Quantifying nitrogen cycling beneath a meander of a low gradient,N‐impacted,agricultural stream using tracers and numerical modelling

In watersheds impacted by nitrate from agricultural fertilizers, nitrification and denitrification may be decoupled as denitrification in the hyporheic zone is not limited to naturally produced nitrate. While most hyporheic research focuses on the 1–2 m of sediment beneath the stream bed, there are a limited number of studies that quantify nitrogen (N) cycling at larger hyporheic scales (10s of metres to kms). We conducted an investigation to quantify N cycling through a single meander of a low gradient, meandering stream, draining an agricultural watershed. Chemistry (major ions and N species) and hydrologic data were collected from the stream and groundwater beneath the meander. Evidence indicates that nearly all the shallow groundwater flowing beneath the meander originates as stream water on the upgradient side of the meander, and returns to the stream on the downgradient side. We quantified the flux of water beneath the meander using a numerical model. The flux of N into and out of the meander was quantified by multiplying the concentration of the important N species (nitrate, ammonium, dissolved organic nitrogen (DON)) by the modelled water fluxes. The flux of N into the meander is dominated by nitrate, and the flux of N out of the meander is dominated by ammonium and DON. While stream nitrate varied seasonally, ammonium and DON beneath the meander were relatively constant throughout the year. When stream nitrate concentrations are high (>2 mg litre^?1), flow beneath the meander is a net sink for N as more N from nitrate in stream water is consumed than is produced as ammonium and DON. When stream nitrate concentrations are low (<2 mg litre^?1), the flux of N entering is less than exiting the meander. On an annual basis, the meander hyporheic flow serves as a net sink for N. Copyright © 2007 John Wiley & Sons, Ltd.     

Extreme supersaturation of nitrous oxide in a poorly ventilated Antarctic lake

Lake Bonney, a permanently ice-covered Antarctic lake, has a middepth maximum N2O concentration of 41.6 micromoles N (>580,000% saturation with respect to the global average mixing ratio of N2O) in its east lobe, representing the highest level yet reported for a natural aquatic system. Atmospheric N2O over the lake was 45% above the global average, indicating that this lake is an atmospheric source of N2O. Apparent N2O production (ANP) was correlated with apparent oxygen utilization (AOU), and denitrification was not detectable, implying that nitrification is the primary source for this gas. The slope of a regression of ANP on AOU revealed that potential N2O production per unit of potential O2 consumed in the east lobe of Lake Bonney is at least two orders of magnitude greater than reported for the ocean. The maximum yield ratio for N2O [ANP/(NO2(-) + NO3-)] in Lake Bonney is 26% (i.e. 1 atom of N appears in N2O for every 3.9 atoms appearing in oxidized N), which exceeds previous reports for pelagic systems, being similar to values from reduced sediments. Areal N2O flux from the lake to the atmosphere is >200 times the areal flux reported for oceanic systems; most of this gas apparently enters the atmosphere through a small moat that occupies approximately 3% of the surface of the lake and exists for approximately 10 weeks in summer.     

The effects of sewage organic matter on biogeochemical processes within mid-shelf sediments offshore Sydney, Australia

A laboratory incubation experiment was conducted using replicate cores collected from a muddy-sand sediment facies offshore Sydney, Australia to determine what components and processes would be affected by the addition of sewage organic matter. Sewage effluent has a solid phase composition of 40% carbon (35% organic carbon), 5% nitrogen, 1% phosphorus and 5% silicate. The molecular C:N:P ratio is 92:10:1, compared to the Redfield ratio of 106:16:1 in marine phytoplankton. Sediment cores were incubated at in situ temperature in a darkened room for periods up to 95 days. Sewage organic matter was added to the cores at three different loads equivalent to 0 (T0), 65 (T1) and 130 (T2) g m⁻² of sediment. Following the addition of sewage organic matter, fluxes of oxygen (into the sediments), ammonia and phosphate (from the sediments) increased, reflecting an enhanced organic carbon supply to the sediments. Oxygen penetrated to a depth of 6 mm in the ambient cores, but the sediment oxygen content was severely depleted following the addition of the sewage-derived organic matter. Sediment porewater data, together with nutrient flux data indicate that oxygen reduction, nitrate reduction and sulphate reduction occurs within these sediments. Following the addition of sewage organic matter, increases in total nitrogen, total phosphate and total organic carbon were measured to depths of 5 cm in the sediments, suggesting that bioturbation influences nutrient and organic carbon distributions. Additionally, irrigation of the surficial sediments may play an important role in the metabolism of organic matter. These results indicate that oxygen penetration, oxygen fluxes, nitrate concentrations within porewaters, ammonia flux rates, and solid phase concentrations of total organic carbon and nutrients may be useful indicators of sediments affected by high rates of organic matter deposition onto Sydney's offshore sediments. The EPA has recently predicted maximum deposition rates of sewage particulate matter to be approximately 1 g m⁻² day⁻¹. Because of the similarities in CNP ratios of sewage organic matter and marine organic matter, the effects of sewage organic matter and marine organic matter inputs to coastal sediments may not be easily distinguishable.     

Impacts of milkfish (Chanos chanos) aquaculture on carbon and nutrient fluxes in the Bolinao area,Philippines

Sediment oxygen consumption, TCO2 production and nutrient fluxes across the sediment-water interface were measured in sediments within and along a transect from four fish pens with production of milkfish (Chanos chanos) in the Bolinao area, The Philippines. The four fish pens were each representing a specific period in the production cycling. There was a positive linear relationship between the rates of sedimentation inside the fish pens and the sediment oxygen consumption indicating that the benthic processes were controlled by the input of organic matter from fish production. The nutrient fluxes were generally higher inside the fish pens, and nitrate was taken up (1.7-5.8 mmol m(-2) d(-1)) whereas ammonium (1-22 mmol m(-2) d(-1)) and phosphate (0.2-4.7 mmol m(-2) d(-1)) were released from the sediments. The sediments were enriched in organic matter with up to a factor 4 compared to outside. A mass balance for one crop of milkfish was constructed based on production data and on measured fluxes of nutrients in the fish pens to assess the loss of carbon and nutrients to the environment. There was a loss to the surroundings of carbon and nitrogen of 51-68% of the total input, whereas phosphorus was buried in the sediments inside the fish pens which acted as net sinks of phosphorus. The results obtained suggest that fish pen culture as practiced in the Bolinao area, leads to even greater impacts on benthic carbon and nutrient cycling than those found in suspended cage cultures.     

Atmospheric nitrogen deposition from a remote source enriches macroalgae in coral reef ecosystems near Green Turtle Cay, Abacos, Bahamas

Over the past several decades, the fixation of "new" nitrogen to the biosphere has doubled. For the early 21st century, the most significant rate increases in atmospheric nitrogen deposition are predicted for developing nations. Wet nitrogen deposition was assessed on the remote island of Green Turtle Cay, Bahamas in a dry and wet season from January to July 2000. Episodic deposition of nitrate (approximately 1-137 microM) and ammonia (approximately 2-122 microM) represented a mean deposition rate of approximately 0.2 mgDIN m(-2) yr(-1). Wet deposition of nitrogen to the climatologically-linked east coast of Florida is approximately 4 times greater than the estimated annual wet nitrogen deposition value at Green Turtle Cay, suggesting the continental US as a principal airshed for this loading source. Short-term bioassays of macroalgal productivity with a 5% rainfall solution caused depressed net productivity and increased dark respiration, well known "transient metabolic" responses by nutrient-limited tropical macroalgae. Wet deposition of inorganic nitrogen from episodic rainfall events may provide up to 20% of the "new" nitrogen necessary to meet growth demands of macroalgae on coral reefs near Green Turtle Cay.     

Quantification of nitrate storage in the vadose (unsaturated) zone: a missing component of terrestrial N budgets

National terrestrial nitrogen budgets for many developed countries have been calculated as part of the management of impacts of N on the environment, but these rarely represent the subsurface explicitly. Using estimates of vadose zone travel time and agricultural nitrate loading, we quantify, for the first time, the total mass of nitrate contained in the vadose zone of aquifers in England and Wales. This mass peaked in 2008 at 1400 kt N (800 to >1700 kt N from sensitivity analyses), which is approximately 2.5 to 6 times greater than saturated zone estimates for this period and indicates that the subsurface is an important store of reactive nitrogen. About 70% of the nitrate mass is estimated to be in the Chalk, with the remainder split between the Permo‐Triassic sandstones, the Jurassic Oolitic limestones and minor aquifers. Current controls on fertilizer application mean that the vadose zone is now a nitrate source, and in 2015 we estimate the net flux from the unsaturated zone to groundwater to be 72 kt N/a. The mass of nitrate in the vadose zone should be included in future terrestrial nitrogen budgets at national and global scales to improve ecosystem management. British Geological Survey © NERC 2015. Hydrological Processes © 2015 John Wiley & Sons, Ltd.     

An experimental investigation of nitrogen gas produced during denitrification

In situ denitrification relies on indigenous microorganisms to reduce nitrate to N(2) gas. However, when initial nitrate concentrations are large, produced gas volumes also can be very large, potentially resulting in reduced water saturation and hydraulic conductivity in the treatment zone. In this study, we investigated the fate of N(2) and other gases produced during denitrification in a laboratory flow cell containing packed sediment. Denitrifying activity was stimulated by additions of nitrate and ethanol. Microbial activity was monitored by measuring nitrate, nitrite, and ethanol concentrations; gas saturations were measured during the experiment using a gamma imaging system. Biomass was measured using phospholipid fatty acid analysis of sediment samples. Bioenergetic calculations calibrated to measured nitrate consumed and biomass produced predicted that 1.2 L N(2) gas/L water should have been produced following the addition of 100 mM nitrate. However, the maximum measured gas saturation was only 23%, indicating substantial gas loss from the sediment pack. Temporal gamma images and visual observations confirm that small gas bubbles formed in the sediment pack coalesced into larger bubbles and migrated upward through gas-filled channels to the sediment pack surface. Although gas saturations increased, there was no significant change in sediment pack hydraulic conductivity. These results suggest that in permeable reactive barriers used for in situ denitrification, gas production will not necessarily lead to unlimited gas accumulation in the pore space and that the effects of gas production on water saturation and hydraulic conductivity may be relatively minor.     

Nitrogen retention in a floodplain backwater of the upper Mississippi River (USA)

Backwaters connected to large rivers retain nitrate and may play an important role in reducing downstream loading to coastal marine environments. A summer nitrogen (N) inflow-outflow budget was examined for a flow-regulated backwater of the upper Mississippi River in conjunction with laboratory estimates of sediment ammonium and nitrate fluxes, organic N mineralization, nitrification, and denitrification to provide further insight into N retention processes. External N loading was overwhelmingly dominated by nitrate and 54% of the input was retained (137 mg m⁻² day⁻¹). Ammonium and dissolved organic N were exported from the backwater (14 and 9 mg m⁻² day⁻¹, respectively). Nitrate influx to sediment increased as a function of increasing initial nitrate concentration in the overlying water. Rates were greater under anoxic versus oxic conditions. Ammonium effluxes from sediment were 26.7 and 50.6 mg m⁻² day⁻¹ under oxic and anoxic conditions, respectively. Since anoxia inhibited nitrification, the difference between ammonium anoxic–oxic fluxes approximated a nitrification rate of 29.1 mg m⁻² day⁻¹. Organic N mineralization was 64 mg m⁻² day⁻¹. Denitrification, estimated from regression relationships between oxic nitrate influx versus initial nitrate concentration and a summer lakewide mean nitrate concentration of 1.27 mg l⁻¹, was 94 mg m⁻² day⁻¹. Denitrification was equivalent to only 57% of the retained nitrate, suggesting that another portion was assimilated by biota. The high sediment organic N mineralization and ammonium efflux rate coupled with the occurrence of ammonium export from the system suggested a possible link between biotic assimilation of nitrate, mineralization, and export.     

湖光岩玛珥湖春季浮游植物对溶解态氮的吸收

利用15N稳定同位素示踪技术,采用现场挂瓶培养的方法测定了湖光岩玛珥湖浮游植物群落对铵态氮、硝态氮和尿素态氮的吸收速率,研究了湖光岩玛珥湖浮游植物群落氮吸收及其吸收动力学特征.结果表明:湖光岩玛珥湖共检测到浮游植物7门54种(包括变种和变型),主要为蓝藻门、硅藻门和绿藻门种类,分别占浮游植物总量的44.68%、26.70%和19.21%,其中水华微囊藻(Microcystis flos-aquae)与铜绿微囊藻(Microcystis aeruginosa)为绝对优势种,优势度分别为0.39与0.28.湖光岩玛珥湖浮游植物群落对铵态氮的绝对吸收速率最高,分别是对硝态氮、尿素态氮绝对吸收速率的5.8和4.2倍,占3种溶解态氮总吸收量的73.3%.铵态氮、硝态氮和尿素态氮的相对优先指数分别为2.907、0.190和1.192,说明浮游植物群落优先吸收铵态氮,其次为尿素态氮,最后为硝态氮.铵态氮、硝态氮和尿素态氮的周转时间分别为3.72、57.03和9.07 h.湖光岩玛珥湖浮游植物对溶解态氮的吸收可用Michaelis-Menten酶动力学方程描述,最大比吸收速率表现为铵态氮尿素态氮硝态氮,亲和力表现为硝态氮铵态氮尿素态氮.湖光岩玛珥湖浮游植物群落对铵态氮具有较高的吸收潜力,并且对硝态氮具有一定的亲和力,具备利用硝态氮的能力.     

A retrospective analysis of trace metals, C, N and diatom remnants in sediments from the Mississippi River delta shelf

The development of oil and gas recovery offshore of the Mississippi River delta began in shallow water in the 1950s, expanded into deeper waters, and peaked in the 1990s. This area of the outer continental shelf (OCS) is the historical and present location of >90% of all US OCS oil and gas production and reserves. The juxtaposition of its 4000 producing platforms, recovering $10 billion yr(-1) of oil, gas and produced water in the same area where about 28% of the US fisheries catch (by weight) is made and near 40% of the US coastal wetlands, makes this an area worth monitoring for regional pollutant loading. This loading may come from several sources, including sources related to OCS development, but also from the Mississippi River watershed. In this context, any contaminant loading on this shelf may be neither detectable nor significant against a background of climatic or biological variability. We examined the sedimentary record for indicators of industrial byproducts from OCS oil and gas development and of industrial products entering via the Mississippi River, primarily using vanadium (V) and barium (Ba) concentrations normalized for aluminum (Al). Barium is primarily used in drilling muds in the form of barite, whereas V is an important strengthening component of metal alloys, including steel. The fluctuations in the accumulation of Ba, but not V, were coincidental with the presumed use of barite. The fluctuations in V concentration in the sediments were coincidental with the national consumption of V. Copper (Cu), cadmium (Cd) and zinc (Zn) concentrations in sediments fluctuate coincidentally with V, not Ba, thus indicating that the dominant source of these trace metals in offshore sediments were derived from riverine sources, and were not primarily from in situ industrial processes releasing them on the shelf. This is not to suggest that local site-specific contamination is not a significant management or health concern. The low oxygen (hypoxia; < or = 2 mg l(-1)) zone that consistently covers much of this continental shelf's bottom layer in summer is attributed to nitrate loading from the Mississippi River. Increased nitrogen loading from river to shelf stimulates diatom production whose loading to the bottom layer and subsequent metabolism results in oxygen being depleted faster than it is replaced. In the last two decades there has been an increased accumulation of organic matter in sediments near the mouth of the Mississippi River. This coupling between river water, surface water and bottom water has recently expanded westward of the Atchafalaya River delta towards the Texas coast. The accumulation of biogenic silica (BSi) and carbon in dated sediments is coincidental with variations in riverine nitrate flux, but not with either V or Ba accumulation rates. These analyses indicate that both OCS development and riverine sources exert strong influences on the sediment constituents offshore, and that these influences may be independent of one another.     

武汉月湖沉积物不同形态氮含量与转换途径的垂直变化

描述了2005年6月月湖4个采样点沉积物不同形态氮含量、净硝化速率、净氮矿化速率与硝酸还原酶活性的垂直分布．亚表层（5—10cm）交换态NO3＋-N含量最高,而有效态氮与交换态NH4＋-N含量最低,故具临界意义．有效态氮多以交换态NH4＋-N的形式贮存于表层（0—5cm）与底层（〉10cm）,且底层含量较高,这种分布与缺氧状态有关．表层沉积物总氮和有机态氮含量、净硝化速率、净氮矿化速率与硝酸还原酶活性均高,间隙水中NH4＋-N浓度亦取峰值,而溶解态N03--N浓度最低,据此提出氮循环的基本过程：有机态氮经矿化与硝化产生NH4＋-N与NO3-N,同时导致有利于NH4＋-N生成的缺氧状态,并促使部分NO3-N异化还原为NH4＋-N,二者共同构成表层间隙水中丰富的NH4＋-N源,总之,富营养化湖泊表层沉积物富含有机态氮,故为氮生物地球化学循环的最为活跃的层面,而NH4＋-N则为最具有效性且含量最高的形态．     

Effects of Caragana Korshinskii Kom. Cover on runoff,sediment yield and nitrogen loss

Soil erosion is serious in the Loess Plateau of China. Deposition of the eroded sediment in lakes or rivers may lead to eutrophication, because the sediment carries a lot of nutrients. Field experiments were conducted to study soil erosion and loss of nitrogen (N) from a 15o hillslope with 30% (low) or 80% (high) coverage of the shrub Caragana korshinskii Kom. A bare soil plot was used for the comparison. The results showed that Caragana korshinskii cover significantly reduced runoff and soil erosion. In comparison to the bare soil plot, the vegetation covered plots had about 20% less runoff and 65% less sediment. In general, the concentration of N in both runoff water and the eroded sediment decreased with time and approached a steady value. However, the species of nitrate nitrogen (NO3) was exceptional which increased with time slightly. The soil erosion caused an N loss of about 250 mg/m2 for the bare soil plot, the low coverage of Caragana korshinskii reduced the N loss by 20% and the high coverage of Caragana korshinskii reduced the N loss by 40%. Moreover, the amount of total N in eroded sediment was 2 to 3 times higher than the value in runoff water. In the total N loss, the organic N was about 75-80%. Nevertheless, inorganic N in runoff water was 5 to 10 times higher than the value in eroded sediment. The species of NO3 was obviously higher than the species of ammonium nitrogen (NH4). NO3 was the main species of inorganic N loss and was about two thirds of total. The organic N was the main species of N in the eroded sediment.     

富营养化湖泊沉积物有机质矿化过程中碳、氮、磷的迁移特征

采用室内培养的方法,以富营养化湖泊太湖为例,研究了沉积物有机质矿化过程中碳、氮、磷的迁移特征.结果表明,在沉积物中的有机质矿化过程中,碳以溶解性无机碳释放至水中,同时以CH4和CO2形式释放至大气中,培养结束时,CH4和CO2累积排放含量分别为1492.21和498.96 mg/g(dw),其中CH4占气态碳的89.16%(以C质量计);此外,大量的氮、磷营养盐释放至上覆水体,水中总氮、总磷和铵态氮的最高浓度分别是初始浓度的62.16、28.16和139.45倍,而硝态氮浓度在整个培养过程中逐渐下降,培养末期浓度是初期的0.21倍;厌氧条件下,沉积物有机质的矿化,不仅可以生成大量的CH4、CO2气体,还能够促使沉积物中铵态氮和磷的释放;而沉积物有机质矿化释放的碳、氮、磷营养元素又能加剧湖泊富营养化程度,促进湖泊水体的初级生产力,从而增加湖泊沉积物有机质输入.这样的循环方式可能是湖泊富营养化自维持的重要机制之一.     

双向环形水槽模拟变化水位和流速下洞庭湖沉积物氮释放特征

浅水湖泊生态系统中的沉积物—水界面是湖泊内源氮释放的重要界面,而水动力因素是改变沉积物氮释放的重要因素.三峡大坝修建以后,长江中下游通江湖泊的水动力条件发生了明显的变化.通过采集洞庭湖湖口区域的沉积物和水样,在双向环形水槽动力模拟装置内模拟湖泊水位和流速的变化,探讨湖泊沉积物氮在沉积物和水系统中的二次释放特征.结果表明,随着扰动强度的增加,上覆水悬浮物浓度增大,上覆水中总氮浓度增加,沉积物向上覆水释放氮的强度增强,水动力条件的改变所引起的沉积物内源氮释放不容忽视.在该模拟实验条件下,沉积物存在最适扰动水位(20cm),此水位下上覆水中悬浮物浓度最低,总氮浓度最小.水动力条件的改变对上覆水和沉积物—水界面处铵态氮和硝态氮浓度的影响并不明显,孔隙水中铵态氮与硝态氮之间发生形态的转化.     

湖滨带芦苇恢复过程中沉积物氮赋存形态及含量变化:以巢湖为例

水生植被对于维持水生态系统结构和功能稳定性具有举足轻重的作用,而重建水生植物被认为是污染湖泊生态修复的重要手段.氮素是水生态系统重要的限制性元素之一,根着挺水植物生长发育无疑将深刻地影响着沉积物氮的迁移转化过程,但水生植物不同生长阶段对沉积物氮的需求和植物代谢强度均不同,目前对挺水植物完整生长过程中沉积物氮组分及含量变化认识仍十分不足.本研究通过为期120d的沉积物柱芯培养和水槽模拟试验,探究巢湖芦苇恢复完整生长过程中沉积物总氮(TN)、无机氮(TIN)与可转化态氮(TF-N)的变化及其关键调控因子.结果表明,芦苇完整生长过程将持续激发沉积物氮活性,沉积物TIN与TF-N含量逐渐增加,而沉积物TN和非可转化态氮(NTF-N)含量显著降低.模拟试验期间,指数型增长的芦苇生物量提高了沉积物铵态氮(NH_4~+-N)和硝态氮(NO_3~--N)含量,但亚硝态氮(NO_2~--N)含量却逐渐降低;与第0天相比,第120天沉积物离子交换态氮(IEF-N)、碳酸盐结合态氮(CF-N)、铁锰氧化态氮(IMOF-N)和有机态及硫化物结合态氮(OSF-N)含量分别增加了 1.10、3.40、3.60和1.40倍,这主要受芦苇吸收利用、根系代谢强化根际沉积物氧化还原电势和改变pH微环境共同驱动.在第120天,沉积物NH_4~+-N和NO_3~--N含量急剧升高,分别是第90天的9.43和2.22倍,表明芦苇衰亡凋落过程将向沉积物释放大量的TIN,故需要综合采取湖泊物理—生态工程手段来有效管控芦苇枯落物,从而提升水生植被修复效果并构建长效稳态机制.     

Spatial Variability of Nitrate and Ammonium in Pleistocene Aquifer of Central Yangtze River Basin

It becomes increasingly important and challenging for nitrogen pollution prevention to identify key controls for spatial variability of nitrogen in groundwater that could be affected by multiple factors, including anthropogenic input, groundwater flow, and local geochemistry. This study characterized spatial variability of both nitrate and ammonium in the Pleistocene aquifer of central Yangtze River Basin and assessed the effect of various factors in controlling nitrate and ammonium levels based on multiple statistical approaches (correlation, geostatistics, multiple liner regression). The results indicate that nitrate is mostly influenced by Cl⁻ that represents anthropogenic input, while Eh representing local redox state is a secondary variable influencing nitrate concentrations. The groundwater with elevated nitrate concentrations are estimated to occur mainly in areas with higher-permeability near-surface sediments which can facilitate more anthropogenic nitrate transport and less nitrate removal owing to more oxidized state. Ammonium is mostly correlated to Eh, followed by dissolved organic carbon (DOC), but only DOC improves significantly the accuracy of co-kriging prediction model. The groundwater with elevated ammonium concentrations are estimated to occur mainly in areas with more organic-rich sediments within or around the aquifer which can facilitate more ammonium release owing to natural organic matter consumption accompanying strong reducing conditions. The regional groundwater flow is not a factor significantly controlling nitrate or ammonium levels owing to flat topography and sluggish lateral flow.     

Relating nitrogen sources and aquifer susceptibility to nitrate in shallow ground waters of the United States

Characteristics of nitrogen loading and aquifer susceptibility to contamination were evaluated to determine their influence on contamination of shallow ground water by nitrate. A set of 13 explanatory variables was derived from these characteristics, and variables that have a significant influence were identified using logistic regression (LR). Multivariate LR models based on more than 900 sampled wells predicted the probability of exceeding 4 mg/L of nitrate in ground water. The final LR model consists of the following variables: (1) nitrogen fertilizer loading (p-value = 0.012); (2) percent cropland-pasture (p < 0.001); (3) natural log of population density (p < 0.001); (4) percent well-drained soils (p = 0.002); (5) depth to the seasonally high water table (p = 0.001); and (6) presence or absence of a fracture zone within an aquifer (p = 0.002). Variables 1-3 were compiled within circular, 500 m radius areas surrounding sampled wells, and variables 4-6 were compiled within larger areas representing targeted land use and aquifers of interest. Fitting criteria indicate that the full logistic-regression model is highly significant (p < 0.001), compared with an intercept-only model that contains none of the explanatory variables. A goodness-of-fit test indicates that the model fits the data well, and observed and predicted probabilities of exceeding 4 mg/L nitrate in ground water are strongly correlated (r2 = 0.971). Based on the multivariate LR model, vulnerability of ground water to contamination by nitrate depends not on any single factor but on the combined, simultaneous influence of factors representing nitrogen loading sources and aquifer susceptibility characteristics.     

Atmospheric nitrogen deposition to the North Sea

The groups of nitrogen components that contribute most to the eutrophication of the sea are ammonia, nitrogen oxides and their respective reaction products. The emission of ammonia is mainly from animal husbandry, whereas the emission of nitrogen oxides is mainly from combustion processes. Ammonia reacts in the atmosphere to form particulate ammonium. Nitrogen oxides react to form nitric acid, nitrous acid, particulate nitrate and peroxyacetyl nitrate. The various nitrogen components have different physical and chemical properties. Therefore, their dry and wet deposition velocities differ. The dry deposition velocity of gas to the sea depends on its solubility, reactivity, concentration in seawater and the wind speed. The dry deposition velocity of a particle depends on its size and the wind speed. The wet removal rate is a function of meteorological conditions. For a gas it further depends on the solubility and reactivity of the gas. For a particle, it further depends on the particle's ability to act as a condensation nucleus for particles. The total deposition of ammonia and its reaction products to the North Sea is about 140 kt N a⁻¹. The total deposition of nitrogen oxides and their reaction products is about 200 kt N a⁻¹.     

The large-scale flux of nutrients from land to water and the eutrophication of lakes and marine waters

The large-scale flux of nitrogen and phosphorus in modern society is coupled to the exponentially growing world population. During the last three decades there has been a several-fold growth in use of nitrogen (N) and phosphorus (P), for example, for agriculture production, in chemical products such as in detergents, and as food additives. Feeding a growing population means increased human nutrient excretion.

The mobility of N differs from that of P. For the easily mobile nitrate ion, river exports are positively correlated to the size of local human populations. P on the other hand is fixed in different systems, at least temporarily. Modern agro-ecosystems accumulate about 60% of the annual input of P. The Stockholm region representing an urban ecosystem served by dephosphatation in sewage, accumulated about 80% of imported P during 1990. With increasing accumulation there is an apparent risk for increasing non-point source pollution.

In some shallow recipient lakes the sediments have been saturated with P. Even after the reduction of external loading these lakes are exporting more P than they are importing. Lakes normally are traps for P.

In order to prevent damage to natural resources and to stop the large-scale flux of nutrients from land to water, the world population growth rate must be decreased and a number of measures taken: agricultural structures must be changed including new concepts for the use of fertilizers; new municipal sewage treatment processes must be developed permitting high-degree recycling of nutrients; unnecessary use of nutrients must be stopped, for example, in detergents and by reducing nutrient additives in food and drinks. Cola drinks, for example, contain 180 mg P l⁻¹, a concentration about 350 times higher than that of the effluent (0.5 mg P l⁻¹) from modern sewage treatment plants operating with dephosphatation. Financial resources and effort are required to stop large-scale eutrophication of marine waters.     

History of anthropogenic nitrogen input to the German Bight/SE North Sea as reflected by nitrogen isotopes in surface sediments,sediment cores and hindcast models

The German Bight/SE North Sea is considered a hot-spot of river-induced eutrophication, but the scarce observational data of river nitrate loads prior to the 1970s complicate the assessment of target conditions for environmental management and legislation. Stable nitrogen isotope ratios (δ¹⁵N) in sediment records can be used to decipher historical river nitrate contributions. To better constrain pre-1970s conditions, we determined δ¹⁵N in archive sediment samples (1950–1969) and dated cores from the Helgoland depositional area. We also modeled the δ¹⁵N in past situations (1960 and 1860) using an N-isotope-tracking ecosystem model. The modeled spatial distribution of δ¹⁵N in sediments for 1960 conditions and the observed spatial pattern of δ¹⁵N in archive sediment samples (1950–1969) represent a period of moderate eutrophication. The modeled spatial distribution of δ¹⁵N in sediments for 1860 conditions (pre-industrial) showed a moderate δ¹⁵N gradient from the Elbe river mouth (δ¹⁵N<4‰) to the open sea (δ¹⁵N∼5‰). This pattern contrasts with the δ¹⁵N pattern in modern surface sediments, which exhibits a steep and inverted δ¹⁵N gradient from the Elbe river mouth (δ¹⁵N>9‰) to the open sea (δ¹⁵N<7‰). Modeled δ¹⁵N for 1860 conditions are consistent with δ¹⁵N values observed in dated sediment cores that span the last 900 years. Value of δ¹⁵N in sediment cores increased from approximately 1860 to 2000 by 2.5‰. The increasing trend reflects changes in the abundance and isotopic composition of riverine nitrate loads caused by anthropogenic activities. Sensitivity tests suggest that loads and isotopic ratios of nitrogen forms other than nitrate (ammonium and organic nitrogen) have minor impact on the modeled surface sediments, despite their higher abundance in the riverborne loads in the past. Our results suggest that eutrophication of the German Bight predates the 1960 period of documented rapidly increasing river loads. Pre-industrial levels of δ¹⁵N modeled with 28% of the modern annual (1990–1999) atmospheric loads and 10% of the modern annual river loads agree best with levels of δ¹⁵N (∼6‰) observed in sediments of the cores dated to 1860.