stochastic precipitation events, which ultimately impact long-term nitrate contamination trends. Overall, the isotope data affirmed a subtle decadal-scale shift in agricultural practices from manure increasingly towards fertilizer nitrate sources; nevertheless, poultry-derived N remains a predominant source of nitrate contamination. Because the aquifer does not generally support denitrification, remediation of the Abbotsford–Sumas aquifer is possible only if agricultural N sources are seriously curtailed, a difficult proposition due to longstanding high-value intensive poultry and raspberry and blueberry operations over the aquifer.

Relationships Among Nutrient and Sediment Fluxes, Hydrological Variability, Fire, and Land Cover in Coastal California Catchments

AbstractCatchments along California's coast, and those with mediterranean climates elsewhere, experience large seasonal and interannual variations in rainfall, and occasional fires; land cover varies from natural vegetation to agricultural and urban development. Multiyear sampling of streams, with intensive collections during rainfall, and continuous measurements of stage, was used to determine the magnitude and variation of concentrations and fluxes of nutrients and suspended sediments during rainfall events and per year from catchments bordering the Santa Barbara Channel. Annual rainfall varied from 18 to 122 cm in water years 2002 through 2015. Hydrological variability, catchment topography, and land cover have strong statistical relationships to nutrient and suspended sediment fluxes. The link between nutrient fluxes and runoff during storms was strongest in sites with greater portions of impervious surfaces. The occurrence of fires followed by above average rainfall resulted in highest ammonium and suspended sediment fluxes. Nitrate fluxes during large storms in undeveloped catchments following wildfire were similar in magnitude to storm fluxes generated in catchments with intensive agriculture. Decreasing storm fluxes were observed with increasing cumulative discharge within a water year, which is consistent with the reduction of flux from each subsequent storm as the rainy season progressed.

Identification of sources and transformations of nitrate in the Xijiang River using nitrate isotopes and Bayesian model

Coupled nitrogen and oxygen isotopes of nitrate have proven useful in identifying nitrate sources and transformation in rivers. However, isotopic fractionation and low-resolution monitoring limit the accurate estimation of nitrate dynamics. In the present study, the spatio-temporal variations of nitrate isotopes (15N and 18O) and hydrochemical compositions (NO3− and Cl−) of river water were examined to understand nitrate sources in the Xijiang River, China. High-frequency sampling campaigns and isotopic analysis were performed at the mouth of the Xijiang River to capture temporal nitrate variabilities. The overall values of δ15N-NO3− and δ18O-NO3− ranged from +4.4‰ to +14.1‰ and from −0.3‰ to +6.8‰, respectively. The results of nitrate isotopes indicated that NO3− mainly originated from soil organic nitrogen (SON), chemical fertilizer (CF), and manure and sewage wastes (M&S). The negative correlation of nitrate isotopic values with NO3−/Cl− ratios suggested the importance of denitrification in NO3− loss. The results of Bayesian model with incorporation of isotopic fractionation during the denitrification showed that SON and CF contributed to the most (72–73%) nitrate in the wet season; whereas approximately 58% of nitrate was derived from anthropogenic inputs (M&S and CF) in the dry season. The nitrate flux was 2.08 × 105 tons N yr−1 during one hydrologic year between 2013 and 2014, with 86% occurring in the wet season. Long-term fluctuations in nitrate flux indicated that nitrate export increased significantly over the past 35 years, and was significantly correlated with nitrate concentrations. The seasonal pattern of nitrate dynamics indicated the mixing of nitrified NO3− and denitrified NO3− between surface flow and groundwater flow under different hydrological conditions. Overall, the present study quantitatively evaluates the spatio-temporal variations in nitrate sources in a subtropical watershed, and the high-frequency monitoring gives a better estimate of nitrate exports and proportional contributions of nitrate sources.

Hedgerows reduce nitrate flux at hillslope and catchment scales via root uptake and secondary effects

Agricultural contamination of groundwater with nitrate (NO3−) is one of the most widespread and pressing environmental issues. The preservation and planting of hedgerows around agricultural fields can reduce NO3− flux, but the efficacy of hedgerows depends on the amount of NO3− in soil and groundwater, hydrological flowpath and timing, and biogeochemical conditions surrounding and below roots. Quantifying these parameters is a major challenge, usually requiring involved and destructive fieldwork. Here, we present a new analytical method to characterize NO3− stratification using water chemistry sampled during piezometer slug tests. We tested this method with a network of wells in a hillslope intersected by an oak hedgerow during high- and low-water conditions, respectively spring and autumn. We found that hedgerows had a strong seasonal effect on near-surface NO3− dynamics in the proximity of the root system, reducing annual hillslope-level fluxes by 26 to 63%, comparable to NO3− removal from cover crop techniques. Hedgerow root uptake accounted for two-thirds of this reduction, with the remaining third attributable to secondary effects, potentially hedgerow-induced microbial retention or denitrification due to increased organic carbon and heterogeneous redox conditions in the rooting zone. However, a simple scaling exercise suggested that at the catchment level, hedgerow NO3− removal has a smaller effect (ca 1–10% reduction of annual flux), due to the large legacy of NO3− in the aquifer from past fertilizer application. These results suggest that while hedgerows cannot immediately solve problems of past groundwater contamination, protection and reestablishment of hedgerow networks could substantially accelerate recovery of groundwater quality on decadal timescales.

A new look at the multi-G model for organic carbon degradation in surface marine sediments for coupled benthic–pelagic simulations of the global ocean

Abstract. The kinetics of particulate organic carbon (POC) mineralization in marine surface sediments is not well constrained. This creates considerable uncertainties when benthic processes are considered in global biogeochemical or Earth system circulation models to simulate climate–ocean interactions and biogeochemical tracer distributions in the ocean. In an attempt to improve our understanding of the rate and depth distribution of organic carbon mineralization in bioturbated (0–20 cm) sediments at the global scale, we parameterized a 1-D diagenetic model that simulates the mineralization of three discrete POC pools (a multi-G model). The rate constants of the three reactive classes (highly reactive, reactive, refractory) are fixed and determined to be 70, 0.5 and ∼ 0.001 yr−1, respectively, based on the Martin curve model for pelagic POC degradation. In contrast to previous approaches, however, the reactivity of the organic material degraded in the seafloor is continuous with, and set by, the apparent reactivity of material sinking through the water column. Despite the simplifications of describing POC remineralization using G-type approaches, the model is able to simulate a global database (185 stations) of benthic oxygen and nitrate fluxes across the sediment–water interface in addition to porewater oxygen and nitrate distributions and organic carbon burial efficiencies. It is further consistent with degradation experiments using fresh phytoplankton reported in a previous study. We propose that an important yet mostly overlooked consideration in upscaling approaches is the proportion of the reactive POC classes reaching the seafloor in addition to their reactivity. The approach presented is applicable to both steady-state and non-steady state scenarios, and links POC degradation kinetics in sedimentary environments to water depth and the POC rain rate to the seafloor.

Physical and biological drivers of sediment oxygenation and denitrification in a New Zealand intermittently closed and open lake lagoon

ABSTRACTIntermittently closed and open lakes and lagoons (ICOLLs) are shallow estuarine ecosystems, many of which show eutrophication symptoms. The physical and biological drivers of sediment oxygenation and denitrification were examined in a New Zealand ICOLL using in situ enclosures for one year. Denitrification was seasonally and spatially variable, and higher denitrification rates were driven by organic matter availability, temperature and nitrate flux. The bulldozing invertebrates generally deepened oxygen penetration in 18% of oxygen profiles, consistent with their effect in multiple regressions. Chironomid larvae (Chironomidae) dominated the benthic community, and their tubular burrows modified oxygen penetration into the sediment in 18% of profiles, potentially affecting denitrification in the ICOLL through bioirrigation. Our study shows that macroinvertebrates support increased sediment oxygenation; therefore the use of artificial opening regimes in many ICOLLs may have negative effects on macroinvertebrate community composition and may consequently inhibit denitrification.

Flux-Mediated Topochemical Growth of Platelet-Shaped Perovskite LiNbO3 Single Crystals from Layered Potassium Niobate Crystals

Well-defined, platelet-shaped LiNbO3 single crystals were prepared by a flux-mediated topochemical reaction from platelet K4Nb6O17 crystals with a mixture of LiNO3 solute and alkali metal nitrate (LiNO3, NaNO3, and KNO3) fluxes at 600 °C. Crystallographic structural characterizations revealed that the LiNbO3 crystals inherited the K4Nb6O17 crystal shape with well-developed {012} faces. The topochemical reaction in molten KNO3 promoted smooth surface formation, in contrast to LiNO3 and NaNO3, which formed rough surfaces. We further found that the dissolution and deposition reaction occurs repeatedly in the vicinity of the LiNbO3 crystal and KNO3 flux interface. It is considered that the KNO3 flux hybridized with the LiNO3 solute provides a moderate solubility and dissolution rate suitable for crystal growth. We also achieved reed-shaped LiNbO3 crystal growth by applying the same technique to the KNb3O8 crystal template.

Tracking nitrate sources in the Chaohu Lake, China, using the nitrogen and oxygen isotopic approach.

The Chaohu Lake is highly polluted and suffers from severe eutrophication. Nitrate is a key form of nitrogen that can cause water quality degradation. In this study, hydrochemical and dual isotopic approaches were utilized to identify the seasonal variation of nitrate sources in the Chaohu Lake and its inflowing rivers. The average nitrate concentrations ([NO3-]) of the lake and its inflowing rivers were 89.9 and 140.8μmolL-1, respectively. The isotopic values of δ15N-NO3- and δ18O-NO3- in the lake ranged from - 1.01 to + 16.67‰ and from - 4.39 to + 22.20‰, respectively. The two major rivers had distinct isotopic compositions, with average δ15N-NO3- values of + 17.52 and + 3.51‰, and average δ18O-NO3- values of + 2.71 and + 7.47‰ for the Nanfei River and Hangbu River, respectively. The results show that soil organic ammonium and urban wastewater discharge were the main sources of nitrate in the Chaohu Lake, and nitrate assimilation was an important process affected [NO3-] and isotopic compositions, especially in the western Chaohu Lake. The elevated [NO3-] and δ15N-NO3- values in the western Chaohu Lake indicate the high influence of human activities. Urban wastewater discharge was the primary nitrate source in the Nanfei River and soil organic ammonium was the main source in the Hangbu River. Although nitrate from direct atmospheric deposition was low, its strong flushing effect can substantially improve riverine [NO3-] and nitrate loading from terrestrial ecosystem. The relatively high nitrate fluxes from the Hangbu River indicate that nitrogen loading from agricultural watershed is unneglectable in watershed nitrogen sources reduction strategies.

Rates of nitrification and nitrate assimilation in the Changjiang River estuary and adjacent waters based on the nitrogen isotope dilution method

Being supplied from both terrestrial inputs and internal regeneration, nitrate is usually in excess in the Changjiang River estuary (CRE) and adjacent waters (CREAW). As significant reactions in the nitrogen cycle, nitrate assimilation and nitrification rates were calculated by field incubation experiments following the isotope dilution method during June and November 2014. Besides this, distribution of other field parameters in the CREAW were also investigated. The results showed that the nitrate assimilation rates were higher in nearshore areas and lower in offshore areas. The nitrate assimilation rates were also higher during June, at 0.3–11.9 μmol L−1 d−1, whereas the rates were 0–3.2 μmol L−1 d−1 in November. The highest rate was observed in the surface water of the estuary, where the chlorophyll-a (chl-a) concentration reached 11.02 μg L−1. In addition, the phytoplankton community structure affected the nitrate assimilation rates, and dinoflagellates presented weaker nitrate assimilation abilities than those of diatoms. By contrast, the nitrification rates were higher in nearshore areas in June but higher in offshore areas in November. The nitrification rates were 0–4.1 μmol L−1 d−1 and 0–3.6 μmol L−1 d−1 in June and November, respectively. At most sites, the nitrification rates were positively correlated with the ammonium concentrations and were higher in November, which might be attributable to the higher temperature. Moreover, a theoretical calculation was used to study the regional nitrate flux throughout the vertical water column. The results showed that a gradual supplement from nitrification might replenish the nitrate consumed by assimilation far from the CRE. The overall result was that terrestrial input remained the primary source of estuarine nitrate; however, the role of internal nitrate regeneration, which would be effective for primary production in the CREAW, should also be highlighted as a source of nitrate, especially in offshore areas.

Identifying the Driving Factors of Water Quality in a Sub-Watershed of the Republican River Basin, Kansas USA.

Studies have shown that the agricultural expansion and land use changes in the Midwest of the U.S. are major drivers for increased nonpoint source pollution throughout the regional river systems. In this study, we empirically examined the relationship of planted area and production of three dominant crops with nitrate flux in the Republican River, Kansas, a sub-watershed of Mississippi River Basin. Our results show that land use in the region could not explain the observed changes in nitrate flux in the river. Instead, after including explanatory variables such as precipitation, growing degree days, and well water irrigation in the regression model we found that irrigation and spring precipitation could explain >85% of the variability in nitrate flux from 2000 to 2014. This suggests that changes in crop acreage and production alone cannot explain variability in nitrate flux. Future agricultural policy for the region should focus on controlling both the timing and amount of fertilizer applied to the field to reduce the potential leaching of excess fertilizer through spring time runoff and/or over-irrigation into nearby river systems.

Impact of particulate sediment, bentonite and barite (oil-drilling waste) on net fluxes of oxygen and nitrogen in Arctic-boreal sponges

To meet the increasing global energy demand, expanding exploration for oil and gas reserves as well as associated drilling activities are expected in the Arctic-boreal region where sponge aggregations contribute to up to 90% of benthic biomass. These deep-water sponges along with their microbial endobionts play key roles in the nitrogen cycling in Arctic-boreal ecosystems. This study aimed to investigate the effects of drilling discharges and associated sediment resuspension events on net fluxes of oxygen, ammonium, nitrate and nitrite in three common deep-water sponge species in the form of explants. Sponges were exposed to suspended bentonite and barite, the primary particulate compounds in drilling waste, as well as suspended natural sediment particles for a period of 33 days (on average 10 mg L−1 for 12 h day−1). The exposure period was followed by a pollution abatement period for a further 33 days. No sponge mortality was observed during the experiment. However, exposure to these particles, especially to barite, led to reduced oxygen consumption by up to 33% that was linearly correlated with reduced nitrite/nitrate release by the sponges. The changes in net fluxes were accompanied by decreased tissue oxygenation by up to 54% within the sponges. These findings reveal the effects of fine particles on sponge metabolic processes by reducing aerobic respiration and microbial nitrification, and possibly by favouring anaerobic processes such as microbial denitrification. Most of the sponge responses recovered to their control levels upon the pollution abatement period, but the effects caused by barite may not be reversible. Our findings provide the first insight into the ecological consequences of oil and gas drilling activities on sponge-mediated nitrogen cycling in the Arctic-boreal region.