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Baseflow is one of the major pathways of runoff in hilly areas, and its contributions to surface water resources and pollutant loads cannot be ignored. In this study, based on water quantity and quality data from 1988 to 2019 in hilly and low rainfall watersheds, we focused on the impact of long-term baseflow on nitrogen load using the load allocation based on the baseflow separation method. We also constructed a nitrogen balance model for the Chaohe River Basin of China from 2012 to 2021 to analyze the nitrogen accumulation in the basin. We used the baseflow nitrogen load lag analysis method to study the lag characteristics of the baseflow discharge process and analyzed the response and periodicity of baseflow nitrogen to precipitation and soil accumulation using time delay analysis. The results showed that the contribution rate of baseflow nitrogen reached 69% and showed a slight increasing trend from 1988 to 2019. The effects of changes in precipitation and nitrogen accumulation on the baseflow contribution was observed after 1–2 and 2 yr, respectively. After nitrogen accumulation, it entered the river channel through baseflow, which was already the main and continuous source of nitrogen in rivers in hilly areas. [ABSTRACT FROM AUTHOR]

Partial nitritation-anammox (PN/A) is a cost-effective technology in high ammonia–nitrogen wastewater treatment. However, PN/A is prone to instability as the ammonia–nitrogen sharply fluctuates. In this study, a packed bed reactor is employed to construct a single-stage PN/A system to investigate the operational characteristics and explore the denitrification mechanism. The effluent NH4+-N concentration, ammonia nitrogen removal rate (ARE), and total nitrogen removal rate (TNR) could be sustained at about 60 mg/L, 80%, and over 70%, respectively, when the influent nitrogen load rate (NLR) is changed from 0.733 to 0.879 kg-N/m3/day. Both ARE and TNR are decreased when NLR continues increasing to 1.026 kg-N/m3/day. The influent NLR decreases from 0.879 to 0.147 kg-N/m3/day, and ARE and TNR reached 98% and 85.4%, respectively. Therefore, the denitrification effect of the reactor could be recovered, and the excellent nitrogen removal capacity could be obtained within a wide range of influent NLR. Moreover, the high-throughput sequencing and metagenomic testing indicate that the Proteobacteria and Planctomycetes that the PN/A functional strains (i.e., ammonia-oxidizing bacteria (AOB) and anammox bacteria (AnAOB)) account for 38.8% in the sludge. The relative abundance of Nitrospira containing the nitrite-oxidizing bacteria (NOB) has dropped to 0.01%, and the functional gene nxr of the nitrite oxidation process is also inhibited. The relative expression of the functional gene is dominated by the short-range nitritation and anammox oxidation, which demonstrates that the nitrogen removal is mainly dominated by nitritation-anammox. [ABSTRACT FROM AUTHOR]

The energy cost and harmful effect of residual chlorine produced as a by-product in the electrochemical processes are the main obstacles in the extensive use of electrochemical recirculating aquaculture systems (ERAS). In this study, a pilot electrochemical system was used in a shrimp cultivation experiment to investigate the effects of current density, geometric feature, timing, and other parameters on the effective control of inorganic nitrogen, pathogens, and residual chlorine in aquaculture water. A 50-L electrochemical batch reactor (BR) equipped with Ti-RuO2/Ti electrodes could effectively remove inorganic nitrogen (the initial concentration was 4.0 mg/L) and inactivate Vibrio in the aquaculture seawater in 5 min, when a current density of 66 mA/cm2 and electrodes with a surface area of 154 cm2 were used. Air stripping was found to be effective in resolving residual chlorine produced from electrochemical process. In the experiment of shrimp cultivation, the ERAS equipped with a 50-L batch reactor and 500-L shrimp tank effectively kept the inorganic nitrogen concentration in the rearing water to a desirable concentration (0.2 mg/L) when a nitrogen load of 4.3 g (transformed from a daily quota of commercial prawn feeds) was used. By precisely controlling the time at which electrooxidation and air stripping alternated, an electrochemical recirculating aquaculture system could effectively remove inorganic nitrogen and residual chlorine, carry out disinfection, and reduce energy cost. [ABSTRACT FROM AUTHOR]

Purpose: To evaluate the effects of enhanced nitrogen (N) load on turnover of phosphorus (P) in Suaeda salsa marsh in the Yellow River estuary, the variation and allocation of P in the plant-soil system were investigated in the growing season. Methods: This study was explored by field N addition experiment which included four load levels: NNL (no N load treatment, 0 gN m−2 yr−1), LNL (low N load treatment, 3.0 gN m−2 yr−1), MNL (medium N load treatment, 6 gN m−2 yr−1), and HNL (high N load treatment, 12 gN m−2 yr−1). Results: In most sampling periods, the TP contents in soils in the LNL, MNL, and HNL treatments showed a decreasing trend compared to the NNL treatment. With increasing N load levels, the contents of non-available decomposed P in soils decreased by 3.08 ~ 5.22%, while those of easily available and moderately available P increased by 0.36 ~ 8.92% and 1.21 ~ 24.24%, respectively. The variations of NaOH-Po, Resin-Pi, and NaHCO3-Po contents primarily rested with the duration of N import, while those of NaOH-Pi, Residual-P, NaHCO3-Pi, and HCl-Pi contents mainly depended on the levels of N load. Dissimilar variations of TP contents in plant tissues in the growing season were observed among N load levels, which were dependent on the structure and function of organs and the nutrient function of P at distinct growth stages. The P absorption coefficients (PAC) of plants in N load treatments increased by 31.82 ~ 36.36%, while the P utilization coefficients (PUC) decreased by 4.96 ~ 13.33%. Conclusion: This paper found that the S. salsa was very likely to adjust its growth rhythm and P absorption/utilization capacity to adapt N load environment. Particularly, as N load reached medium level, the P supply relationships between aboveground and belowground tissues and the P transferences in the plant-soil system were greatly altered, indicating that the S. salsa might have a special response to the medium N enrichment. Thus, the potential impacts of medium N load level on S. salsa marsh should be emphasized in the future. [ABSTRACT FROM AUTHOR]

To regulate the water level and minimize the occurrence of water eutrophication in shallow lakes, dams and gates are often constructed in rivers. However, this practice may result in a deterioration of water quality in some estuaries. In the present study, using the correction of Nemerow pollution index (CNPI) and a redundancy analysis (RDA), water samples from different dammed rivers around Taihu Lake were compared to assess the pollution risk and identify the factors responsible for water eutrophication. The average total nitrogen (TN), total phosphorus (TP), total organic carbon (TOC) concentrations, and chemical oxygen demand (CODMn) were 2.45 ± 2.28, 0.08 ± 0.06, 43.01 ± 18.75, and 10.78 ± 4.86 mg L−1, respectively. The CNPI values indicated that approximately 76.47% of the estuarine water was moderately polluted (1 < CNPI < 7.28). A positive correlation was observed between dam construction and nutrient concentrations (e.g., rTN = 0.38, p < 0.05; rTP = 0.89, p < 0.01). Under the effects of dam construction, land use change, estuary shape, and meteorological conditions, there was a clear spatial variation of the TN concentrations. Dams that were closed all year round accelerated the TN accumulation in the water around them. The pollution risk in a trumpet-shaped estuary was higher than that in other regions (t = 2.92, p = 0.02). Endogenous release of pollutants was an important factor that may have a priming effect on algal blooms and should be given more attention. In Wuli Lake, exogenous pollution was the dominant pollutant source. A total of 74.49% of the nitrogen losses with the runoff into the estuarine water in 2018 were derived from urban domestic sewage and constructed land, with the load being 4.40 times higher than in 2000. The RDA results revealed that dam construction was the main factor (43.70%) affecting water quality, while meteorological conditions, land use types, estuary shape, and other factors contributed 56.30%. Scientific regulation and control of dam operation is important to protect the water environment of Taihu Lake. [ABSTRACT FROM AUTHOR]

Pollution with excess nutrients deteriorate the water quality of the Baltic Sea. The effect of combined land use and climate scenarios on nitrate leaching and nitrogen (N) loads to surface waters from two Baltic Sea catchments (Norsminde in Denmark and Kocinka in Poland) was explored using different models; the NLES and Daisy models for nitrate leaching, and MIKE SHE or MODFLOW/MT3DMS for N transport. Three Shared Socioeconomic Pathways (SSP1, SSP2 and SSP5) defined change in land use and agricultural activities. The climate change scenarios covered 2041–2060 compared with 1991–2010 under RCP8.5, applying four different climate models. Increases in predicted N-load from climate change vary from 20 to 60% depending on climate model. SSPs moderate these N-load changes with small changes for SSP1 to large increases for SSP5, with greater increases for Norsminde than Kocinka due to land use differences. This stresses needs for new measures and governing schemes to meet sustainability targets. [ABSTRACT FROM AUTHOR]

Key message: Growth of C. japonica under highly irrigated and N-loaded soil is noticeable because of N-induced increase of needle mass and alleviation of high irrigation-induced reduction in photosynthesis. Abstract: To clarify the combined effects of change in precipitation amount and increasing nitrogen load on Japanese forest trees, 2-year-old seedlings of Cryptomeria japonica were treated with combinations of four irrigation levels (80, 100, 120, or 140% of annual precipitation) and three nitrogen loads (15, 50, or 100 kg ha year). The 100% of annual precipitation corresponded to 1520 mm year which was the average annual precipitation in the habitats of C. japonica in Japan. High irrigation amount significantly enhanced the whole-plant dry mass of the seedlings. The N load significantly enhanced the extent of high irrigation-induced increase in needle dry mass. As a result, the irrigation-induced increase in the whole-plant dry mass was greater in the treatment with the highest N. However, increase in needle dry mass induced by high irrigation enhanced plant water consumption and caused dehydration and oxidative damage in needles, thus resulting in reduced net photosynthetic rate. The high irrigation-induced reduction in net photosynthetic rate was alleviated by N load. This alleviation was not caused by N load-induced changes in antioxidative capacity but by a N load-induced increase in ribulose 1,5-bisphosphate carboxylase/oxygenase activity. These results indicate that an increase in the whole-plant dry mass of C. japonica seedlings grown in highly irrigated and N-loaded soil was attributed to N load-induced enhancement of high irrigation-induced increase in needle dry mass and alleviation of high irrigation-induced reduction in net photosynthetic rate. [ABSTRACT FROM AUTHOR]

To investigate the long-term nitrogen treatment efficiency in vertical-flow (VF)-horizontal-flow (HF) hybrid constructed wetlands (CWs), the nitrogen removal efficiency under different seasons, N loads, and three operating stages (representing age of the wetland) were evaluated over a 12-year period. The average total nitrogen (TN) removal efficiencies in the effluent during the operation period were in the following order: summer (75.2 %) > spring (73.4 %) ≒ autumn (72.6 %) > winter (66.4 %). The removal efficiencies of TN in summer, autumn, and spring were generally higher than those in winter. At different stages of operation (years), the average TN removal rates in the effluent were in the following order: middle stage (73.4 %; years 2006-2009) > last stage (72.0 %; years 2010-2013) > beginning stage (70.1 %; years 2002-2005). In VF-HF CWs, the amount of average TN removal (mg N m day) over the 12-year period was in the order of summer (5.5) ≒ autumn (5.1) > spring (4.3) ≒ winter (4.2) for the VF bed and in the order of summer (3.5) ≒ spring (3.5) ≒ autumn (3.3) > winter (2.7) for the HF bed, showing that the amount of TN removal per unit area (m) in summer was slightly greater than that in other seasons. The amount of TN removal in the VF bed was slightly greater than that in the HF bed. Using three-dimensional simulation graphs, the maximum TN removal rate was at inflow N loads below 2.7 g m day in the summer season, whereas the minimum TN removal rate was at inflow N loads below 1.4 g m day in the winter season. Consequently, the TN removal efficiency was very stable over the 12 years of operation in VF-HF hybrid CWs. Results demonstrate that the VF-HF hybrid CWs possess good buffer capacity for treating TN from domestic sewage for extended periods of time. [ABSTRACT FROM AUTHOR]

The nitrogen stable isotopic signature (δN) of sediment is a powerful tool to understand eutrophication history, but its interpretation remains a challenge. In a large-scale comparative approach, we identified the most important drivers influencing surface sediments δN of 65 lakes from two regions of Canada using proxies that reflect watershed nitrogen (N) sources, internal lake microbial cycling and productivity. Across regions, we found that water column total nitrogen (TN), %N in the sediments and lake morphometric variables were the best predictors of sedimentary δN, explaining 66 % of its variation. Significant relationships were also found between sediment δN and human-derived N load ( $$ R_{{{\text{adj}} .}}^{2} $$ = 0.23, p < 0.001), the latter being a strong predictor of TN ( $$ R_{{{\text{adj}} .}}^{2} $$ = 0.68, p < 0.001). Despite a relatively strong overall relationship, variation partitioning revealed an interesting difference in the dominant variable that influenced regional δN. Alberta lake sedimentary δN signature was dominated by human derived N load. In contrast, internal processing appeared to be more important in Quebec lakes, where sediment δN was best explained by %N in the sediments and lake volume. Overall, our findings support the use of δN in paleolimnological investigations to reconstruct changing N sources to lakes but also highlight that regions may have distinctive drivers. Interpretations of sediment δN are likely to be strongest when multiple lines of evidence are employed and when placed in a regional context. [ABSTRACT FROM AUTHOR]

To diagnose environmental nitrogen (N) load from food consumption and to suggest preventive measures, this study identified relationships between nitrogen load from food consumption and driving factors by examining six representative countries and regions for the period 1970-2009 as an example. The logarithmic mean Divisia index technique was used to disassemble nitrogen load growth into four driving factors: population, economic activity, food intensity of the economy, and nitrogen content of food. In all study areas, increased economic activity was the main factor driving nitrogen load increase. The positive effect of population growth was relatively small but not negligible and changes in food intensity had a decreasing effect on nitrogen load. Changes in nitrogen content of food varied between areas. Broad strategies to reduce and mitigate nitrogen loading and decouple nitrogen load from economic growth in both developed and developing countries are suggested. [ABSTRACT FROM AUTHOR]

The natural aging process of Chesapeake Bay and its tributary estuaries has been accelerated by human activities around the shoreline and within the watershed, increasing sediment and nutrient loads delivered to the bay. Riverine nutrients cause algal growth in the bay leading to reductions in light penetration with consequent declines in sea grass growth, smothering of bottom-dwelling organisms, and decreases in bottom-water dissolved oxygen as algal blooms decay. Historically, bay waters were filtered by oysters, but declines in oyster populations from overfishing and disease have led to higher concentrations of fine-sediment particles and phytoplankton in the water column. Assessments of water and biological resource quality in Chesapeake Bay and tributaries, such as the Potomac River, show a continual degraded state. In this paper, we pay tribute to Owen Bricker's comprehensive, holistic scientific perspective using an approach that examines the connection between watershed and estuary. We evaluated nitrogen inputs from Potomac River headwaters, nutrient-related conditions within the estuary, and considered the use of shellfish aquaculture as an in-the-water nutrient management measure. Data from headwaters, nontidal, and estuarine portions of the Potomac River watershed and estuary were analyzed to examine the contribution from different parts of the watershed to total nitrogen loads to the estuary. An eutrophication model was applied to these data to evaluate eutrophication status and changes since the early 1990s and for comparison to regional and national conditions. A farm-scale aquaculture model was applied and results scaled to the estuary to determine the potential for shellfish (oyster) aquaculture to mediate eutrophication impacts. Results showed that (1) the contribution to nitrogen loads from headwater streams is small (about 2 %) of total inputs to the Potomac River Estuary; (2) eutrophic conditions in the Potomac River Estuary have improved in the upper estuary since the early 1990s, but have worsened in the lower estuary. The overall system-wide eutrophication impact is high, despite a decrease in nitrogen loads from the upper basin and declining surface water nitrate nitrogen concentrations over that period; (3) eutrophic conditions in the Potomac River Estuary are representative of Chesapeake Bay region and other US estuaries; moderate to high levels of nutrient-related degradation occur in about 65 % of US estuaries, particularly river-dominated low-flow systems such as the Potomac River Estuary; and (4) shellfish (oyster) aquaculture could remove eutrophication impacts directly from the estuary through harvest but should be considered a complement-not a substitute-for land-based measures. The total nitrogen load could be removed if 40 % of the Potomac River Estuary bottom was in shellfish cultivation; a combination of aquaculture and restoration of oyster reefs may provide larger benefits. [ABSTRACT FROM AUTHOR]

The Dutch National Monitoring Programme for Effectiveness of the Minerals Policy (LMM) was initiated to allow detection of a statutory reduction in nitrate leaching caused by a decreasing N load. The starting point, or baseline, was taken as the nitrate concentration of the upper metre of groundwater sampled on 99 farms in the 1992-1995 period in the sandy areas of the Netherlands, where predominantly grass and maize grow. We found here that a reduction in nitrate leaching of more than 20% in future would almost certainly be detected with the LMM. Detecting downward trends due to decreasing N load will require nitrate concentrations to also be related to soil drainage, precipitation excess leading to groundwater recharge and to location. Furthermore, we found that about 16% of the N load in the Dutch sandy regions was being leached to the upper metre of groundwater in the 1992-1995 period. The critical N load in approximately 1990 for exceeding the EC limit value for nitrate, NO3, (50 mg L-1) in the upper metre of groundwater for the mean situation for grassland, maize and arable land in the sandy area was found to be 210 kg ha-1 a-1. Because manure management has been altered, the critical load found will be lower than the current critical load. [ABSTRACT FROM AUTHOR]

The total nitrogen mass balance was studied over a two-year period in two reservoirs located in south-eastern Poland. The observed nitrogen retention (Nret) in the reservoirs ranged from 0.18 to 2.12 g m-2d-1 (mean 0.83 g m-2d-1) in case of the Rzeszów Reservoir and from -0.02 to 0.41 g m-2d-1 (mean 0.13 g m-2d-1) in case of the Solina Reservoir, while the percentage retention (Nret) ranged from 7 to 42% (mean 22%) and -15 to 73% (mean 35%), respectively. A good relationship between Nret and nitrogen loads was observed for the Solina Reservoir. The data obtained by the author and collected from the researches of 6 other reservoirs located in southern Poland were used to develop an empirical model suitable to predict the nitrogen seasonal dynamics in a reservoir. The model allows for the determination of both the annual mean Nret value and its seasonal variation in a reservoir. [ABSTRACT FROM AUTHOR]

Large quantities of the nitrate leaching below the crop root-zone, particularly during the wet season, in the tropical Far North Queensland (FNQ) of Australia, may be entering off and on-site the water bodies. The objectives of this study were to (i) provide quantitative information on NO3-N in the shallow fluctuating groundwater (GW) that develops during the rainy season (January through May) in the Johnstone River Catchment (JRC) of FNQ and (ii) determine whether this NO3-N is potentially transportable to creeks/streams. The NO3-N concentration and GW table heights were monitored, at least at weekly intervals, in 6 piezometers, installed to 8.5–12.0 m depth in 6 different soil types under fertilized sugarcane (Saccharum Officinarum-S) in the JRC during the 1999 rainy season. Depending on the location of piezometers on the landscape and time of water sampling, the GW table fluctuated between from 1.5 to 11.5 m above the piezometer bottom. The NO3-N concentration in the fluctuating GW also showed spatio-temporal dynamics and it ranged from 0.60 to 3.70 mg L-1. The NO3-N adsorbed at anion exchanges sites, up to 10 m depth, ranged from 154 to 3956 kg ha-1, compared with 21 kg ha-1 under rainforest. In the fluctuating GW, the NO3-N concentration increased with increasing GW table height and the NO3-N adsorbed at anion exchange sites (R2 = 0.96). The NO3-N load in the GW ranged from 40 to 110 kg ha-1 and it increased with increasing GW table and GW NO3-N concentration. The estimated N-load in the GW that was discharged into creeks/streams when the GW receded ranged from 21 to 81 kg ha-1. The results provide evidence that a (i) a major proportion of the NO3-N that was leaching below the sugarcane root-zone entered the shallow GW that developed during the rainy season, and (ii) a significant proportion of the NO3-N in the GW was transported to creeks/streams when the GW table receded. [ABSTRACT FROM AUTHOR]

Changes in environmental conditions within a river basin in South Central Sweden (1400 km2) and impacts on riverine nitrogen (N) transport were evaluated. A historical database was compiled and the process-based HBV-N model used to estimate flow normalised N loads in 1885, 1905, 1927, 1956, 1976, and 1994, using a standard climatological record (1985–1994). The study shows the value of process-based modelling in environmental impact assessment, by making it possible to assess and integrate the effect of a number of factors, both with regard to human impact and natural climatic variability. Factors taken into account include: the effects of land use, agricultural practices, atmospheric deposition, human dietary intake, use of flush toilets, lowering of lakes, building of dams, and climatic variability. For all years studied, agriculture was the overriding source of N, and changes in riverine N over time mainly reflected changes in land use and agricultural practices. In spite of decreasing N-leaching from agriculture, the net load remained fairly constant between 1885 and 1927, due to reduced N retention. Drainage of agricultural land had a dominating impact on reducing N retention, which increased the N loads, while the effects of the lowering of lake levels and dam building were less pronounced. Household N emission per capita was higher in 1994 than in 1927, as the increased consumption of meat and dairy products alone resulted in a higher increase of the emission than was compensated for with wastewater treatment improvement. In addition, introduction of flush toilets increased the emission from households. In total, the net load in 1976 was twofold higher than that in 1885, 1905 and 1927, due to increased leaching from agriculture, wastewater emission, and atmospheric deposition on lake surfaces. Finally, the impact of climatological variability was assessed, using a 110-yr climatological record. The choice of 10-yr period of climatological data was the factor that had the largest impact on calculated N load. [ABSTRACT FROM AUTHOR]