Nitrate Flux Research Articles

Spatio-temporal distribution of reactive nitrogen species in relation to wheat cultivation in Bangladesh

Farmers generally use more nitrogen fertilizer than others for crop production in Bangladesh because of its visible growth symptoms. Such practice is responsible for extra reactive N (Nr) load to the environment, but data are not available. Nitrous oxide (N2O) data were collected from a field trial following static closed-chamber technique, which were used for calibration and validation of DeNitrification and DeComposition model along with soil clay fraction, pH, bulk density and organic carbon contents. The model was well fitted and estimated about 364 g N2O–N ha−1 emission in Rajshahi region and only 15 g N2O–N ha−1 in Barisal region. District-wise N2O–N emissions varied from < 1–15.96 t season−1. In 2011–2016, N2O–N emissions from wheat fields were about 103–129 t yr−1 in Bangladesh. The model estimated nitric oxide (NO), ammonia (NH3) and nitrate (NO3) fluxes varied from 0.012 to 0.447, 7 to 12.5 and 0 to 4.7 kg N ha−1, respectively, under ambient temperature condition. In about 79% yield variabilities were explainable by N2O emission. In dominant wheat growing areas, if sowing is started from 15 to 30 November, N2O emission could be reduced by 8–40% with 5–13% reduction in yields compared to 10 November sowing. In similar areas and same sowing date with 1.5 °C temperature rise, N2O emission may increase by 8–45% and wheat yield might reduce by about 4–8%. Time of seeding and other cultural management in wheat cultivation would be the main avenue for reducing Nr loads to the environment.

Reductions in nitrogen oxides over the Netherlands between 2005 and 2018 observed from space and on the ground: Decreasing emissions and increasing O3 indicate changing NOx chemistry

Satellite measurements of tropospheric NO2 columns are valuable for monitoring long-term changes in air quality. However, direct linkage of satellite-derived NO2 trends with changes in underlying NOx emissions and NOx surface concentrations is complicated by the contribution of background NO2 to the column, by changes in the chemical regime wherein emissions take place, and by data sampling differences. Here we study the 2005–2018 changes in nitrogen oxides concentrations over the polluted Netherlands. We use the QA4ECV OMI NO2 retrievals, RIVM surface measurements of NO, NO2, and O3, wet deposition fluxes of nitrate, and NOx emissions reported in two European inventories (EMEP and TNO-MACC-III). We interpret the observed changes in concentrations with simulations by CLASS, a box model accounting for boundary layer dynamics and chemistry. Nationally averaged, OMI column NO2 and RIVM surface NO2 concentrations are reduced by 30% and 32% respectively between 2005 and 2018. This is in line with Dutch national NOx emissions from the TNO-MACC-III and EMEP inventories showing decreases of 32%–39%, respectively, between 2005 and 2018. There is no indication that the decrease in NO2 concentrations slows down after 2010. The observed reductions in nitrogen oxides differ between winter and summer and turn out to be modulated by ozone chemistry. The RIVM surface measurements show a stronger reduction in NOx than in NO2, accompanied by an increase in O3 of 4–6 ppbv, which is due to diminished NO-titration following the lower NOx emissions, especially in winter. CLASS simulations confirm that daytime O3 increases have shifted the NO–NO2 equilibrium more towards NO2, explaining the weaker reductions in NO2 (−30%) than in NOx (−40%) concentrations over time. The O3 increases occur both during day and during night, and have likely shortened the NOx lifetimes both in summer (via faster OH + NO2+M during daytime) and in winter (via faster nighttime N2O5 formation and subsequent hydrolysis). Our findings suggest a reduction in anthropogenic NOx emissions of approximately 30% in the southern part of the Netherlands, and propose that interpreting satellite NO2 trends as a proxy for trends in NOx emissions is well-possible over a high-NOx region, but requires careful analysis in terms of changes in the chemical regimes and NOx lifetime.

Fabrication of gas-permeable polyvinylidene fluoride (PVDF) hollow-fiber membrane by dry-jet wet spinning and its application in membrane biofilm reactors

This study presents the usability of produced gas-permeable hollow fiber membranes in a membrane biofilm reactor. Different morphologies of the membranes were obtained by adding poly(vinylpyrrolidone) or propionic acid to the dope solution during the fabrication step. It was found that contact angles and oxygen-gas diffusion coefficients membranes with additives increased compared to the pristine membrane as Young’s modulus decreased. The membrane with the highest oxygen gas diffusion coefficient (5 wt% PVP K10) was tested in hydrogen and oxygen-based membrane biofilm reactors (MBfRs) in terms of nitrate reduction and ammonium oxidation, respectively. The results showed that H2-based MBfR achieved nitrate flux of 3.31 g N/m2. d while the flux of H2 utilized for NO3–N reduction was 1.18 gH2/m2. d. On the other hand, the flux of NH4-N was 6.16 g N/m2. d while the flux of O2 utilized for NH4-N oxidation reached to a maximum value of 28.14 gO2/m2. d in the O2-based MBfR system. The results showed that the hollow fiber gas-permeable membrane fabricated by dry-jet wet spinning method with poly(vinylpyrrolidone) or propionic acid as additives of dope solutions could be an inexpensive approach to make it prevalent the membrane biofilm technology.

Radium and nitrogen isotopes tracing fluxes and sources of submarine groundwater discharge driven nitrate in an urbanized coastal area

The quantitative evaluations of nutrients delivered by submarine groundwater discharge (SGD) have been widely conducted worldwide, but sources of nutrients in the discharged submarine groundwater remain unclear. Identifying these sources of nutrients is essential to the protection and management of marine ecological environments. This study aims to evaluate the magnitudes of SGD and the associated nitrate in the Guangdong-Hongkong-Macao Greater Bay Area (GHM Greater Bay Area), China, and identify the sources of SGD-driven nitrate in this region using radioactive radium (Ra) isotopes (223Ra, 224Ra, and 228Ra) and stable nitrogen (N) and oxygen (O) isotope composition of nitrate (δ15N-NO3− and δ18O-NO3−). The results of the Ra mixing model show that the estimated SGD and the associated nitrate fluxes into the Greater Bay Area are (9.15 ± 1.26) × 108 m3/d and (3.77 ± 0.52) × 107 mol/d, respectively, both of which are comparable to the contributions from the Pearl River. Combing NO3− dual isotopic signatures of sampled coastal groundwater and five kinds of potential nitrate sources, we found that ammonium (NH4+) fertilizer and natural soil N are the two main sources of nitrate in discharged submarine groundwater and rivers. No anthropogenic inputs from manure or sewage waste were identified. This study provides significant insights into the establishment of effective management strategies for controlling SGD-nutrients into the bay and protecting the marine ecological environment.

Seasonal soil nitrogen dynamics affect yields of lowland rice in the savanna zone of West Africa

AbstractBackground: Rice production in low‐input systems of West Africa relies largely on nitrogen supply from the soil. Especially in the dry savanna agro‐ecological zone, soil organic N is mineralized during the transition period between the dry and the wet seasons. In addition, in the inland valley landscape, soil N that is mineralized on slopes may be translocated as nitrate into the lowlands. There, both in‐situ mineralized as well as the laterally translocated nitrate‐N will be exposed to anaerobic conditions and is thus prone to losses.Aim: We determined the dynamics of soil NO3‐N along a valley toposequence during the dry‐to‐wet season transition period and the effects of soil N‐conserving production strategies on the grain yield of rainfed lowland rice grown during the subsequent wet season.Methods: Field experiments in Dano (Burkina Faso) assessed during two consecutive years the temporal dynamics and spatial fluxes of soil nitrate along a toposequence. We applied sequential and depth‐stratified soil nitrate analysis and nitrate absorption in ion exchange resin capsules in lowlands that were open to subsurface interflow and in those where the interflow from the was intercepted. During one year only we also assessed the effect of pre‐rice vegetation on conserving this NO3‐N as well as on N addition by biological N2 fixation in legumes using δ15N isotope dilution. Finally, we determined the impact of soil N fluxes and their differential management during the transition season on growth, yield and N use of rainfed lowland rice.Results: Following the first rainfall event of the season, soil NO3‐N initially accumulated and subsequently decreased gradually in the soil of the valley slope. Much of this nitrate N was translocated by lateral sub‐surface flow into the valley bottom wetland. There, pre‐rice vegetation was able to absorb much of the in‐situ mineralized and the laterally‐translocated soil NO3‐N, reducing its accumulation in the soil from 40–43 kg N ha−1 under a bare fallow to 1–23 kg N ha−1 in soils covered by vegetation. Nitrogen accumulation in the biomass of the transition season crops ranged from 44 to 79 kg N ha−1 with a 36–39% contribution from biological N2 fixation in the case of legumes. Rice agronomic performance improved following the incorporation as green manure of this “nitrate catching” vegetation, with yields increasing up to 3.5 t ha−1 with N2‐fixing transition seasons crops.Conclusion: Thus, integrating transition season legumes during the pre‐rice cropping niche in the prevailing low‐input systems in inland valleys of the dry savanna zone of West Africa can temporarily conserve substantial amounts of soil NO3‐N. It can also add biologically‐fixed N, thus contributing to increase rice yields in the short‐term and, in the long‐term, possibly maintaining or improving soil fertility in the lowland.

Plasma membrane calcineurin B-like calcium-ion sensor proteins function in regulating primary root growth and nitrate uptake by affecting global phosphorylation patterns and microdomain protein distribution.

The collective function of calcineurin B-like (CBL) calcium ion (Ca2+ ) sensors and CBL-interacting protein kinases (CIPKs) in decoding plasma-membrane-initiated Ca2+ signals to convey developmental and adaptive responses to fluctuating nitrate availability remained to be determined. Here, we generated a cbl-quintuple mutant in Arabidopsis thaliana devoid of these Ca2+ sensors at the plasma membrane and performed comparative phenotyping, nitrate flux determination, phosphoproteome analyses, and studies of membrane domain protein distribution in response to low and high nitrate availability. We observed that CBL proteins exert multifaceted regulation of primary and lateral root growth and nitrate fluxes. Accordingly, we found that loss of plasma membrane Ca2+ sensor function simultaneously affected protein phosphorylation of numerous membrane proteins, including several nitrate transporters, proton pumps, and aquaporins, as well as their distribution within plasma membrane microdomains, and identified a specific phosphorylation and domain distribution pattern during distinct phases of low and high nitrate responses. Collectively, these analyses reveal a central and coordinative function of CBL-CIPK-mediated signaling in conveying plant adaptation to fluctuating nitrate availability and identify a crucial role of Ca2+ signaling in regulating the composition and dynamics of plasma membrane microdomains.

A Fluorescence Resonance Energy Transfer-Based Analytical Tool for Nitrate Quantification in Living Cells.

Nitrate (NO3–) is a critical source of nitrogen (N) available to microorganisms and plants. Nitrate sensing activates signaling pathways in the plant system that impinges upon, developmental, molecular, metabolic, and physiological responses locally, and globally. To sustain, the high crop productivity and high nutritional value along with the sustainable environment, the study of rate-controlling steps of a metabolic network of N assimilation through fluxomics becomes an attractive strategy. To monitor the flux of nitrate, we developed a non-invasive genetically encoded fluorescence resonance energy transfer (FRET)-based tool named “FLIP-NT” that monitors the real-time uptake of nitrate in the living cells. The developed nanosensor is suitable for real-time monitoring of nitrate flux in living cells at subcellular compartments with high spatio-temporal resolution. The developed FLIP-NT nanosensor was not affected by the pH change and have specificity for nitrate with an affinity constant (Kd) of ∼5 μM. A series of affinity mutants have also been generated to expand the physiological detection range of the sensor protein with varying Kd values. It has been found that this sensor successfully detects the dynamics of nitrate fluctuations in bacteria and yeast, without the disruption of cellular organization. This FLIP-NT nanosensor could be a very important tool that will help us to advance the understanding of nitrate signaling.

Nitrate in Streams During Winter Low‐Flow Conditions as an Indicator of Legacy Nitrate

AbstractWinter low‐flow (LF) conditions in streams provide a potential opportunity to evaluate the importance of legacy nitrate in catchments due to the dominance of slow‐flow transport pathways and lowered biotic activity. In this study, the concentration, flux, and trend of nitrate in streams during winter low‐flow conditions were analyzed at 320 sites in the conterminous United States. LF flow‐normalized nitrate concentrations varied from &lt;0.1 to &gt;20 mg‐N L−1 and LF conditions contributed between 2% and 98% of the winter nitrate flux. LF nitrate concentrations generally exceeded 2.5 mg‐N L−1 in the upper Midwest, with smaller regions of high LF nitrate concentrations in eastern Texas and along the northern mid‐Atlantic coast. Groundwater was inferred to be the primary or sole contributor of nitrate to streams during winter LF conditions at 140 of our 320 sites. Among these 140 sites, nitrate from groundwater comprised 45% or more of the winter nitrate flux at a quarter of the sites. Among the same 140 sites, concentrations of nitrate in streams during winter LF conditions generally increased between 2002 and 2012 at sites where 40% or more of the winter flux was from groundwater, suggesting that concentrations of nitrate in the contributing groundwater system were increasing. Using metrics developed herein, we characterize the potential importance of legacy nitrate at sites in this study and discuss methods to characterize sites with fewer samples than required by our models or at sites without continuous stream discharge measurements.

Linking nitrate dynamics to water age in underground conduit flows in a karst catchment

Although numerous studies on nitrate transport and transformation in karst catchments have been reported, many challenges in understanding nitrate fluxes remain due to the unique architecture of karst critical zones and complex hydrological processes. Water age is an important descriptor of hydrological function providing insight to into flow paths and water sources at the catchment scale. This offers a potential opportunity to better understand the spatio-temporal variations in nitrate dynamics in karst catchments. We linked the nitrate dynamics of underground conduit flow to water age extracted from a robust tracer-aided model for a karst catchment in southwestern China. The results show that nitrate dynamics in underground conduit flow are controlled by the coupling of nitrate supply and of flow paths in the karst catchment. High contributions of drainage from small fractures water leads to low nitrate concentrations in underground conduit flow during the dry season. This reveals that the small fractures may be another important “hot spot” for denitrification leading to nitrate removal in the karst critical zone. The switching between transport limited and supply limited conditions cause the marked variations in nitrate concentration of underground conduit flow during wet season. Although rapid infiltration via large fractures and sinkholes have strong dilution effects on nitrate in underground conduit flow at beginning of rainfall, more nitrate was transported out of the catchment with drainage during rainfall events due to the mobilization of soil nitrate by rainfall. Meanwhile, the capacity to transport nitrate out of catchment can be potentially activated when the ‘old’ water is displaced by rainfall under high wetness condition.

DOC and nitrate fluxes from farmland; impact on a dolostone aquifer KCZ

DOC and nitrate in farmland represent key chemical species that determine the water quality in the Karst Critical Zone (KCZ). The work reported here focuses on quantifying fluxes of these species in an experimental farm site (University of Leeds Farm, UK) overlying a dolomitic karst aquifer of Permian age. In this research, the Transect Method was applied for the first time to farmland by combining hydrochemical data from soil and groundwater for computation of mass fluxes. The Transect Method, developed for management of industrially contaminated sites, was applied to a farm source due to the presence of localised contamination from application of pig slurry.Required inputs for our approach include concentrations of nitrate and DOC in soil water and groundwater, net recharge flux (here derived from a MODFLOW-2005 model) and local hydraulic gradient and conductivity measurements. Key outputs are fluxes and downstream groundwater concentrations of DOC and nitrate. Downstream concentrations were validated against direct groundwater measurements, demonstrating the veracity of the approach. The approach shows that the localised contamination has a significant impact on both concentrations of nitrate and DOC in groundwater, although the DOC impact is greater, because the upstream land uses also produce nitrate as a result of agricultural practices that are widespread in the region.The results of the study also constrain the zone vulnerable to contamination to the upper ∼40 m below the ground surface. Future modelling efforts on solute contaminant transport should focus on this shallow vulnerability zone (0–40 mBGL) and the Transect Method applied in this work can be used to define boundary conditions.Hence, following this research, we envisage to export a generic approach that combines physical flow parameters and hydrochemical analyses for computation of subsurface mass fluxes using the Transect Method, to identify the degree of impact of specific point sources and to support conceptualization and modelling of contaminant transport in the KCZ of farm areas.

Vertical fluxes of nutrients enhanced by strong turbulence and phytoplankton bloom around the ocean ridge in the Luzon Strait

Steep oceanic ridges and tidal currents in the Luzon Strait generate some of the world’s strongest turbulent mixing. To evaluate the impacts of the turbulence intensity on the marine ecosystem, we carried out measurements of microstructure turbulence and biogeochemical hydrography along 21°N in the Luzon Strait during the R/V Hakuho Maru cruise, KH-17-5-2, in November 2017. We found a turbulent kinetic energy dissipation rate exceeding O(10−7) W kg−1 and vertical eddy diffusivity exceeding O(10−3) m2 s−1, two orders of magnitude larger than those in the open ocean, above a shallow sub-ridge on the eastern ridge of the Luzon Strait. In addition, a clear chlorophyll a bloom was identified in the surface layer above the sub-ridge from in situ measurements and satellite observations. High values of nitrate (4.7 mmol N m−2 d−1) and phosphate (0.33 mmol P m−2 d−1) fluxes estimated near the base of the surface chlorophyll a bloom strongly suggest that enhanced turbulent mixing promotes nutrient supply to the euphotic zone and generates new production within the surface layer, contributing to the formation of a quasi-permanent local chlorophyll a bloom north of Itbayat Island on the eastern ridge.

Internal Tide‐Driven Tracer Transport Across the Continental Slope

AbstractThe role of the internal tide in driving tracer transport across the continental slope is examined using simplified layered theory, channel model experiments, and observational diagnostics of near shelf‐edge moorings. The effect of the internal tide is interpreted in terms of its Stokes' drift, which is separated into two distinct components: a bolus component, driven by the covariance of layer thickness and the velocity, and a shear component, driven by the velocity following the movement of an interface. For a three‐layer ocean, in the model experiments and observations, the onshore propagation of an internal tide drives a Stokes' transport directed onshore in the surface and the bottom layers and directed offshore in the pycnocline. This reversing structure is due to the bolus component dominating near the boundaries, while the shear component dominates at the pycnocline. In the observational diagnostics, the Stokes' transport is not canceled by the Eulerian transport, which is mainly directed along bathymetric contours. The Stokes' drift of the internal tide then provides a systematic on shelf tracer transport if there is a tracer sink on the shelf, carried in the surface or bottom layers. Conversely, the tracer transport is directed offshore if there is a tracer source on the shelf with plumes of shelf tracer expected to be carried offshore along the pycnocline. This tracer transport as a result of the internal tide is diagnosed for heat, salt, and nitrate. The depth‐integrated nitrate flux is directed onto the shelf supplying nutrients to the productive shelf seas.

Turbulent mixing and vertical nitrate flux induced by the semidiurnal internal tides in the southern Yellow Sea

Based on mooring observations carried out in the southern Yellow Sea during late summer 2017, this study focuses on the characteristics of the internal tide-induced turbulent mixing and the influences on the vertical transport of nitrates. We observe clear semidiurnal pycnocline displacements with an amplitude of ~3.5 m during 25 h. The baroclinic velocity and velocity shear are mainly controlled by the low-mode semidiurnal internal tide. The turbulence profiling measurements reveal that the measured turbulent kinetic energy dissipation rates ε are largely enhanced at the thermocline (ε ~2.0 × 10−8 Wkg−1) where the maximum displacements occur. The depth of the enhanced ε and velocity shear coincide with each other which all follow the displacements of isopycnals induced by the internal tide, indicating the role of local velocity shear in generating the strong turbulence. Further analysis shows that the observed interior ε is in good agreement with the MacKinnon-Gregg parametrization model which is formulated based on the wave-wave interaction theory. The strong turbulence has induced large vertical turbulent diffusivity (Kρ ~1.0 × 10−5 m2s−1) in the nitracline. The estimated vertical nitrate fluxes across the nitracline are 0.07–0.44 mmolNm−2d−1. Considering the fact that the temporal variation of vertical nitrate fluxes is mainly determined by the trend of Kρ, our results suggest that the internal tides which have generated large nitracline Kρ may play an important role in transporting nitrates upward, supporting the growth of phytoplankton in the summer subsurface chlorophyll maximum zone at least near our measurement location where internal tides may prevail.

Shorebirds Affect Ecosystem Functioning on an Intertidal Mudflat

Ecosystem functioning and services have provided a rationale for conservation over the past decades. Intertidal muddy sediments, and the microphytobenthic biofilms that inhabit them, perform crucial ecosystem functions including erosion protection, nutrient cycling and carbon sequestration. It has been suggested that predation on sediment macrofauna by shorebirds may impact biofilms, and shorebirds are known to consume biofilm, potentially causing significant top-down effects on mudflat ecosystem functioning. We carried out an exclusion experiment on the Colne Estuary, Essex, to examine whether shorebird presence significantly affects sediment erodibility measured with a Cohesive Strength Meter (CSM) and microphytobenthos biomass measured using PAM fluorescence (Fo) and chlorophyll a content. We also tested for treatment effects on sediment-water nutrient fluxes [nitrate, nitrite, ammonia, phosphate and dissolved organic carbon (DOC)] during periods of both dark and light incubation. Excluding shorebirds caused statistically significant changes in regulating and provisioning ecosystem functions, including mudflat erodibility and nutrient fluxes. The presence of shorebirds lowered the sediment critical erosion threshold τcr, reduced nitrate fluxes into the sediment under illumination, lowered nitrate efflux, and reduced phosphate uptake, compared to sediments where birds were excluded. There were no significant differences in macrofauna community composition within the sediment between treatments after 45 days of bird exclusion, suggesting a direct link between shorebird presence or absence and the significant differences in biofilm-related variables. This study introduces previously unknown effects of shorebird presence on ecosystem functions within this system and highlights an area of shorebird science that could aid joint conservation and human provisioning action.

Turbulence and vertical nitrate flux adjacent to the Changjiang Estuary during fall

Turbulent microstructure and nitrate measurements from one repeated sampling station and two transects were analyzed focusing on the internal-wave (IW) induced turbulence and vertical nitrate flux adjacent to the Changjiang Estuary during fall. The repeated sampling station has a subsurface nitrate maximum zone locating within the stratified interior layer. The subsurface nitrate maximum is clearly separated from the periodic hydrographic changes at the bottom boundary layer which is induced by tidal advection of the intruded Taiwan Warm Current. Turbulence in the stratified interior layer is largely driven by the local velocity shear which shows a semidiurnal cycle. The driving mechanism is suggested as the periodic occurrence of IWs with a period of ~2.5 h and an amplitude of ~8 m. Consecutive turbulence casts demonstrate the occurrence of large turbulent kinetic energy dissipation rate ε (10−6 Wkg−1) at the trailing edge of the wave which is the first observational evidence of the IW-induced internal turbulence in this region. The calculated turbulent vertical nitrate flux at the base of the surface boundary layer reveals a daily-mean value of 1.0 (0.4, 2.9) mmolm−2d−1, capable of supplying about half of the nitrate required for new production there during fall. Calculations also suggest that the vertical supply of nitrate is dominated by episodic strong mixing events driven by IWs which have increased the eddy diffusivity and thus the nitrate flux by one order of magnitude. The presented results provide important implications for understanding the nutrient cycling and new production off the Changjiang Estuary.

Dry deposition of particulate matter and its associated soluble ions on five broadleaved species in Taichung, central Taiwan

Many studies have estimated particulate matter (PM) removal by urban trees using dry deposition models; however, few studies have quantified the accuracy of their results. Thus, this study investigated the dry deposition of PM and its associated soluble ions in five broadleaved species in three districts of Taichung, central Taiwan, through field experiments. The total suspended particulate (TSP) dry deposition flux on leaf surfaces varied with sampling time, site, and tree species. By contrast, single-factor effects were observed for PM10 and PM2.5. The average dry deposition velocities of TSPs, PM10, and PM2.5 were 0.63, 0.062, and 0.028 cm s−1, respectively. Moreover, the dry deposition velocities of sulfate and nitrate were estimated to be 0.186 and 0.194 cm s−1, respectively. A significant relationship was observed between the ambient concentration and the dry deposition flux for all size fractions of PM. By contrast, weak and negative correlations were found between particle deposition velocity and wind speed. The measured PM2.5 dry deposition velocity was approximately equal to the dry deposition velocity obtained with the i-Tree model (0.03 cm s−1), which indicated the promising application potential of i-Tree in Taiwan. Compound and rough leaves, such as leaves of the Taiwan golden-rain tree, intercepted a high amount of PM2.5, whereas the pongam tree, which has thin leaves and wax surfaces, exhibited the lowest TSP interception. Species difference mostly occurred in the dry deposition flux of nitrate rather than sulfate; however, the interception of sulfate by trees revealed the possibility of the long-range transport of air pollutants. The results of this study elucidate the dry deposition of PM and its associated soluble ions in real-world situations.

Dataset: Quantification of nitrate fluxes in natural streams in northwestern Mexico using ion exchange resins

Dataset: Quantification of nitrate fluxes in natural streams in northwestern Mexico using ion exchange resins

Assessment of Nutrient Supply by a Tidal Jet in the Northern Strait of Georgia Based on a Biogeochemical Model

AbstractWe present a coupled three‐dimensional biological‐physical model for the Salish Sea and evaluate it by comparison to nitrate, silicate, and chlorophyll observations. It accurately reproduces nitrate concentrations with Willmott skill scores, root‐mean‐square error, and bias ranging from 0.84–0.95, 4.02–6.5 μM, and −2.33–1.84 μM, respectively, compared to three independent discrete sample data sets. A prominent feature of the model output is a tidal jet emanating from Discovery Passage producing a downstream plume of elevated surface nitrate. The signal is present from April to September, when surface nitrate is otherwise drawn down. It has a weak but statistically significant correlation to Discovery Passage tidal velocity (R=0.37, p&lt;0.01). Within the turbulent jet and associated plume, the average rate of vertical nitrate supply due to mixing and advection across a depth of roughly 6 m is 0.46 μmol m−2 s−1 between 15 May and 20 August 2015, compared to 0.10 μmol m−2 s−1 for the northern Strait of Georgia as a whole. Close to Discovery Passage, where velocities and shear are strongest, the majority of the vertical nitrate flux is due to mixing. As velocities weaken downstream, vertical advection becomes more important relative to mixing, but vertical velocities also decrease. The tidal pulses out of Discovery Passage drive waves that contribute net upward nitrate flux as far south as Cape Lazo, 40 km away. The nitrate supply drives new production, consistent with existing observations. Similar dynamics have been described in many other tidally influenced coastal systems.

Dynamic assessment of the impacts of global warming on nitrate losses from a subsurface-drained rainfed-canola field

The impact of global warming on water and nitrate losses from a rainfed-canola cropping system under various artificial drainage systems was assessed using an integrated field-modeling approach. Four subsurface drainage systems with different drain depths (Dx) and spacings (Ly), including D0.90L30, D0.65L30, D0.65L15, and Bilevel (with a drain spacing of 15 m and alternate drain depths of 0.65 and 0.90 m), were considered. The HYDRUS (2D/3D) model was first calibrated and validated using data collected for all drainage systems during the 2015–2016 and 2016–2017 canola cropping cycles, respectively, and then applied to simulate water/nitrate losses for different drainage systems under meteorological conditions predicted assuming expected future global warming. Future weather data were downscaled from 20 general circulation models and four RCP scenarios for the mid 21st century (for 2041–2070). The model capability of representing experimental field data was evaluated using the mean bias error (MBE), the normalized root mean square error (nRMSE), and the model efficiency (EF). The HYDRUS (2D/3D) model provided reliable description of soil water contents (MBE=-0.5 % to 0.2 %, nRMSE = 0.005−0.034%, and EF = 0.73−0.99), drainage fluxes (MBE= -21.7 × 10−3 to 24.9 × 10−3 mm d-1, nRMSE = 0.23−0.37%, and EF = 0.69−0.85), soil nitrate concentrations (MBE= -0.002 to 1.00 mg cm−3, nRMSE = 0.08−0.18%, and EF = 0.51−0.88), and nitrate fluxes (MBE= -0.97 to 0.72 mg cm-1 d-1, nRMSE = 0.35−0.57%, and EF = 0.77−0.87). The modeling results indicate that climate change will cause an increase of up to 148 % in average daily drainage fluxes and up to 125 % in average daily nitrate fluxes compared to the base case. This will result in an increase of 4–125 % in seasonal nitrate losses from various drainage systems, with the lowest and highest projections for the D0.65L15 and D0.65L30 systems, respectively. The HYDRUS-simulated results indicate that the D0.65L15 system is environmentally safer than the other evaluated drainage systems for predicted global warming conditions concerning water/nitrate losses.

A risk-based groundwater modeling framework in coastal aquifers: a case study on Long Island, New York, USA

A methodology is proposed to define indices for quantifying risks under the threat of reducing in groundwater levels, the existence of saltwater intrusion (SWI), and an increasing nitrate contamination load in submarine groundwater discharge (SGD). The proposed methodology considers coastal regions under geological heterogeneity and it is tested on a groundwater system in Nassau County of Long Island, New York (USA). The numerical model is constructed with the SEAWAT code. The parameter uncertainty of this model is evaluated by coupling the Latin hypercube sampling method (as a sampling algorithm) and Monte Carlo simulation to consider the uncertainty in both hydraulic conductivity and recharge rate. The indices are presented in spatial maps that classify areas of risk to potential threats. The results show that two of the water districts have a high risk under conditions of decreasing groundwater level. Salinity occurs in the southern and southwestern parts of the Nassau County aquifer and a considerable area of high risk of SWI is identified. Furthermore, the average SGD rate with the associated fluxes of nitrate is estimated as 81.4 million m3/year (average 0.8 tons of nitrate through SGD per year), which can adversely affect the quality of life in the local coastal ecosystems. The framework developed in this study could help the water district managers to identify high-risk areas for short-term and long-term planning and is applicable to other coastal settings.