Nitrate Export Research Articles

Best management practices scenario analysis to reduce agricultural nitrogen loads and sediment yield to the semiarid Mar Menor coastal lagoon (Spain)

Agriculture is a major source of diffuse pollution, where nitrogen and sediment pollution of water bodies are its main associated environmental impacts. Best management practices are effective tools for preventing and minimizing water pollution. Water quality models and model-based scenario analyses are useful tools for assessing impacts of best management practices and to identify appropriate strategies on the watershed scale. This study was conducted in the southernmost Mar Menor basin, one of the largest saltwater coastal lagoons in Europe and threatened by diffuse nutrient and sediment export from the agricultural landscape. This study evaluates the impact of several management practices on nitrogen and sediment loads, and horticultural crop yield, to identify an appropriate management strategy on the watershed scale. Both structural and nonstructural management practices scenarios were evaluated: three scenarios representing field operations, two coastal line buffers, a new fertilizer management strategy and a change in the productive cultivation system from three-crop rotation to two-crop rotation. Each management practice reduced a certain type of diffuse pollution and, therefore, a combined set of changed management practices is necessary to cope with all agricultural pollution types. Contour farming, combined with hedgerow field borders, was effective in sediment yield and surface organic nitrogen export reduction terms, while improved fertilizer management reduced surface nitrate export and leaching with minimal impacts on crop yields.

Identifying the sources of nitrate contamination using a combined dual isotope, chemical and Bayesian model approach in a tropical agricultural river: Case study in the Mun River, Thailand

Environmental issues triggered by increasing nitrate in agricultural river has become global concern. Identifying nitrate sources and transformation are crucial for water sources protection and eliminating nitrate contamination in an agricultural watershed. In this study, chemical and dual isotopic compositions of nitrate were employed to trace the nitrate sources and transformation processes, and proportional contribution of NO3− source were estimated by SIAR based on Bayesian model. NH4+ concentrations in middle Mun and lower Mun in wet season were significantly higher than NO3−, suggesting enhanced runoff processes by flood promote agricultural fertilized NH4+ leaching into the river. Higher Cl− concentration and NO3−/Cl− indicated that manure and sewage was the dominate nitrate source in the Lam Takhong River and the upper Mun. The overall values of δ15N-NO3− and δ18O-NO3− ranged from −3.9‰ to +16.6‰ and from −5.2‰ to +40.0‰, respectively. The results of nitrate isotopes indicated that NO3− mainly originated from soil N nitrogen, chemical fertilizer, and manure and sewage wastes. Spatially, soil N and chemical fertilizer contributed the most nitrate in the mainstream of lower Mun, middle Mun, and the Lam Takhong River; whereas over 60% of nitrate was derived from manure and sewage in the upper Mun. The spatial variation of water discharge and rainfall, together with the nitrate concentration and isotopes inferred that the nitrate sources and transformations in rain-fed river in tropical zone were distinguished from other rivers. High water discharge driven by rainfall events accelerated the nitrate export and the contribution of atmospheric deposition in wet season, and enlarged the contribution of manure and sewage in dry season. This study provided an example for further researches and approaches to assess the effects of tropical climate and agriculture on nitrate accumulation in watershed.

Snowmelt periods as hot moments for soil N dynamics: a case study in Maine, USA.

The vernal transition represents the seasonal transition to spring, occurring as temperatures rise at the end of winter. With rapid snowmelt, microbial community turnover, and accelerated nutrient cycling, this is a critical but relatively under-studied period of ecosystem function. We conducted a study over two consecutive winters (2015-2016) at the Bear Brook Watershed in Maine to examine how changing winter conditions (warming winters, reduced snow accumulation) altered soil nitrogen availability and stream N export during winter and the vernal transition, and how these patterns were influenced by ecosystem N status (N-enriched vs. N-limited). Of the two study years, 2016 had a warmer winter with substantially less snow accumulation and a discontinuous snowpack-and as a result, had a longer vernal transition and a snowpack that thawed before the vernal transition began. Across both years, snowmelt triggered a transition, signaled by increased ammonium concentrations in soil, decreased soil nitrate concentrations due to flushing by meltwater, and increased stream nitrate exports. Despite the contrasting winter conditions, both years showed similar patterns in N availability and export, differing only in the timing of these transitions. The vernal transition has conventionally been considered a critical period for biogeochemical cycling, because the associated snowmelt event triggers physicochemical and biochemical changes in soil systems. This was consistent with our results in 2015, but our data for 2016 show that this may not always hold true, and instead, that warmer, low-snow winters may demonstrate a temporal asynchrony between snowmelt and the vernal transition. We also show that ecosystem N status is a strong driver of the seasonal N pattern, and the interaction of N status and changing climate must be further investigated to understand ecosystem function under our current predicted trajectory of warming winters, declining snowfall, and winter thaw events.

Long‐Term Trends in Acid Precipitation and Watershed Elemental Export From an Alpine Catchment of the Colorado Rocky Mountains, USA

AbstractAcid deposition associated with precipitation is an ecological problem that has affected watersheds in industrialized parts of North America and Europe, but remote landscapes, such as the Colorado Rocky Mountains, have also been impacted. The deposition of strong acids, including nitric and sulfuric acids, has decreased substantially over the past 30 years at Niwot Ridge, a high‐alpine watershed of Colorado. The pH of precipitation has followed these declines and has increased to ~5.5. Meanwhile, NH4+, another important constituent of acid/base balance, and a fertilizing nutrient, has more than doubled in precipitation between 1984 and 2017. A statistical model of long‐term water chemistry concentrations and loads revealed a variety of trends in watershed export of acids and bases, some of which were unexpected, and remain unexplained. For example, despite declining sulfate deposition, sulfate export has increased. On the other hand, watershed nitrate export has remained constant despite decreases in atmospheric deposition, while watershed NH4+ export has increased, albeit minimally, relative to total nitrogen export. Watershed pH and alkalinity appear to have stabilized in response to decreased acid precipitation, which may be explained by geochemical processes, such as carbonate weathering. Overall, in this high‐alpine watershed, atmospheric deposition is trending toward preindustrial conditions, and there is a need to conduct targeted, process‐based studies to determine the mechanisms underlying these trends. The unexpected watershed responses that we identify here require a new framework for understanding acid precipitation recovery.

Improving the effectiveness of saturated riparian buffers for removing nitrate from subsurface drainage.

A saturated riparian buffer (SRB) is an edge-of-field conservation practice that reduces nitrate export from agricultural lands by redistributing tile drainage as shallow groundwater and allowing for denitrification and plant uptake. We propose an approach to improve the design of SRBs by analyzing a tradeoff in choosing the SRB width, and we apply the approach to six sites with SRBs in central Iowa. A larger width allows for more residence time, which increases the opportunity for removing nitrate that enters the buffer. However, because the SRBs considered here treat only a portion of the tile flow when it is large, for the same difference in hydraulic head, a smaller width allows more of the total tile flow to enter the buffer and therefore treats more of the drainage. By maximizing the effectiveness of nitrate removal, defined as the ratio of total nitrate removed by the SRB to total nitrate leaving the field in tile drainage, an equation for the optimal width was derived in terms of soil properties, denitrification rates, and head difference. All six sites with existing SRBs considered here have optimal widths smaller than the current width, and two are below the minimum width listed in current design standards. In terms of uncertainty, the main challenges in computing the optimal width for a site are estimating the removal coefficient for nitrate and determining the saturated hydraulic conductivity. Nevertheless, including a width that accounts for site conditions in the design standards would improve water quality locally and regionally.

Analyzing impacts of seasonality and landscape gradient on event-scale nitrate-discharge dynamics based on nested high-frequency monitoring

Increasingly available high-frequency data during storm events, when hydrological dynamics most likely activate nitrate storage-flux exchanges, reveal insights into catchment nitrate dynamics. In this study, we explored impacts of seasonality and landscape gradients on nitrate concentration-discharge (C-Q) hysteresis patterns in the Selke catchment, central Germany, which has heterogeneous combinations of meteorological, hydrogeological and land use conditions. Three nested gauging stations established along the main Selke River captured flow and nitrate export dynamics from the uppermost subcatchment (mixed forest and arable land), middle subcatchment (pure steep forest) and lowermost subcatchment (arable and urban land). We collected continuous high-frequency (15-min) discharge and nitrate concentration data from 2012 to 2017 and analyzed the 223 events detected at all three stations. A dominant hysteresis pattern in the uppermost and middle subcatchments was counter-clockwise and combined with an accretion effect, indicating many proximal and mobilized distal nitrate sources. However, 66% of all events at the catchment outlet experienced a dilution effect, possibly due to mechanisms that vary seasonally. During wetting/wet periods (October-March), it was combined mainly with a counter-clockwise pattern due to the dominance of event runoff volume from the uppermost and middle subcatchments. During drying/dry periods (April-September), however, it was combined mainly with a clockwise pattern due to occasional quick surface flows from lowland near-stream urban areas. In addition, the clockwise hysteresis occurred mainly from May-October during mostly drying/dry periods at all three sites, indicating little distal nitrate transport in response to the low terrestrial hydrological connectivity, especially in the lowermost dry and flat subcatchment. This comprehensive analysis (i.e., clockwise vs. counter-clockwise, accretion vs. dilution) enables in-depth analysis of nitrate export mechanisms during certain periods under different landscape conditions. Specific combination of C-Q relationships could identify target locations for agricultural management actions that decrease nitrate output. Therefore, we strongly encourage long-term multisite and high-frequency monitoring strategies in heterogeneous nested catchment(s), which can help understand process mechanisms, generate data for physical-based water-quality modeling and provide guidance for water and agricultural management.

Optimal Transport for Assessing Nitrate Source‐Pathway Connectivity

AbstractExcessive nitrate threatens a wide range of water resources, aquatic habitats, and sensitive infrastructure. Despite this problem, tracing a nutrient from its eventual fate back to its origin remains an elusive challenge due to heterogeneity in how nutrient sources and hydrologic pathways are connected. Typically, this problem is underdetermined (i.e., too many unknowns, not enough equations) and cannot be solved with existing methodologies. The theory of optimal transport allows for the solution of underdetermined systems, and here we construct a novel formulation for its use in water quality modeling. Our objective was to develop an optimal transport modeling framework—coupled to Bayesian source unmixing, loadograph pathway separation, and geospatial connectivity analysis—to apportion nitrate loading from three sources (soil, fertilizer, and manure) across three pathways (quick, intermediate, and slow), resulting in nine possible source‐pathway couplings (soil‐quick, soil‐intermediate, …, manure‐slow). We apply this model to a 30 month elemental (NO3−) and isotopic (δ15N and δ18O) nitrate data set from a karst watershed in Kentucky, USA. Modeling results indicate that—of the nine possible source‐pathway couplings—nearly 60% of nitrate export is facilitated by just three: fertilizer‐quick (16.4%), manure‐intermediate (15.4%), and soil‐slow (27.2%). Further, we reinforce the need to explicitly consider heterogeneity in source‐pathway connectivity as homogeneous assumptions lead to erroneous inferences. The applicability of the model, its input requirements, and transferability to other sites is discussed. Lastly, we simulated two land management scenarios (field buffers and septic repair) and demonstrate how optimal transport can be used to test nutrient reduction strategies.

Rainfall and conduit drainage combine to accelerate nitrate loss from a karst agroecosystem: Insights from stable isotope tracing and high-frequency nitrate sensing

Understanding where nitrate is mobilized from and under what conditions is required to reduce nitrate loss and protect water quality. Low frequency sampling may inadequately capture hydrological and biogeochemical processes that will influence nitrate behavior. We used high-frequency isotope sampling and in-situ nitrate sensing to explore nitrate export and transformation in a karst critical zone. Nitrate was mobilised during light rainfall, and transferred from soil layers to the karst matrix, where some nitrate was retained and denitrified. Nitrate isotopic composition changed rapidly during the rising limb of events and slowly during the falling limb. The main nitrate source was synthetic fertiliser (up to 80% during event flow), transported by conduit flow following high rainfall events, and this contribution increased significantly as discharge increased. Soil organic nitrogen contribution remained constant indicating at baseflow this is the primary source. Isotope source appointment of nitrate export revealed that synthetic fertilizer accounted for more than half of the total nitrate export, which is double that of the secondary source (soil organic nitrogen), providing valuable information to inform catchment management to reduce nitrate losses and fluvial loading. Careful land management and fertilizer use are necessary to avoid nitrate pollution in the karst agroecosystem, for example by timing fertilizer applications to allow for plant uptake of nitrate before rainfall can flush it from the soils into the karst and ultimately into catchment drainage.

The Shallow and Deep Hypothesis: Subsurface Vertical Chemical Contrasts Shape Nitrate Export Patterns from Different Land Uses

Eutrophication has threatened water resources worldwide, yet mechanistic understanding on controls of nutrient export remains elusive. This work tests the shallow and deep hypothesis: subsurface vertical chemical contrasts regulate nitrate export patterns under different land use conditions. We synthesized data from 228 watersheds and used reactive transport modeling (500 simulations) under broad land use, climate, and geology conditions. Data synthesis indicated that human perturbation has amplified chemical contrasts in shallow water (e.g., soil water) versus deep waters (e.g., groundwater), inducing primarily flushing patterns (concentrations increase with streamflow) in agriculture lands and dilution patterns (concentrations decrease with streamflow) in urban watersheds. Results revealed a quantitative relationship between export patterns and shallow-versus-deep concentration contrasts, underscoring the often-overlooked role of nutrient distribution over depth. Results challenge the commonly held perception that legacy stores in agricultural lands induce chemostasis where concentrations vary negligibly with streamflow. They suggest that nitrate concentrations from agricultural lands will escalate during large hydrological events, which can exacerbate nutrient export problems as flooding events intensify in the future climate.

Nitrate Removal Across Ecogeomorphic Zones in Wax Lake Delta, Louisiana (USA)

AbstractHuman activities have increased nitrate export from rivers, degrading coastal water quality. At deltaic river mouths, the flow of water through wetlands increases nitrate removal, and the spatial organization of removal rates influences coastal water quality. To understand the spatial distribution of nitrate removal in a river‐dominated delta, we deployed 23 benthic chambers across ecogeomorphic zones with varying elevation, vegetation, and sediment properties in Wax Lake Delta (Louisiana, USA) in June 2018. Regression analyses indicate that normalized difference vegetation index is a useful predictor of summertime nitrate removal. Mass transfer velocity were approximately three times greater on a vegetated submerged levee (13 mm hr−1), where normalized difference vegetation index was greatest, compared to other locations (4.6 mm hr−1). Two methods were developed to upscale nitrate removal across the delta. The flooded‐delta method integrates spatially explicit potential removal rates across submerged portions of the delta and suggests that intermediate elevations on the delta—including submerged levees—are responsible for 70% of potential nitrate removal despite covering only 33% of the flooded area. The channel network method treats the delta as a network of river channels and suggests that although secondary channels are more efficient than primary channels at removing received nitrate, primary channels collectively contribute more to overall removal because they convey more of the total nitrate load. The two upscaling methods predict similar rates of nitrate removal, equivalent to less than 4% of nitrate entering the delta. To protect coastal waters against high nitrate loads, management policies should aim to reduce upstream nutrient loads.

Nitrate removal and young stream water fractions at the catchment scale

AbstractDespite extensive research on nitrate export and removal, nutrient contamination remains a major threat to water bodies worldwide. At the local scale, nitrate removal is governed by biogeochemical conditions that vary in space and time, making integration to entire landscapes critical. Water transit times have often been used to describe solute transport, but the relation between water age and nitrate removal at the catchment scale is still poorly understood. We test the hypothesis that nitrate removal peaks when the fraction of young water in discharge is at its minimum, because nitrate removal occurs mostly under dry conditions where deeper, older groundwater dominates streamflow. We tested this hypothesis by exploring a detailed water quality record from the Kervidy–Naizin catchment (FR) and comparing the dynamics of nitrate to those of a conservative solute (chloride). We find that estimates of nitrate removal are consistent with previous estimates at the site and they show a good (inverse) correlation with the fraction of streamflow that is younger than 2.5 months. However, this young water fraction cannot be used to predict nitrate removal in the winter–spring period, when no removal is observed regardless of streamflow age. While this leads us to reject our hypothesis during the winter period, it also suggests that water age distributions and their correlation with nitrate removal can possibly reveal distinct sources of stream water at different hydrologic regimes and relevant biogeochemical reactions.

The role of corn and soybean cultivation on nitrate export from Midwestern US agricultural watersheds

Accumulation of nitrogen (N) in reservoirs has negative effects, generating algal blooms, creating hypoxic zones, and degrading drinking water quality. Agricultural land use, specifically corn and soybean cultivation, contribute excess N to aquatic systems. This work examines whether the nitrate as nitrogen (NO3–N) export (load) from a watershed is correlated to the percentage of land devoted to growing corn or soybeans. To investigate potential relationships, Annual NO3–N load from within 10 watersheds in the US Midwest were calculated using discharge (Q) and NO3–N concentration data obtained from USGS gauging stations. The NO3–N load data were coupled with crop data from the watersheds. Watershed areas ranged from 106 to 154,767 km2. Corn was grown on between 14.3 and 56.1% of the land, while soybeans accounted for 7.2–45.4%. Crop percentages were compared to NO3–N loads per area, the quotient of annual NO3–N load (kg) to the watershed area (km2), from 2008 to 2017. The analyses of the collective data showed a positive trend between the percentage of land cultivated and the NO3–N exported. However, as the percentage of corn cultivated increased within the individual watersheds, NO3–N load per area decreased. At both the collective and individual scale, increases in soybean cultivation resulted in increases in NO3–N export. Extended cultivation of the same crop lead to higher NO3–N export. Strategies to decrease NO3–N export need to focus on enhanced crop rotation.

Diffuse nitrogen pollution in a forest-dominated watershed: Source, transport and removal

Along with gradually controlled point source discharges, diffuse nitrogen pollution has become an important cause of eutrophication. Diffuse nitrogen export from forests is an inherent characteristic, and increasing human disturbance aggravates this process. To control diffuse nitrogen pollution effectively, it is of great significance to determine the source, transport path and removal method of diffuse nitrogen export in forest-dominated watersheds. Long-term variations in the nitrogen concentrations indicate that nitrate and dissolved organic nitrogen (DON) are the main diffuse nitrogen export forms in the Buerhatong River watershed (BRW). Thus, the Soil and Water Assessment Tool (SWAT) hydrological model is conducted to simulate diffuse nitrate and DON export. First, main source of diffuse nitrate and DON export is determined by the fitting results of nitrogen contribution and forest/farmland cover. Then, the relationship between nitrogen export from forest/farmland and main hydrologic streamflow is explored. Finally, the role of wetlands in the diffuse nitrogen load removal is analyzed, and sensitivity results by Monte Carlo simulation of nitrogen export from wetlands, forests and farmlands are compared. The average annual diffuse nitrate and DON export values are 1.19 and 0.43 kg ha−1 yr−1, respectively, from 2014 to 2017. When forest cover and farmland cover are higher than 70% and 5%, farmlands and forests are the main sources of diffuse nitrate and DON export, respectively. Lateral flow is the dominant transport path of DON export from forests and nitrate export from farmlands, with correlation coefficient of 0.73 and proportion of 98%, respectively. Wetlands have the ability to reduce the diffuse nitrogen load, and their efficiency is related to the relative geographic location. The in situ experimental results show that the nitrate and DON removal rates by wetlands are 24.46% and 11.71%, respectively. The sensitivity of forest and wetland for diffuse nitrate and DON load removal is 0.31, −0.12 and 0.2, −0.21, respectively. Returning forests to wetlands is a good method for diffuse nitrogen removal with food production taken into consideration.

Positive correlation between wood δ15N and stream nitrate concentrations in two temperate deciduous forests

A limitation to understanding drivers of long-term trends in terrestrial nitrogen (N) availability in forests and its subsequent influence on stream nitrate export is a general lack of integrated analyses using long-term data on terrestrial and aquatic N cycling at comparable spatial scales. Here we analyze relationships between stream nitrate concentrations and wood δ15N records (n = 96 trees) across five neighboring headwater catchments in the Blue Ridge physiographic province and within a single catchment in the Appalachian Plateau physiographic province in the eastern United States. Climatic, acidic deposition, and forest disturbance datasets were developed to elucidate the influence of these factors on terrestrial N availability through time. We hypothesized that spatial and temporal variation of terrestrial N availability, for which tree-ring δ15N records serve as a proxy, affects the variation of stream nitrate concentration across space and time. Across space at the Blue Ridge study sites, stream nitrate concentration increased linearly with increasing catchment mean wood δ15N. Over time, stream nitrate concentrations decreased with decreasing wood δ15N in five of the six catchments. Wood δ15N showed a significant negative relationship with disturbance and acidic deposition. Disturbance likely exacerbated N limitation by inducing nitrate leaching and ultimately enhancing vegetative uptake. As observed elsewhere, lower rates of acidic deposition and subsequent deacidification of soils may increase terrestrial N availability. Despite the ephemeral modifications of terrestrial N availability by these two drivers and climate, long-term declines in terrestrial N availability were robust and have likely driven much of the declines in stream nitrate concentration throughout the central Appalachians.

Tracking nitrogen pollution sources in plain watersheds by combining high-frequency water quality monitoring with tracing dual nitrate isotopes

High-frequency water quality measurements at daily resolutions combined with tracing of nitrate δ15N and δ18O isotopes was proposed as a method to quantify nitrogen (N) export from point and nonpoint sources, which is usually difficult to achieve at a watershed scale. The method was applied to two agricultural watersheds located in two nearby plains along the Xifeihe and Nihe Rivers in China. The two watersheds had different crops and planting patterns, in addition to their different urbanization percentages of 25.3% and 14.1%, respectively. Analysis showed that these two watersheds exported approximately 7.8 and 11.2 kg N ha−1y−1 to their receiving rivers. Isotope analysis identified that point sources of sewage/manure contributed majority (53%) of nitrate load to the Xifeihe River, whereas nonpoint sources from soils, fertilizer and atmospheric deposition contributed majority (60%) of nitrate load to the Nihe River. Crop patterns reduced the N export through soybean planting or denitrification in rice paddies despite 200–300 kg N ha−1 fertilizer application to the fields per year. Annual nitrate export patterns of the two watersheds did not show correlations with their corresponding daily flow rates, which had important implications to watershed monitoring design. Nitrogen (N) concentrations were neither chemeostatic nor discharge-correlated except for during high flows in the soybean-wheat agriculture dominant Xifeihe watershed. This indicated that the traditional monitoring protocol for pollution source identification with measurements only conducted during few rainfall events was likely to be insufficient to estimate the total loads of nutrients (e.g., N) from watersheds.

Excess Nitrate Export in Mountaintop Removal Coal Mining Watersheds

AbstractThroughout the Central Appalachian ecoregion, mountaintop removal coal mining (MTM) is the predominant form of land use change. The streams draining MTM impacted watersheds have been reported to contain high stream nitrate (NO3−) concentrations, yet the source and persistence of elevated NO3− remains unknown. Here we compiled data from multiple sources to conduct a regional evaluation of the impact of mining on stream NO3−, examine NO3− persistence after mining cessation, and identify potential N sources. Using water quality data from six large watersheds, we observe elevated NO3− in watersheds with the highest active mining density. At four small MTM watersheds with repeat sampling, we found that high levels of NO3− concentrations declined after mining cessation but remain elevated above reference after several decades. At MTM watersheds, we found annual mass flux of NO3− was 3.68 to 26.4 kg N ha−1 year−1, which is 1 to 2 orders of magnitude higher than a nearby forested reference watershed. Stream water NO3− isotopic ratios at these sites did not match previously suggested NO3− sources such as explosives used during mining and fertilizer applied during reclamation but were highly enriched in both δ15N and δ18O compared to the reference watershed suggesting high rates of NO3− retention. Explosive residue could account for the bulk of watershed NO3− export during active mining phases but other mining related N sources including fertilizer and rock‐derived N, the construction of valley fills, and alterations in watershed NO3‐ cycling likely contribute to the persistence of elevated NO3− export observed in this study.

Estimation of baseflow nitrate loads by a recursive tracing source algorithm in a rainy agricultural watershed

AbstractBaseflow has become an important source of nitrate nonpoint source pollution in many intensive agricultural watersheds. Uncertainties in baseflow nutrient load separation are caused by the effects of hydrometeorological factors on both baseflow recession and baseflow nutrient load recession. These uncertainties have not been addressed well in the existing separating algorithms, which are based on simple baseflow rate–load relationships. In the present study, a recursive tracing source algorithm (RTSA) was developed based on a nonlinear reservoir algorithm and hydrometeorology‐corrected baseflow nutrient load recession parameter. This approach was used to reduce the uncertainty of baseflow nitrate load estimation caused by variations in different load recessions under varying climate conditions. RTSA validation in a typical rainy agricultural watershed yielded Nash–Sutcliffe efficiency, root mean square error‐observation standard deviation ratio, and R2 values of 0.91, 0.30, and 0.91, respectively. The baseflow nitrate–nitrogen (N─NO3–) loads from 2003 to 2012 in the Changle River watershed of eastern China were estimated with the RTSA. The results indicated that baseflow nitrate export accounted for 62.0% of the mean total annual N─NO3– loads (18.0 kg/ha). The total baseflow N─NO3– export was highest in spring (3.6 kg/ha), followed by summer (3.2 kg/ha), winter (2.3 kg/ha), and autumn (2.1 kg/ha). The contribution of baseflow to total nitrate in the stream decreased in the order of winter (69.88%) >spring (66.59%) >autumn (60.36%) >summer (54.04%). The monthly baseflow N─NO3– loads and flow‐weighted concentrations greatly increased during the research period (Mann–Kendall test, Zs > 2.56, p < .01). Without proper countermeasures, baseflow nitrate may represent a serious long‐term risk for water surfaces in the future.

Using structural equation model to estimate nitrate pollution in the Melen Watershed of the Turkey

In this study, Bayesian technique was applied in order toestimate export coefficients for the Melen Watershed.Furthermore, instead of calculating the contributions ofsubwatersheds individually, the whole watershed was consideredfor the estimation of the total load at the outlet of the MelenWatershed using the calculated nitrate export coefficient. TheBayesian approach has the goal of combining prior knowledgewith data to optimally use both sources of information. Success ofthe Bayesian approach is directly proportional to sufficiency ofdata for acquiring the prior information about estimands.Bayesian analysis was conducted through Structural EquationModel (SEM) using AMOS software and posterior informationabout land use based export coefficients was obtained throughMarkov Chain Monte Carlo (MCMC) method. Estimated land usebased nitrate export coefficients are in kg/km2/day unit. Inaddition, monthly river retention value of nitrogen in allsubwatersheds of the Melen Watershed were estimated. Thisinformation was used in order to predict nitrate exportcoefficients appropriately. This study is aimed to be an importantprecedent for other basins that are determined as in priority interms of pollution by The Ministry of Forest and Water Works ofTurkey.

Comparison of nitrate export patterns in forested catchments in Japan and the Northeastern United States: A meta‐analysis

AbstractDespite the numerous field studies on watersheds in temperate forests, little attention has been paid to differences in the seasonality of NO3‐ exports due to different seasonal patterns of precipitation. Factors that cause seasonal variation in NO3‐ exports include the biological conditions involved in the production of NO3‐ and the hydrological conditions of NO3‐ transport and storage. Previous studies described in this paper reveal that differences in the seasonal patterns of precipitation result in differences in the NO3‐ flux between sites in Japan and the Northeastern United States. In Japanese forests affected by the Asian monsoon and are subject to heavy rainfall and high‐flow conditions during the summer, the NO3‐ concentrations in stream water often increase during the summer growing season. This is a major difference from the typical decrease in NO3‐ concentrations that occur in summer in the Northeastern United States, which is not affected by monsoon conditions. In addition, the orogenically active geology differs in Japanese catchments from those in the United States and is composed of younger bedrock and steep topography in the former. It has been shown that the buffering capacity of stream water is higher in Japan than in the United States, resulting in that the degree of acidification of stream water due to the outflow of NO3‐ is also low in Japan. These comparisons with the United States serve not only to indicate the hydrological and biogeochemical uniqueness involved in the export of NO3‐ in Japanese forest catchments but also to provide a more universal understanding of the mechanisms underlying the differences. To elucidate these mechanisms clearly, a comprehensive database is required for both climatic zones that include the seasonality of nitrogen dynamics within the catchments and descriptions of the hydrological processes. Additionally, a process‐based model is needed, based on the results of a meta‐analysis using the database.

Influence of land use and hydrologic variability on seasonal dissolved organic carbon and nitrate export: insights from a multi-year regional analysis for the northeastern USA

Land use/land cover (LULC) change has significant impacts on nutrient loading to aquatic systems and has been linked to deteriorating water quality globally. While many relationships between LULC and nutrient loading have been identified, characterization of the interaction between LULC, climate (specifically variable hydrologic forcing) and solute export across seasonal and interannual time scales is needed to understand the processes that determine nutrient loading and responses to change. Recent advances in high-frequency water quality sensors provide opportunities to assess these interannual relationships with sufficiently high temporal resolution to capture the unpredictable, short-term storm events that likely drive important export mechanisms for dissolved organic carbon (DOC) and nitrate (NO3−–N). We deployed a network of in situ sensors in forested, agricultural, and urban watersheds across the northeastern United States. Using 2 years of high-frequency sensor data, we provide a regional assessment of how LULC and hydrologic variability affected the timing and magnitude of dissolved organic carbon and nitrate export, and the status of watershed fluxes as either supply or transport controlled. Analysis of annual export dynamics revealed systematic differences in the timing and magnitude of DOC and NO3−–N delivery among different LULC classes, with distinct regional similarities in the timing of DOC and NO3−–N fluxes from forested and urban watersheds. Conversely, export dynamics at agricultural sites appeared to be highly site-specific, likely driven by local agricultural practices and regulations. Furthermore, the magnitude of solute fluxes across watersheds responded strongly to interannual variability in rainfall, suggesting a high degree of hydrologic control over nutrient loading across the region. Thus, there is strong potential for climate-driven changes in regional hydrologic cycles to drive variation in the magnitude of downstream nutrient fluxes, particularly in watersheds where solute supply and/or transport has been modified.