Dissolved Inorganic Nitrogen Export Research Articles

A Framework to Quantify Riverine Dissolved Inorganic Nitrogen Exports under Changing Land-Use Patterns and Hydrologic Regimes

Riverine dissolved inorganic nitrogen (DIN), when elevated by human activities (e.g., land-use change), can accelerate the nitrogen cycle and downstream dispersal. However, estimating DIN export coefficients for individual land-use types can be complex due to mosaic land-use patterns and interactions between fertilizers and hydrological processes. We propose a framework that integrates an empirical model, a moving-window method, and an elasticity method to quantify seasonal DIN export coefficients for each land use in the Shixi Creek catchment, southeast China. Our model showed good agreement with field observations according to root mean square error and a normalized objective function. The export coefficients of farmland and forest were the highest (9.16 mg L−1) and lowest (2.91 mg L−1) ones, resulting in annual DIN exports, respectively, for farmland and forests of 1951 kg km−2 yr−1 and 619 kg km−2 yr−1, respectively. Urbanization was a dominant factor influencing DIN export; the export coefficient of built-up areas showed the highest elasticity and highest uncertainty, with abrupt fluctuations from dry to wet years. Our framework revealed the complex role of built-up areas in nitrogen export. Our results can shed light on how to improve riverine N management in a catchment by considering the interactive effects of climate and land use.

Influence of nitrogen inputs, dam construction and landscape patterns on riverine nitrogen exports in the Yangtze River basin during 1980–2015

A quantitative understanding of different N sources and the factors controlling riverine N fluxes is critical for optimizing water pollution control strategies. However, the lack of a comprehensive understanding of N pollution legacy effects and human alteration of landscapes (e.g., dam construction and land use/land cover) hinders development of efficient N management strategies. This study integrated long-term (1980–2015) net anthropogenic nitrogen inputs (NANI) and different forms of N exports, hydroclimate data, dam construction and landscape metrics to elucidate N sources and the factors controlling N fluxes in the Yangtze River basin (YRB). Over the 36-year study period, annual NANI in the YRB increased from 4166 to 8571 kg N km−2 yr−1. Riverine concentrations of total nitrogen (TN), dissolved inorganic nitrogen (DIN) and dissolved organic nitrogen (DON) increased ∼ 3-fold, ∼3-fold and ∼ 36-fold during 1980–2015, respectively, whereas riverine particulate nitrogen (PN) decreased by ∼ 90 %. Such long-term riverine N export dynamics were driven by elevated NANI, landscape configuration change and dam construction activities. Inclusion of these variables in multiple linear regression models explained 91 %, 83 % and 96 % of the annual variability in riverine DIN, DON and PN fluxes, respectively. Estimated riverine TN flux from sum of predicted DIN, DON and PN fluxes showed high agreement with measured values (R2 = 0.92, Nash–Sutcliffe coefficients = 0.91), indicating strong efficacy for the developed regression models. Separation of annual riverine N exports into current year NANI versus legacy source contributions indicated that legacy sources contributed ∼ 49 % (26–82 %), 51 % (<1–78 %), 95 % (87–98 %) and 56 % (36–86 %) of riverine DIN, DON, PN and TN exports, respectively. The release of legacy N from the landscape is associated with increases in dam construction, urban density, and the number of small area farm operations between 1980 and 2015. This study highlights the potential of aggregating multiple historical N input datasets, along with changes in land use and river regulation to model long-term riverine N export dynamics. The results of the study provide a foundation for developing integrated watershed N management strategies that address both current and legacy N pollution.

Investigating the effect of lab bias on long-term stream chemistry data

AbstractLong-term (LT) stream chemistry studies are used to examine changes in and responses to the environment. Much of the data collected over long periods of time goes through changes in instrumentation, methods, and personnel potentially resulting in changing values. A data user must understand these measures of data quality through quality control (QC) results to know with certainty if trends are real or attributable to other factors. We used the Web of Science database search engine to search for LT stream chemistry studies. For each study, we then determined: record or study length; if QC was reported; and if QC was used. We found that 33% of papers reported QC in the method, and 12% presented the QC in the results. Next, we conducted a case study on 46 years of stream chemistry data to evaluate the data with and without the application of QC protocols from two watersheds (WS) at Coweeta Hydrologic Laboratory; WS 7; clear-cut in 1967–77 and adjacent WS 2 which serves as a reference. We focused on nitrogen and sulfur due to their importance in understanding the forest ecosystem response to disturbance (NO3) and acid deposition (SO4). We determined average annual dissolved inorganic nitrogen (DIN) export (NH4 + NO3 = DIN) using three methods for censoring values below the method detection limit (mdl): (1) the found value, (2) the value of zero, and (3) one-half the mdl value. We found that DIN export for WS 2/WS 7 was (1) 66.9/831.4 (g ha−1 yr−1), (2) 45.4/808.0 (g ha−1 yr−1), and (3) 72.1/823.2 (g ha−1 yr−1) using the three censoring methods, and that the export estimate was significantly different for WS 2 but not for WS 7 (P = 0.001). We found that on average stream NH4 concentrations were below the mdl 58% of the time until an instrument change in 1994 resulted in improved mdls resulting in fewer data points below detection. We found increased bias for stream SO4 concentration following an instrumentation change from segmented flow analysis to ion chromatography. As a result, stream SO4 concentration data that were bias-corrected declined more rapidly in WS 2 compared with non-bias-corrected data, but not in WS 7. We conclude that including QC results with LT data is essential to verify data validity and give the data user a full understanding of the results.

Effects of Anthropogenic Disturbances and Climate Change on Riverine Dissolved Inorganic Nitrogen Transport

AbstractNitrogen (N) transport from land to rivers, estuaries, and coastal marine systems has been markedly altered by anthropogenic and climatic drivers over recent decades. In this study, a riverine N transport scheme considering anthropogenic N discharge and water regulation was incorporated into the Land Surface Model of the Chinese Academy of Sciences (CAS‐LSM). Seven groups of simulations using the developed model at the global scale for the period of 1981–2010 were conducted to investigate the effects of anthropogenic disturbances and climate change on riverine dissolved inorganic nitrogen (DIN) transport. It was shown that fertilization and point source pollution have enhanced the DIN fluxes in rivers across the world, especially in western Europe and eastern China. The DIN exports were significantly reduced due to retention by reservoirs and the withdrawal of surface water and groundwater, with a retention efficiency of 50–70%. Climate variability and trends increased or decreased the riverine DIN fluxes depending on the specific hydroclimatic conditions. We further analyzed the contributions of climatic and anthropogenic changes to the riverine DIN changes in four major rivers. The riverine DIN exports in the Mississippi River Basin were affected primarily by fertilization, while the changes in DIN exports of the Danube were dominated by point source pollution and water regulation. The Yangtze River in China was seriously affected by both fertilization and point source pollution, and water regulation played a significant role in reducing DIN exports. Climate variability was the primary factor explaining the interannual variability of DIN exports.

Landsat-Based Indices Reveal Consistent Recovery of Forested Stream Catchments from Acid Deposition

Central European forests suffered from severe, large-scale atmospheric depositions of sulfur and nitrogen due to coal-based energy production during the 20th century. High deposition of acid compounds distorted soil chemistry and had negative effects on forest physiology and growth. Since 1994, continuous data on atmospheric deposition and stream runoff fluxes have provided evidence of ecosystem recovery from acidification. In this study, we combined for the first time mass budget data (sulfur deposition and total dissolved inorganic nitrogen (DIN) export) from the GEOMON monitoring network of headwater catchments with annual trajectories of vegetation indices derived from Landsat remote sensing observations. Time series of selected vegetation indices was constructed from Landsat 5, 7, and 8 using Google Earth Engine. Linear regression between the field data and vegetation indices was analyzed using R software. Biogeochemical responses of the forested catchment to declining acid deposition (driven by SO2 emission reduction) were consistent across all catchments covering various forest stands from different regions of the Czech Republic. Significant correlations were found with total sulfur depositions, suggesting that the forests are continuously and consistently prospering from reductions in acid deposition. Disturbance index (DI) was the only vegetation index that was well-related to changes in forest cover associated with salvage loggings (due to the forest decline) during the 1980s and 1990s. A significant relationship (R2 = 0.82) was found between the change in DI and DIN export in stream water. Regrowth of young forests in these highly affected areas tracks the most pronounced changes in total DIN export, suggesting a prominent role of vegetation in nitrogen retention. With the Landsat-derived DI, we could map decennial changes in forest disturbances beyond the small scale of the catchments to the regional level (demonstrated here for two protected landscape areas). This analysis showed the peak in forest disturbances to have occurred around the mid-1990s, followed by forest recovery and regrowth. Despite the improvement in forest ecosystem functioning over the past three decades in mountainous areas, emerging threats connected to changing climate will shape forest development in the near future.

Seasonality in river export of nitrogen: A modelling approach for the Yangtze River

In China, many estuaries suffer from eutrophication problems such as green tides and hypoxia. This is often a result of human activities on land leading to increased nutrient exports by rivers. River pollution shows seasonal trends that are not well understood. Therefore, the main objective of this study is to improve our understanding of the seasonal variation in river export of dissolved inorganic nitrogen (DIN) by source and at the sub-basin scale. To this end, we modified the existing MARINA model 1.0 (Model to Assess River Input of Nutrient to seAs) to account for seasonality in river export of DIN, and applied it to the Yangtze River. The resulting MARINA model version 1.1 takes a mass-balance approach and accounts for seasonality in human activities (e.g., crop planting and fertilization) and meteorology. The model distinguishes four seasons: winter (December–February), spring (March–May), summer (June–August) and fall (September–November). Our results for Yangtze indicate that N inputs to land and river export of DIN to sea are higher in summer and lower in winter. In spring, summer and fall, diffuse sources from agriculture contribute 43–85% to DIN export. In spring and fall, use of synthetic N fertilizers in cropland is an important source of DIN. In summer, both atmospheric N deposition and synthetic N fertilizers dominate. Animal manure is typically applied on land in spring and fall, contributing then to DIN. In winter, point sources of animal manure are responsible for 34–74% of DIN river export. In general, more DIN is exported to the sea from activities in middlestream and downstream sub-basins. Our results can serve as an example for other large rivers worldwide, and support the formulation of effective strategies to reduce seasonal eutrophication.

Effects of instream processes, discharge, and land cover on nitrogen export from southern Appalachian Mountain catchments

AbstractCatchments with minimal disturbance usually have low dissolved inorganic nitrogen (DIN) export, but disturbances and anthropogenic inputs result in elevated DIN concentration and export and eutrophication of downstream ecosystems. We studied streams in the southern Appalachian Mountains, USA, an area dominated by hardwood deciduous forest but with areas of valley agriculture and increasing residential development. We collected weekly grab samples and storm samples from nine small catchments and three river sites. Most discharge occurred at baseflow, with baseflow indices ranging from 69% to 95%. We identified three seasonal patterns of baseflow DIN concentration. Streams in mostly forested catchments had low DIN with bimodal peaks, and summer peaks were greater than winter peaks. Streams with more agriculture and development also had bimodal peaks; however, winter peaks were the highest. In streams draining catchments with more residential development, DIN concentration had a single peak, greatest in winter and lowest in summer. Three methods for estimating DIN export produced consistent results. Annual DIN export ranged from less than 200 g ha−1 year−1 for the less disturbed catchments to over 2,000 g ha−1 year−1 in the catchments with the least forest area. Land cover was a strong predictor of DIN concentration but less significant for predicting DIN export. The two forested reference catchments appeared supply limited, the most residential catchment appeared transport limited, and export for the other catchments was significantly related to discharge. In all streams, baseflow DIN export exceeded stormflow export. Morphological and climatological variation among watersheds created complexities unexplainable by land cover. Nevertheless, regression models developed using land cover data from the small catchments reasonably predicted concentration and export for receiving rivers. Our results illustrate the complexity of mechanisms involved in DIN export in a region with a mosaic of climate, geology, topography, soils, vegetation, and past and present land use.

Modeling investigation of the nutrient and phytoplankton variability in the Chesapeake Bay outflow plume

The Chesapeake Bay outflow plume (CBOP) is the mixing zone between Chesapeake Bay and less eutrophic continental shelf waters. Variations in phytoplankton distribution in the CBOP are critical to the fish nursery habitat quality and ecosystem health; thus, an existing hydrodynamic-biogeochemical model for the bay and the adjacent coastal ocean was applied to understand the nutrient and phytoplankton variability in the plume and the dominant environmental drivers. The simulated nutrient and chlorophyll a distribution agreed well with field data and real-time satellite imagery. Based on the model calculation, the net dissolved inorganic nitrogen (DIN) and phosphorus (DIP) flux at the bay mouth was seaward and landward during 2003–2012, respectively. The CBOP was mostly nitrogen-limited because of the relatively low estuarine DIN export. The highest simulated phytoplankton biomass generally occurred in spring in the near field of the plume. Streamflow variations could regulate the estuarine residence time, and thus modulate nutrient export and phytoplankton biomass in the plume area; in comparison, changing nutrient loading with fixed streamflow had a less extensive impact, especially in the offshore and far-field regions. Correlation analyses and numerical experiments revealed that southerly winds on the shelf were effective in promoting the offshore plume expansion and phytoplankton accumulation. Climate change including precipitation and wind pattern shifts is likely to complicate the driving mechanisms of phytoplankton variability in the plume region.

Unexpected ecological advances made possible by long‐term data: A Coweeta example

In the 1970s, Forest Service and academic researchers clearcut the forest in Watershed 7 in the Coweeta Basin to observe how far the perturbation would move the ecosystem and how quickly the ecosystem would return to its predisturbance state. Our long‐term observations demonstrated that this view of resistance and resilience was too simplistic. Forest disturbance triggered a chain of ecological dynamics that are still evolving after 40 years. Short‐term pulses in dissolved inorganic nitrogen (DIN) (3 years) and streamflows (4 years) were followed by several years in which the system appeared to be returning to predisturbance conditions. Then however, changes in forest composition triggered a regime change in DIN dynamics from biological to hydrological control as well as persistent high stream DIN levels mediated by climatic conditions. These forest composition changes also led to later reductions in streamflow. These long‐term observations of streamflows, stream DIN concentrations, stream DIN exports, and stand composition have substantially advanced our understanding of forest ecosystem dynamics; and they demonstrate the value of long‐term observational data in revealing ecosystem complexities and surprises, generating new hypotheses, and motivating mechanistic research. Shorter observational records from this experiment would have produced incomplete or erroneous inference. WIREs Water 2018, 5:e1273. doi: 10.1002/wat2.1273This article is categorized under: Science of Water &gt; Hydrological Processes Science of Water &gt; Water Quality Water and Life &gt; Nature of Freshwater Ecosystems

Seasonality of dissolved nitrogen from spring melt to fall freezeup in Alaskan Arctic tundra and mountain streams

Predicting the response of dissolved nitrogen export from Arctic watersheds to climate change requires an improved understanding of seasonal nitrogen dynamics. Recent studies of Arctic rivers emphasize the importance of spring thaw as a time when large fluxes of nitrogen are exported from Arctic watersheds, but studies capturing the entire hydrologic year are rare. We examined the temporal variability of dissolved organic nitrogen (DON) and dissolved inorganic nitrogen (DIN) concentrations in six streams/rivers in Arctic Alaska from spring melt to fall freeze-up (May through October) in 2009 and 2010. DON concentrations were generally high during snowmelt and declined as runoff decreased. DIN concentrations were low through the spring and summer and increased markedly during the late summer and fall, primarily due to an increase in nitrate. The high DIN concentrations were observed to occur when seasonal soil thaw depths were near maximum extents. Concurrent increases in DIN and DIN-to-chloride ratios suggest that net increases from nitrogen sources contributed to these elevated DIN concentrations. Our stream chemistry data, combined with soil thermistor data, suggest downward penetration of water into seasonally thawed mineral soils, and reduction in biological nitrogen assimilation relative to remineralization, may increase DIN export from Arctic watersheds during the late summer and fall. While this is part of a natural cycle, improved understanding of seasonal nitrogen dynamics is particularly important now because warmer temperatures in the Arctic are causing earlier spring snowmelt and later fall freeze-up in many regions.

Modeling the terrestrial N processes in a small mountain catchment through INCA-N: A case study in Taiwan

Riverine dissolved inorganic nitrogen (DIN) is an important indicator of trophic status of aquatic ecosystems. High riverine DIN export in Taiwan, ~3800kg-Nkm−2yr−1, which is ~18 times higher than the global average, urges the need of thorough understanding of N cycling processes. We applied INCA-N (Integrated Nitrogen Catchment Model) to simulate riverine DIN export and infer terrestrial N processes using weekly rainwater and streamwater samples collected at the Fushan Experimental Forest (FEF) of northern Taiwan. Results showed that the modeled discharge and nitrate export are in good agreement with observations, suggesting the validity of our application. Based on our modeling, the three main N removal processes, in the order of descending importance, were plant uptake, riverine N transport and denitrification at FEF. The high plant uptake rate, 4920kg-Nkm−2yr−1, should have led to accumulation of large biomass but biomass at FEF was relatively small compared to other tropical forests, likely due to periodic typhoon disruptions. The low nitrate concentration but high DIN export highlights the importance of hydrological control over DIN export, particularly during typhoons. The denitrification rate, 750kg-Nkm−2yr−1, at FEF was also low compared to other tropical forest ecosystems, likely resulting from quick water drainage through the coarse-loamy top soils. The high DIN export to atmospheric deposition ratio, 0.45, suggests that FEF may be in advanced stages of N excess. This simulation provides useful insights for establishing monitoring programs and improves our understanding N cycling in subtropical watersheds.

Contribution of urban runoff in Taipei metropolitan area to dissolved inorganic nitrogen export in the Danshui River, Taiwan.

A previous study has demonstrated that Danshui River has almost the highest dissolved inorganic nitrogen (DIN) yield in the world and exports most of the DIN in the form of ammonium unlike the world's large rivers. However, the DIN sources are poorly constrained. In this study, the contributions of major sources in the Taipei metropolitan area to the DIN export in the Danshui River were investigated. It is observed that ammonium is the major DIN species in the downstream reaches, resulting from the ammonium-dominated inputs of the effluents of wastewater treatment plants (WWTP) and rain water pumping stations (RWPS). DIN concentrations in the downstream (urban) reaches are substantially elevated. The upstream tributaries annually discharge ∼2709t DIN to the downstream reaches. However, the DIN discharge off the downstream reaches rises to ∼17,918t, resulting from the contribution of RWPS-collected water, i.e., ∼14,632t, and the effluents of two WWTP, i.e., ∼577t. RWPS-collected water inherently contains the contribution of atmospheric deposition, ∼2937t DIN. This finding implies that ∼11,695t (∼66% of the downstream output) DIN flux off the Danshui River is from urban runoff and can be attributed to human activities in the Taipei metropolitan area. To improve the water quality in the Danshui River, water quality controls in urban runoff are important.

The role of seasonal and hydrological conditions in regulating dissolved inorganic nitrogen budgets in a forested catchment in SW Slovenia

During two consecutive years the monitoring of rainfall nitrate (NO3−) and ammonium (NH4+) concentrations, combined with high-frequency measurements of streamwater NO3− concentrations, provided insight into the mechanisms controlling the dissolved inorganic nitrogen (DIN) budgets of the forested Padež stream catchment in SW Slovenia. During both years, the catchment's annual wet atmospheric DIN deposition (12.6 and 13.8kg-N/ha) exceeded the annual DIN export (10.7kg-N/ha and 8.8kg-N/ha). The analysis of the monthly DIN budgets discloses seasonal patterns. In winter and early spring, the catchment behaves as a net source of DIN, whereas during late spring, summer, and early fall the catchment generally behaves as a net sink of the atmospheric DIN. Due to large evapotranspiration in the growth period, most of the DIN remains in the catchment and is flushed during the dormant season. Despite high evapotranspiration and consequently low runoff coefficients characteristic for the Mediterranean climate, large rainfall events in the growth period can cause intensive washout of DIN from the catchment. At a monthly basis, the DIN flushing might exceed the catchment's DIN retention capacity and the catchment might behave as a net source of DIN. Therefore, the hydrological conditions have the ability to shift the catchment's role in regulating the DIN budgets. The high responsiveness of the streamwater NO3− concentration in the growth period indicates a large pool of DIN in the forest soils which can become mobile by runoff formation. Wet atmospheric DIN deposition at the Padež catchment is considerable; highest DIN inputs can be expected when air masses approach the catchment from the southwest to southeast where there are extensive urban areas along the Italian, Slovenian and Croatian coast. However, the Padež catchment does not appear to be approaching N saturation, presumably due to strong internal N cycle in the forest soils.

Assessing the potential to decrease the Gulf of Mexico hypoxic zone with Midwest US perennial cellulosic feedstock production

AbstractThe goal of this research was to determine the changes in streamflow, dissolved inorganic nitrogen (DIN) leaching and export to the Gulf of Mexico associated with a range of large‐scale dedicated perennial cellulosic bioenergy production scenarios within in the Mississippi–Atchafalaya River Basin (MARB). To achieve this goal, we used Agro‐IBIS, a vegetation model capable of simulating the biogeochemistry of row crops, miscanthus and switchgrass, coupled with THMB, a hydrology model capable of simulating streamflow and DIN export. Simulations were conducted at varying fertilizer application rates (0–200 kg N ha−1) and fractional replacement (5–25%) of current row crops with miscanthus or switchgrass across the MARB. The analysis also includes two scenarios where miscanthus and switchgrass (MRX and MRS, respectively) each replace the ca. 40% of maize production currently devoted to ethanol. Across the scenarios, there were minor reductions in runoff and streamflow throughout the MARB, with the largest differences (ca. 6%) occurring for miscanthus at the highest fractional replacement scenarios in drier portions of the region. However, differences in total MARB discharge at the basin outlet were less than 1.5% even in the MRX scenario. Reductions in DIN export were much larger on a percentage basis than reductions in runoff, with the highest replacement scenarios decreasing long‐term mean DIN export by ca. 15% and 20% for switchgrass and miscanthus, respectively. Fertilization scenarios show that significant reductions in DIN leaching are possible even with application rates of 100 and 150 kg N ha−1 for switchgrass and miscanthus, respectively. These results indicate that, given targeted management strategies, there is potential for miscanthus and switchgrass to provide key ecosystem services by reducing the export of DIN, while avoiding hydrologic impacts of reduced streamflow.

Acid rain mitigation experiment shifts a forested watershed from a net sink to a net source of nitrogen

Decades of acid rain have acidified forest soils and freshwaters throughout montane forests of the northeastern United States; the resulting loss of soil base cations is hypothesized to be responsible for limiting rates of forest growth throughout the region. In 1999, an experiment was conducted that reversed the long-term trend of soil base cation depletion and tested the hypothesis that calcium limits forest growth in acidified soils. Researchers added 1,189 kg Ca(2+) ha(-1) as the pelletized mineral wollastonite (CaSiO3) to a 12-ha forested watershed within the Hubbard Brook Experimental Forest in the White Mountains of New Hampshire. Significant increases in the pH and acid-neutralizing capacity of soils and streamwater resulted, and the predicted increase in forest growth occurred. An unanticipated consequence of this acidification mitigation experiment began to emerge a decade later, with marked increases in dissolved inorganic nitrogen (DIN) exports in streamwater from the treated watershed. By 2013, 30-times greater DIN was exported from this base-treated watershed than from adjacent reference watersheds, and DIN exports resulting from this experiment match or exceed earlier reports of inorganic N losses after severe ice-storm damage within the study watershed. The discovery that CaSiO3 enrichment can convert a watershed from a sink to a source of N suggests that numerous potential mechanisms drive watershed N dynamics and provides new insights into the influence of acid deposition mitigation strategies for both carbon cycling and watershed N export.

Net anthropogenic nitrogen inputs (NANI) into the Yangtze River basin and the relationship with riverine nitrogen export

AbstractThis study investigated net anthropogenic nitrogen inputs (NANI, including atmospheric nitrogen deposition, nitrogenous fertilizer use, net nitrogen import in food and feed, and agricultural nitrogen fixation) and the associated relationship with riverine dissolved inorganic nitrogen (DIN) export in the Yangtze River basin during the 1980–2012 period. The total NANI in the Yangtze River basin has increased by more than twofold over the past three decades (3537.0 ± 615.3 to 8176.6 ± 1442.1 kg N km−2 yr−1). The application of chemical fertilizer was the largest component of NANI in the basin (51.1%), followed by net nitrogen import in food and feed (26.0%), atmospheric nitrogen deposition (13.2%), and agricultural nitrogen fixation (9.7%). A regression analysis showed that the riverine DIN export was strongly correlated with NANI and the annual water discharge (R2 = 0.90, p &lt; 0.01). NANI in the Yangtze River basin was estimated to contribute 37–66% to the riverine DIN export. We also forecasted future variations in NANI and riverine DIN export for the years 2013 to 2030, based on possible future changes in human activities and the climate. This work provides a quantitative understanding of NANI in the Yangtze River basin and its effects on riverine DIN export and helps to develop integrated watershed nitrogen management strategies.

Land-based nutrient loading to LMEs: A global watershed perspective on magnitudes and sources

Coastal resource management initiatives in recent years have moved towards ecosystem approaches such as embodied by Large Marine Ecosystems (LMEs). In this study, land-based dissolved inorganic nitrogen (DIN) loading to LMEs was evaluated using a spatially-explicit river export model (Global NEWS 2) for the year 2000 conditions and for a current trends analysis for the year 2050. Watershed export was aggregated by LME to estimate total DIN load and attribution to diffuse and point sources including natural biological fixation, agricultural biological fixation, fertilizer, manure, atmospheric deposition and sewage. Biological fixation in natural landscapes was the primary source of DIN to many LMEs, but in most (73%) LMEs, over half of the total DIN load was related to anthropogenic sources. Most of the anthropogenic DIN load across LMEs was related to agricultural sources especially fertilizer and manure. Fertilizer was the primary source of DIN to LMEs in most of Europe and Asia, while manure was the primary source in most of Central and South America. Agricultural biological fixation, sewage and atmospheric deposition in general supported a minor fraction of the DIN exported to LMEs although each was a dominant source to a few LMEs. If current trends continue, DIN export to coastal systems by 2050 relative to 2000 is predicted to increase by approximately 40–45% from Africa, South America, South Asia and Oceania. Almost half of the total global increase in DIN is from South Asia. Relatively smaller increases are predicted for North America, with slight decreases in Australia and Europe.

Impacts of human activities and climate change on the water environment of Lake Poyang Basin, China

The global mean temperature has risen by 0.85 °C from 1880 to 2012, and this increase may even accelerate in the future. The surface temperatures of large inland water bodies worldwide have been rapidly warming since 1985 with an average rate of 0.045 ± 0.011 °C yr−1 and rates as high as 0.10 ± 0.01 °C yr−1. The global climate change was responsible for an increased contribution of diffuse nutrient losses to the total nutrient loads in water bodies. More and more algae blooms were reported in inland water bodies. Lake Poyang, the largest freshwater lake in China, is experiencing a deterioration of water quality. Ganjiang River, as the longest river in the Lake Poyang catchment, plays an important role in linking between terrestrial and aquatic ecosystems. A large amount of N pollutants is transported through the Ganjiang River to Lake Poyang. It is important to study quantitatively human enhanced N (nitrogen) inputs and DIN (Dissolved Inorganic Nitrogen) exports through rivers in the background of global warming. In this paper, we estimate the inputs of nitrogen (N) and exports of dissolved inorganic nitrogen (DIN) from the Ganjiang River to Lake Poyang for the period 1995-2010 by using the Global NEWS-DIN model. Modeled DIN yields range from 1038 kg N km−2 yr−1 in 1995 to 150 kg N km−2 yr−1 in 2010, showing a decreasing trend. The study demonstrates a varied contribution of different N inputs to river DIN yields during the period 1995-2010. Biological N fixation contributes about 73.3 % of DIN yields before 2005, but the contribution decreases substantially to 28.4 % in 2010. Chemical N fertilizer application and animal manure N inputs together contribute 12.6 % of the river DIN yields, while atmospheric N deposition contributes an average of 5.5 % of DIN yields in the period 1995-2010. Sewage N inputs contributes an average of 5.2 % of DIN yields over the period 1995-2005, while the contribution increases remarkably to 69.7 % in 2010. The trophic state index (TSI) of Lake Poyang agrees well with modeled DIN yields from the Ganjiang River over the whole study period. N inputs from nonpoint sources are approximately 10-54 times that from point sources. Almost all the diffuse DIN inputs were retained in the watershed before entering the lake Poyang. While about half of sewage N inputs were input to Lake Poyang as DIN from the Ganjiang River. Modeled DIN yield from Ganjiang River decreased after 2005 significantly, providing evidence for the recovering capacity of watersheds to retain DIN as inputs decrease as a direct result of human activities. The study also demonstrates a varied contribution of different N inputs to river DIN yields during the study period. The authors also point out that more attention should be kept on reducing sewage N discharge to Lake Poyang in the background of global warming.

Effects of lateral nitrate flux and instream processes on dissolved inorganic nitrogen export in a forested catchment: A model sensitivity analysis

AbstractThe importance of terrestrial and aquatic ecosystems in controlling nitrogen dynamics in streams is a key interest of ecologists studying dissolved inorganic nitrogen (DIN) export from watersheds. In this study, we coupled a stream model with a terrestrial ecohydrological model and conducted a global sensitivity analysis to evaluate the relative importance of both ecosystems to nitrogen export. We constructed two scenarios (“normal” and high nitrate loads) to explore conditions under which terrestrial (lateral nitrate flux) or aquatic ecosystems (instream nutrient processes) may be more important in controlling DIN export. In a forest catchment, although the forest ecosystem controls the nitrogen load to streams, sensitivity results suggested that most nitrogen output from the terrestrial ecosystem was taken up by instream microbial immobilization associated with benthic detritus and retained in detritus. Later the immobilized nitrogen was remineralized as DIN. Therefore, the intra‐annual pattern of DIN concentration in the stream was low in fall and became high in spring. Not only was instream microbial immobilization saturated with the high nitrogen load scenario, but also the net effect of immobilization and mineralization on DIN export was minimized because nitrogen cycling between organic and inorganic forms was accelerated. Overall, our linked terrestrial‐aquatic model simulations demonstrated that stream process could significantly affect the amount and timing of watershed nitrogen export when nitrogen export from the terrestrial system is low. However, when nitrogen export from the terrestrial system is high, the effect of stream processes is minimal.

Mechanisms of basin-scale nitrogen load reductions under intensified irrigated agriculture.

Irrigated agriculture can modify the cycling and transport of nitrogen (N), due to associated water diversions, water losses, and changes in transport flow-paths. We investigate dominant processes behind observed long-term changes in dissolved inorganic nitrogen (DIN) concentrations and loads of the extensive (465,000 km2) semi-arid Amu Darya River basin (ADRB) in Central Asia. We specifically considered a 40-year period (1960–2000) of large irrigation expansion, reduced river water flows, increased fertilizer application and net increase of N input into the soil-water system. Results showed that observed decreases in riverine DIN concentration near the Aral Sea outlet of ADRB primarily were due to increased recirculation of irrigation water, which extends the flow-path lengths and enhances N attenuation. The observed DIN concentrations matched a developed analytical relation between concentration attenuation and recirculation ratio, showing that a fourfold increase in basin-scale recirculation can increase DIN attenuation from 85 to 99%. Such effects have previously only been observed at small scales, in laboratory experiments and at individual agricultural plots. These results imply that increased recirculation can have contributed to observed increases in N attenuation in agriculturally dominated drainage basins in different parts of the world. Additionally, it can be important for basin scale attenuation of other pollutants, including phosphorous, metals and organic matter. A six-fold lower DIN export from ADRB during the period 1981–2000, compared to the period 1960–1980, was due to the combined result of drastic river flow reduction of almost 70%, and decreased DIN concentrations at the basin outlet. Several arid and semi-arid regions around the world are projected to undergo similar reductions in discharge as the ADRB due to climate change and agricultural intensification, and may therefore undergo comparable shifts in DIN export as shown here for the ADRB. For example, projected future increases of irrigation water withdrawals between 2005 and 2050 may decrease the DIN export from arid world regions by 40%.