Fate Of NO3 Research Articles

Seasonal nitrogen sources and its transformation processes revealed by dual-nitrate isotopes in Xiamen Bay, China

The eutrophication load in coastal waters is gradually increasing due to excessive nitrate (NO3−) input caused by intense human activities. The tracing of NO3− sources as well as their biogeochemistry is crucial for the development of effective measures to alleviate the eutrophication load. In this study, seasonal seawater samples from Xiamen Bay were investigated to explore the sources and fates of NO3− using dual nitrate isotopes. The results indicated that the NO3− is less affected by biological processes and mainly influenced by the terrestrial sewage discharges (48%) during rainy seasons. Contrarily, as runoff input decreases and external seawater intrusion increases, the assimilation of phytoplankton dominates the process of nitrate removal during dry seasons. This study suggests that the increase in the input of terrestrial sewage nutrients is responsible for the increase in eutrophication in Xiamen Bay, providing a basis for controlling water eutrophication and regional environmental governance.

Tracing the Sources and Fate of NO3- in the Vadose Zone-Groundwater System of a Thousand-Year-Cultivated Region.

Excess nitrate (NO3-) loading in terrestrial and aquatic ecosystems can result in critical environmental and health issues. NO3--rich groundwater has been recorded in the Guanzhong Plain in the Yellow River Basin of China for over 1000 years. To assess the sources and fate of NO3- in the vadose zone and groundwater, numerous samples were collected via borehole drilling and field surveys, followed by analysis and stable NO3- isotope quantification. The results demonstrated that the NO3- concentration in 38% of the groundwater samples exceeded the limit set by the World Health Organization. The total NO3- stock in the 0-10 m soil profile of the orchards was 3.7 times higher than that of the croplands, suggesting that the cropland-to-orchard transition aggravated NO3- accumulation in the deep vadose zone. Based on a Bayesian mixing model applied to stable NO3- isotopes (δ15N and δ18O), NO3- accumulation in the vadose zone was predominantly from manure and sewage N (MN, 27-54%), soil N (SN, 0-64%), and chemical N fertilizer (FN, 4-46%). MN was, by far, the greatest contributor to groundwater NO3- (58-82%). The results also indicated that groundwater NO3- was mainly associated with the soil and hydrogeochemical characteristics, whereas no relationship with modern agricultural activities was observed, likely due to the time delay in the thick vadose zone. The estimated residence time of NO3- in the vadose zone varied from decades to centuries; however, NO3- might reach the aquifer in the near future in areas with recent FN loading, especially those under cropland-to-orchard transition or where the vadose zone is relatively thin. This study suggests that future agricultural land-use transitions from croplands to orchards should be promoted with caution in areas with shallow vadose zones and coarse soil texture.

Identification of the sources and fate of NO3−-N in shallow groundwater around a plateau lake in southwest China using NO3− isotopes (δ15N and δ18O) and a Bayesian model

Pollution by NO3−-N seriously threatens the quality of shallow groundwater (SG) around Erhai Lake, which is the 2nd largest source of freshwater in the plateau area in southwest China; further, NO3−-N affects the lake water quality and human health. We collected SG samples during the dry and wet seasons in 2018 and 2019, and the potential NO3−-N sources and their fates were identified in SG by NO3− isotopes and hydrochemical methods. Our results showed that the NO3−-N concentrations in the SG in the wet season in farmland were far higher than those in the dry season in residential areas. The high variation in δ15N–NO3- and δ18O–NO3- (from −12.78‰ to +18.10‰ and −27.62‰ to +23.07‰, respectively, in the farmland and from −5.34‰ to +34.54‰ and −20.04‰ to +17.47‰, respectively, in the residential area) indicated multiple NO3−-N sources in the SG. The NO3−-N in the farmland mainly originated from chemical nitrogen fertilizer (NF, 36%), soil nitrogen (SN, 33%) and manure and sewage (M&S, 24%) in the dry season and from SN (61%) and NF (33%) in the wet season. The NO3−-N in the residential area mainly originated from M&S (57%), SN (23%) and NF (14%) in the dry season and from SN (50%), NF (25%) and M&S (24%) in the wet season. Nitrogen transformation was dominated by denitrification in the SG. The most polluted SG area was observed on the east bank of Erhai Lake, NO3−-N mainly originated from NF. But the NO3−-N pollution slowed down from high altitude to lakeside and had multiple NO3−-N sources on the west bank of Erhai Lake. The SG was contaminated by nitrogen from NF, SN and M&S along the flow path and flowed into Erhai Lake. Therefore, reducing soil nitrogen concentrations and chemical nitrogen fertilizer applications and improving sewage facilities are significant ways to mitigate nitrate pollution in the SG.

Isotopic and chemical evidence for nitrate sources and transformation processes in a plateau lake basin in Southwest China

In recent decades, multiple occurrences of algal blooms have substantially deteriorated water quality, especially for the nutrient budget in plateau lakes. Specifically, NO3− pollution has critically threatened groundwater quality, thus increasing human health risk if groundwater serves as a drinking source. To identify the origin and fate of NO3− in a plateau lake basin, we utilized nitrate isotope natural abundance and water chemistry information under land use frameworks and groundwater flow information. In December 2018, we collected water samples from aquifers (n = 33), rivers (n = 2), soil (n = 7), and lakes (n = 4) in Chenghai Lake basin, Southwest China. Our results showed that nearly 41% of groundwater samples failed to meet the drinking water standard of WHO and China (GB/T 5749-2006) of 50 mg/L for NO3− during the dry season. The high variation of δ15N-NO3− (from −3.3 to +41.3‰) and δ18O-NO3− (from −6.4 to +13.6‰) indicated multiple N sources and N cycling processes. Our analysis revealed that 16%–80% of nitrate in groundwater was derived from accumulated soil N, whereas 13%–76% was contributed from manure/landfill leachate. The contribution from atmospheric nitrogen deposition to aquifers was less than 3%. Manure/landfill and soil nitrogen were the primary N sources, contributing for 38.9% and 35.3% to N loading in lake. As for river water, soil nitrogen contributed for 69.7% and 37.2% in R1 and R2, respectively. The denitrification process significantly affects nitrate attenuation of N sources in aquifers. An increasing trend in NO3− concentration was noticed along the groundwater flow path (A-A’) from mountain area to lake. Among different pathways, distinct nitrate sources loading downwards to the aquifers were observed in massive farmlands and residential areas. Thus, the information on both land-use and groundwater flow pathways is indispensable for modelling nitrate sources and transformation processes using the dual isotope approach.

Quantification of nitrate sources and fates in rivers in an irrigated agricultural area using environmental isotopes and a Bayesian isotope mixing model

Nitrate (NO3−) pollution in rivers caused by intensive human activities is becoming a serious problem in irrigated agricultural areas. To identify NO3− sources and reveal the impact of irrigation projects on NO3− pollution in rivers, the hydrochemistry and isotopes of irrigation water from the Yellow River (IW) and river water (RW), and potential source samples were analyzed. The mean NO3− concentrations in the IW and RW were 24.4 mg/L and 49.9 mg/L, respectively. Approximately 45.2% of RW samples (n = 31) exceeded the Chinese drinking water standard for NO3− (45 mg/L). The δ15N and δ18O values, combined with the Cl−/Na+, SO42−/Ca2+ ratio distributions, indicate that the NO3− in the RW mainly originated from chemical fertilizers, manure and sewage. A Bayesian model showed that manure and sewage contributed the most to the overall NO3− levels of the IW. In the RW, chemical fertilizers and IW contributed the most to the overall NO3− levels. The mean nitrate contribution to the RW from the combination of chemical fertilizers and IW is estimated to be 51.6%. Nitrogen from manure and sewage, soil N and precipitation also contributed. The NO3− pollution in rivers was largely influenced by the irrigation regime, with a large amount of nitrogen in chemical fertilizer lost because of low utilization efficiency and subsequent transfer, via irrigation runoff, into the rivers. This study suggests that with a detailed assessment of the sources and fate of NO3−, effective reduction strategies and better management practices can be implemented to control NO3− pollution in rivers.

Using major ions and δ15N-NO3− to identify nitrate sources and fate in an alluvial aquifer of the Baiyangdian lake watershed, North China Plain

In semi-arid regions, most human activities occur in alluvial fan areas; however, NO3(-) pollution has greatly threatened the shallow groundwater quality. In this paper, δ(15)N-NO3(-) and multi-tracers were used to identify the origin and fate of NO3(-) in groundwater of the Baiyangdian lake watershed, North China Plain. The investigation was conducted in two typical regions: one is the agricultural area located in the upstream of the watershed and another is the region influenced by urban wastewater in the downstream of the watershed. Results indicate that the high NO3(-) concentrations of the upstream shallow groundwater were sourced from fertilizer and manure or sewage leakage, whilst the mixture and denitrification caused the decrease in the NO3(-) concentration along the flow path of the groundwater. In the downstream, industrial and domestic effluent has a great impact on groundwater quality. The contaminated rivers contributed from 45% to 76% of the total recharge to the groundwater within a distance of 40 m from the river. The mixture fraction of the wastewater declined with the increasing distance away from the river. However, groundwater with NO3(-) concentrations larger than 20 mg l(-1) was only distributed in areas near to the polluted river or the sewage irrigation area. It is revealed that the frontier and depression regions of an alluvial fan in a lake watershed with abundant organics, silt and clay sediments have suitable conditions for denitrification in the downstream.

Oxygen exchange with water alters the oxygen isotopic signature of nitrate in soil ecosystems

Combined oxygen (O) and nitrogen (N) stable isotope analyses are commonly used in the source determination of nitrate (NO3−). The source and fate of NO3− are studied based on distinct O and N isotopic signatures (δ18O and δ15N) of various sources and isotopic effects during NO3− transformation processes, which differ between sources like fertilizer, atmospheric deposition, and microbial production (nitrification). Isotopic fractionation during production and consumption of NO3− further affects the δ18O and δ15N signal. Regarding the δ18O in particular, biochemical O exchange between O from NO3− and H2O is implicitly assumed not to affect the δ18O signature of NO3−. This study aimed to test this assumption in soil-based systems. In a short (24 h) incubation experiment, soils were treated with artificially 18O and 15N enriched NO3−. Production of NO3− from nitrification during the incubation would affect both the 18O and the 15N enrichment. Oxygen exchange could therefore be studied by examining the change in 18O relative to the 15N. In two out of the three soils, we found that the imposed 18O enrichment of the NO3− declined relatively more than the imposed N15–NO3− enrichment. This implies that O exchange indeed affected the O isotopic signature of NO3−, which has important implications for NO3− source determination studies. We suggest that O exchange between NO3− and H2O should be taken into consideration when interpreting the O isotopic signature to study the origin and fate of NO3− in ecosystems.

Nitrate reduction in streambed sediments: Effects of flow and biogeochemical kinetics

The effect of retention time on redox sequences along the hydrological flow path of groundwater discharging through low‐relief coastal stream sediments and the subsequent impact on the fate of NO3− carried in the groundwater was examined in two intact cores. Rates of denitrification were determined for the organic‐rich streambed sediments, and a macroscopic, multispecies, reactive transport model based on multiple Monod kinetics was developed to interpret and extend the experimental results. Regionalized sensitivity analysis and parameter estimation were used to determine a set of parameters that best describe the experimental data for one column. The calibrated model successfully replicated the spatial profiles of nitrate under both steady and transient conditions in the second column operated under different conditions. A dimensionless form of the model was used to examine how coupled biogeochemical reactions and hydrological transport processes operate within the stream sediments could be understood in terms of Peclet (ratio of advection to dispersion) and Damkohler numbers (the ratio of the characteristic time of transport to the characteristic time for reaction). At the study site, the Peclet number and the Damkohler numbers for both oxygen and nitrate are high (Pe = 25, DaN = 47.5, and DaO = 40). When Pe > 5, Damkohler numbers explain observed variations in nitrate removal rates; as the flow rate increases, the solute residence time in the reactive zone is shortened resulting in a lesser extent of reaction, such that more NO3− is delivered to the stream water.

Nitrogen‐15 and Oxygen‐18 Natural Abundance of Potassium Chloride Extractable Soil Nitrate Using the Denitrifier Method

In agroecosystems, most isotopic investigations of NO3− involve the use of tracers that are artificially enriched in 15N. Although the dual isotope composition of NO3−— δ15N and δ18O is especially beneficial for understanding the origin and fate of NO3−, its use for KCl‐extractable soil NO3− has been hampered by the lack of a suitable analytical technique. Our objective was to test whether the denitrifier method, whereby NO3− is reduced to N2O before mass spectrometric analysis, can be used to determine the N and O isotopic composition of NO3− from 2 M KCl soil extracts. Several internationally accepted NO3−standards were dissolved in 2 M KCl, the conventional extractant for soil inorganic N, and inoculated with the bacterial strain Pseudomonas aureofaciens (ATCC no. 13985). The standard deviation of the NO3− standards analyzed did not exceed 0.2‰ for δ15N and 0.3‰ for δ18O values. After appropriate corrections, differences between our measured and consensus δ15N and δ18O values of standard NO3− generally were within the standard deviations given for the consensus values. Both δ15N and δ18O values were reproducible among separate analytical runs. The method was also tested on genuine 2 M KCl extracts from unfertilized and fertilized soils. Depending on N fertilization, the soils had distinct δ15N and δ18O values, which were attributed to amendment with NH4NO3 fertilizer. Hence, our data indicate that the denitrifier method provides a fast, reliable, precise, and accurate way of simultaneously analyzing the natural abundances of 15N and 18O in KCl‐extractable soil NO3−

Modeling flow and nitrate fate at catchment scale in Brittany (France).

In the intensive pig-farming (Sus scrofa) area of Brittany (western France), many surface and subsurface water resources are contaminated by nitrate (NO3) with concentrations that chronically exceed the European Community 50 mg L(-1) drinking standard. To ensure sustainable water supply, the fate of NO3 must be considered in both surface water and ground water. The fate of N was investigated in a Britain catchment, the Coët-Dan watershed, with an integrated management tool: the hydrological SWAT model coupled with the ground water model MODFLOW, and its companion contaminant and solute transport model MT3DMS. The model was validated with respect to water quantity during a 6-yr period and for the NO3 concentration during a 44-mo period, at two gauging stations in the catchment. The coupled models reproduced accurately the measurements. At the basin outlet, the Nash-Sutcliffe coefficients were 0.88 for monthly flow for the entire period and 0.87 for monthly N load. Alternative scenarios were simulated and showed potential benefits of decreasing manure application from 210 to 170 kg N ha(-1) as required by the European Commission Nitrates Directive.

Effect of the addition of nitrate to milk on the formation of volatile N -nitrosamines and apparent total N-nitroso compounds

Saint-Paulin, a French semihard cheese, was manufactured from milk with and without added NO3 –. Reference and experimental cheeses were analysed at different stages of ripening for the presence of apparent total N-nitroso compounds (ATNC) by chemical denitrosation and chemiluminescence detection (thermal energy analyser). The fate of NO3 – and NO2 –was also scrutinized. The levels of ATNC and of volatile N-nitrosamines were followed during ripening. After 60 days of ripening, ATNC were detected at concentrations of 27 μg NO-N/kg and 64 μg NO-N/kg in the cheeses obtained from milk with and without added NO3 –, respectively. The NO3 – contents of both cheeses were nearly the same, and below 5 mg/kg. The occurrence of NO2 –was not observed. No relationship was found between the NO3 – contents and the contamination of the cheeses by volatile N-nitrosamines. At the beginning of ripening, the cheeses manufactured with added NO3 - showed high levels of ATNC (mean value 5800 μg N-NO/kg with large variations of 570–15210 μg NO-N/kg). These ATNC contents rapidly decreased during the first days of ripening (96%), but decreased much more slowly during the following days. An hypothetical mechanism, attempting to explain the disappearance of ATNC, involving the formation of alkylated species in the cheese was proposed.

A Tracer Test to Determine the Fate of Nitrate in Shallow Groundwater

AbstractThe effectiveness of shallow groundwater areas to serve as a sink for NO3 is affected by many biological and physical properties. However, the direct impact of these properties on the fate of NO3 in shallow groundwater is not well understood, especially where the soils are intermittently saturated. This study was conducted to assess in situ reaction and transport of NO3‐N in an intermittent shallow groundwater system. Tracer experiments were conducted within an imposed constant flowing shallow groundwater. A constant‐head, single injection well technique was adapted for this study using multilevel soil water samplers placed at 14 locations around the center injection well. The use of Br as a tracer for NO3‐N in these constant‐flow experiments provided the means to assess in situ NO3‐N removal both with and without added C. In experiments without added C, an average NO3 removal rate of 0.33 g N m−2 d−1 was estimated. In a second experiment with dextrose added as an added C source, an average NO3 loss rate 1.06 g N m−2 d−1 was observed. The observed response to added dextrose indicates that the N removal processes were primarily microbial in origin, i.e., the NO3 was denitrifled or immobilized into microbial biomass.