Nonpoint Source Nitrogen Pollution Research Articles

Roles of Hydrology and Transport Processes in Denitrification at Watershed Scale

AbstractRainfall runoff and leaching are the main driving forces that nitrogen, an important non‐point source (NPS) pollutant, enters streams, lakes, and groundwater. Hydrological and transport processes thus play a pivotal role in NPS nitrogen pollution. Existing hydro‐environmental models for nitrogen pollution often over‐simplify the within‐watershed processes. It is unclear how such simplification affects the pollution assessment regarding the formation and distribution of denitrification hot spots—which is important for the design of land‐based countermeasures. To study this problem, we developed a model, DHSVM‐N, and its variant, DHSVM‐N_alt. DHSVM‐N is developed by integrating nitrogen‐related processes of SWAT into a comprehensive process‐based hydrological model, the Distributed Hydrology Soil and Vegetation Model (DHSVM). DHSVM‐N includes detailed representations of nitrate transport process at a fine spatial resolution with good landscape connectivity to accommodate interactions between hydrological and biogeochemical processes along the flow travel pathways. Because of the lack of spatially distributed observational data for validation, a model‐to‐model comparison study is conducted. Through comparison studies on a representative catchment using SWAT, DHSVM‐N and DHSVM‐N_alt, we quantify the critical roles of hydrological processes and nitrate transport processes in modeling the denitrification process. That is, the capabilities to give reasonable soil moisture estimates and to account for essential processes that take place along flow pathways are keys to simulate denitrification hot spots and their spatial variation. Furthermore, DHSVM‐N results show that terrestrial denitrification from hotspots alone can reach as high as 36% of the annual stream nitrate export of the watershed.

Using the paddy eco-ditch wetland system to reduce nitrogen non-point source pollution discharge from paddies: A consecutive six-year study in the Yunnan Plateau, China

Nitrogen non-point source pollution from paddies occurs in a wide area to the detriment of the environment. Paddies are usually connected with eco-ditches and wetlands in the dominant rice planting regions in southern China, forming a paddy eco-ditch wetland system. This system could effectively reduce nitrogen discharge from the source (paddies) to the process (eco-ditches and wetlands). However, the rules and mechanisms of nitrogen discharge reduction still need to be clarified. Here, a consecutive six-year (2016–2021) study with different irrigation modes and nitrogen application regimes in paddies, different plant types and overflow conditions in eco-ditches and wetlands was conducted. Results showed that applying alternate wetting and drying irrigation, along with 135 kg ha−1 total nitrogen application divided into three times in paddies, reduced 35.4% nitrogen discharge compared to the continuous flooding irrigation along with 193 kg ha−1 total nitrogen application. The average nitrogen discharge reduction rates of Pontederia cordata, Canna indica, and Zizania latifolia ditches under natural overflow were 23.9%, 14.2%, and 18.7%, respectively. Likewise, those of Pontederia cordata, Thalia dealbata Fraser, and Schoenoplectus tabernaemontani wetlands were 33.6%, 17.1%, and 22.9%, respectively. The average nitrogen discharge reduction rate of eco-ditches and wetlands reached more than 50% after 3-day hydraulic retention, which was significantly higher than that under dynamic overflow. Reducing the water depth, overflow velocity, and nitrogen concentration could effectively increase the nitrogen discharge reduction rate of eco-ditches. Furthermore, the discharge reduction effect of ammonium nitrogen in eco-ditches was better than that of nitrate nitrogen. The reduction rate of the paddy eco-ditch wetland system on nitrogen discharge from paddies ranged from 32.3% to 64.9%. Pontederia cordata ditch and wetland had the best performance of nitrogen discharge reduction due to the robust root system. In conclusion, the optimal strategy to reduce the nitrogen non-point source pollution from paddies using the paddy eco-ditch wetland system was applying the alternate wetting and drying irrigation and 135 kg ha−1 nitrogen in paddies, extending the overflow path in eco-ditches, and prolonging the hydraulic retention time in wetlands.

Strategies for the management of water and nitrogen interaction in seed maize production; A case study from China Hexi Corridor Oasis Agricultural Area

The oasis agricultural area of the Hexi Corridor in China supports industrial-scale seed maize production which depends on significant inputs of water and fertilizer, however, the effects of water-nitrogen interaction on seed maize yield and its components, and on water and nitrogen use efficiency in seed maize are unknown, making optimization of inputs impossible. Here, we conducted a study to address these unknowns. Specifically, we established a two-factor split-plot experiment in the Zhangye area of the Hexi Corridor with three irrigation levels of low, medium, and sufficient water supply (50%, 75%, and 100% of the local typical irrigation quota), and three nitrogen application rates of low (160 kg ha−1), medium (280 kg ha−1), and high nitrogen (400 kg ha−1); three irrigation levels without nitrogen fertilizer were used as the control. We found that there was a significant interaction between water-nitrogen combinations, all of which had a significant effect on seed maize yield and its components, and water and nitrogen use efficiency. The yield of seed maize increased with the increase in irrigation quota, but when the irrigation quota reached 100% of the traditional, yield exhibited an increasing and then decreasing trend with the increase in nitrogen application. During the 4-year test period, the maximum yield of seed maize was observed in sufficient water and medium-nitrogen treatment, reaching an average of 9509.38 kg ha−1. Lower nitrogen lowered water productivity, while lower irrigation quotas limited nitrogen utilization. The relationship between irrigation quota, nitrogen application, and seed maize yield was binary quadratic, and the coefficient of determination, R2, reached 0.963–0.985 indicating that the model predicted the yield of seed maize well. The main factor analysis of yield increase showed that the effect of irrigation quota was greater than that of nitrogen application. The optimal combination of water-nitrogen for the highest target yield was obtained with an irrigation quota of 100% of the local typical and a nitrogen application rate of 292.0 kg ha−1-383.2 kg ha−1. This result can be used as a reasonable water and fertilizer management guideline and a technical reference for the seed maize industry in the oasis agricultural area of the Hexi Corridor with objectives of stable and high yields, and reductions in the risk of nitrogen non-point source pollution.

Assessing the response of non-point source nitrogen pollution to land use change based on SWAT model

In recent years, with the growth of the population and the continuous expansion of agricultural land, non-point source (NPS) pollution has gradually become the primary cause of deteriorating water quality in the aquatic environment. Compared to point source pollution, NPS pollution is more diffuse, complex in its mechanisms, and challenging to pinpoint its sources. This study utilized the SWAT-Land Use Update Tool (SWAT-LUT) to dynamically update multi-year land use and land cover (LULC) data into the SWAT model to investigate the differences in nitrogen pollution sources in Jincheng City under different LULC scenarios. Two models were constructed in this study: SWAT-UNI, which utilized static 1997 LULC data, and SWAT-MULTI, which incorporated dynamic LULC data from 1997 to 2022. During the calibration period, SWAT achieved R2 and NSE values exceeding 0.82 for the daily streamflow simulation results, and these values remained above 0.76 during the validation period. Additionally, the Patch-generating Land Use Simulation (PLUS) model was employed to forecast the land use evolution in Jincheng City from 2022 to 2032 to explore the future response of NPS nitrogen pollution. From 1997 to 2022, significant changes were observed in agricultural land, forested land, and grassland areas within Jincheng City. Agricultural land and forested land increased by 3.29% and 4.71% of the total area of Jincheng City, respectively, while grassland decreased by 10.4%. In the prediction of land use evolution from 2022 to 2032, the evolutionary trends remained similar to previous patterns, albeit with a slightly decelerated pace. Simulation results indicated that the top three sources of nitrogen pollution in Jincheng City's water bodies in 1997 were atmospheric deposition (39.8%), nitrogen fertilizer application (29.8%), and soil nitrogen reservoirs (21.4%). With the continuous expansion of agricultural land, nitrogen pollution from nitrogen fertilizer application accounted for 35.6% of the TN (Total Nitrogen) load in water bodies in 2022, surpassing atmospheric deposition to become the dominant factor. The contribution of soil nitrogen reservoirs to nitrogen pollution in water bodies within Jincheng City showed a continuous upward trend over the twenty-five years, resulting in a total nitrogen load of 565.1 tons in 2022, ranking second and becoming a crucial aspect in pollution control efforts. Regarding seasonal distribution, the crop growing season (March to September) was identified as the critical period for controlling nitrogen pollution from nitrogen fertilizer application, while the autumn and winter seasons were crucial for controlling nitrogen pollution from atmospheric deposition and soil nitrogen reservoirs. The predictive results for future NPS nitrogen pollution indicate a continual increase in annual TN inflow into the river from nitrogen fertilizer application and soil nitrogen reservoirs, reaching 1841.6 tons in 2032, accounting for 65.2% of the total inflow. This research contributes to supporting decision-making for NPS pollution control measures in Jincheng City.

Characteristics of Nitrogen in Overlying Water and Sediment of Typical Agricultural Drainage Ditches during Different Periods in a Freezing-Thaw Area of China

In the Sanjiang Plain, agricultural drainage ditches effectively alleviate agricultural non-point source nitrogen pollution. However, limited information is known about the characteristics of bidirectional trans-ports of nitrogen between sediment and overlying and pore water in different patterns of ditches undergoing seasonal freezing-thawing cycles. It is vital to better understand nitrogen interception and purification by ecological ditches. In order to clarify the interception of ecological ditches on internal and external nitrogen, overlying water and sediment samples of two typical agricultural ditches were collected and analyzed in Sanjiang Plain during the growing seasons of 2015–2017. The results indicated that the N-NO3− in overlying water, which was higher than N-NH4+, was the dominant inorganic nitrogen, whilst, in the sediment, N-NH4+ was much higher than N-NO3−, which should be attributed to the soil’s adsorption of N-NH4+. In contrast to the dryland ditch, the paddy ditch had a more significant amount of inorganic nitrogen both in overlying water and sediment, which means that the non-point source nitrogen pollution caused by paddy fields was more severe than that of drylands. Compared with dryland ditches, N-NH4+ in the sediment of pad ditches seemed to be much easier to migrate to a deeper layer, which may cause a greater risk of nitrogen pollution to groundwater. Both in the overlying water and the sediment of ditches, nitrogen content fluctuated during different periods, and inter-annual variation was noticeable, which results means that estimation or prediction of the non-point source pollution output needs to extend the monitoring period and increase sampling frequency to reduce the great uncertainty. The findings may provide a foundation for forecasting agricultural nitrogen pollution and guide best management practices (BMPs) of non-point source nitrogen pollution control in seasonally frozen areas.

The role of precise management in mitigating non-point source nitrogen pollution of a reservoir region: Environmental profits and economic benefits

Non-point source pollution is a worldwide water environment pollution problem, which could be effectively solved with the concept and method of precision management. In this study, an integrated optimization decision system (IODS) was developed for supporting precise management of non-point source nitrogen pollution under multiple uncertainties. Several kinds of methods were innovatively coupled into the IODS, including material flow analysis (MFA), Gumbel copula model (GCM), path analysis (PA), and interval multi-objective chance constraint programming (IMOCCP). The IODS could (i) identify the main sources of non-point source pollution through the MFA; (ii) estimate the pollution status of different zones within the entire watershed using the GCM and PA; (iii) incorporate the pollution sources and pollution status to the IMOCCP as decision variables and development scenarios (i.e., S1 to S3), respectively. The settlement of the IMOCCP model could provide targeted optimal solutions to zones of different pollution status, with minimized non-point source nitrogen pollution and maximized economic benefit. Our research demonstrates great advantage of precise management in obtaining more environmental profits and economic benefits. Specifically, the precise management makes the system benefit increased by about [89.86%, 96.10%], [87.35%, 95.00%], and [28.23%, 60.40%] from S1 to S3, respectively. While the nitrogen loading decreased by about [23.40%, 81.84%], [66.51%, 145.72%], and [385.05%, 623.16%], correspondingly. This work may offer inspiration and guidance for the practice of precise management in mitigating the non-point source pollution.

The Characteristics of Non-point Source Nitrogen Pollution in the Lower Reaches of Huangshui Basin in Western China Based on SWAT Model

The Characteristics of Non-point Source Nitrogen Pollution in the Lower Reaches of Huangshui Basin in Western China Based on SWAT Model

[Classification and Identification of Non-point Source Nitrogen Pollution in Surface Flow of the Shangwu River Watershed in the Qiandao Lake Region].

Accurate quantification of non-point source pollution is an important step for non-point source pollution control and management at the watershed scale. Considering the non-point source pollution from baseflow, an improved export coefficient model (IECM) on a weekly scale was established based on the traditional export coefficient model (ECM), which was then used to estimate the surface flow non-point source total nitrogen (TN) loads contributed by different land use types of the Shangwu River watershed in the Qiandao Lake Region. The results showed that IECM performed well for the predictions of TN loads in the studied watershed, with the Nash-Sutcliffe efficiency coefficient (NSE) and R2values of 0.82 and 0.77 (P<0.01) for the calibration period and 0.87 and 0.84 (P<0.01) for the validation period, respectively. The IECM estimated TN exports through surface flow and baseflow were 5.74 kg·(hm2·a)-1and 9.85 kg·(hm2·a)-1 from the Shangwu River watershed in the period of Nov. 2020 to Oct. 2021, which accounted for 36.80% and 63.20% of the corresponding streamflow TN load, respectively. Without consideration of the baseflow non-point source TN pollution, the ECM-estimated surface flow TN loading was 54.21% higher than that estimated by IECM. Obviously, attributing baseflow non-point source pollution to surface flow directly would lead to a serious load overestimation of surface flow. According to IECM, the estimated TN export intensity through surface flow from paddy fields, grasslands, woodlands, rainfed croplands, and residential lands was 10.95, 5.42, 5.20, 12.34, and 2.77 kg·(hm2·a)-1, respectively, which accounted for 5.80%, 4.00%, 26.55%, 0.38%, and 0.03% of the corresponding total streamflow TN loads. Therefore, the future management of non-point source nitrogen pollution in the studied watershed should focus mainly on the prevention and management of groundwater non-point source pollution and control of load export from surface flow on cultivated land (paddy fields and rainfed croplands).

Tidal Flushing Rather Than Non-Point Source Nitrogen Pollution Drives Nutrient Dynamics in A Putatively Eutrophic Estuary.

The effects of nonpoint source nutrients on estuaries can be difficult to pinpoint, with researchers often using indicator species, monitoring, and models to detect influence and change. Here, we made stable isotope measurements of nitrogen and carbon in sediment, water column particulates, primary producers, and consumers at 35 stations in the reportedly eutrophic Barnegat Bay (New Jersey) to assess N sources and processing pathways. Combined with water quality and hydrological data, our C and N isoscapes revealed four distinct geographic zones with diverging isotopic baselines, indicating variable nutrient sources and processing pathways. Overall, the carbon stable isotopes δ13C) reflected the terrestrial-marine gradient with the most depleted values in the urban and poorly flushed north of the estuary to the most enriched values in the salt marsh-dominated south. In contrast, the nitrogen stable isotope values δ15N) were most enriched near the oceanic inlets and were consistent with offshore δ15N values in particulate organic matter. Several biogeochemical processes likely alter δ15N, but the relatively lower δ15N values associated with the most urbanized area indicate that anthropogenic runoff is not a dominant N source to this area. Our findings stand in contrast to previous studies of similar estuaries, as δ15N signatures of biota in this system are inversely correlated to population density and nutrient concentrations. Further, our analyses of archival plant (Spartina sp., Phragmites australis) and shell (Geukensia demissa, Ilyanassa obsoleta) samples collected between 1880 and 2020 indicated that δ15N values have decreased over time, particularly in the consumers. Overall, we find that water quality issues appear to be most acute in the poorly flushed parts of Barnegat Bay and emphasize the important role that oceanic exchange plays in water quality and associated estuarine food webs in the lagoon.

An agro-hydrological process-based export coefficient model for estimating monthly non-point source loads in a semiarid agricultural area

Export coefficient models (ECM) with few parameters and simple structures have been widely used to estimate nonpoint source (NPS) pollution loads. However, they are unsuitable to simulate characteristics and distributions of NPS pollution over semiarid agricultural watersheds of intensive irrigation, infrequent precipitation, and shallow groundwater. In this study, field monitoring was conducted to identify key impact factors of total nitrogen (TN) loads from NPS pollution over a typical semiarid agricultural watershed in northern China. Sources and characteristics of TN loads were examined by integrating the monitoring, a Pearson correlation analysis, an agro-hydrologic model, and a genetic algorithm into an ECM framework. This study quantitatively revealed the relationship between agro-hydrologic features and NPS nitrogen pollution in semiarid agricultural areas; it also simplified quantification of nutrient distributions in various seasons under pollution-source diversity. An accurate ECM was constructed to simulate variation of TN loads with seasons. Compared with traditional ones, the developed model reduces the simulation error of TN loads from 16.2% to 8.9% and improves R2 from 0.072 to 0.692. Simulation results show that the dominant TN nutrient source, i.e., farmland, accounts for 49% of annual TN loads. If water use efficiency increases from 40% to 50%, the largest reduction of TN loads would occur on farmland (about 14.4%), followed by grassland (12.0%), construction land (11.5%), bare land (11.0%), and woodland (8.3%). Improving water use efficiency and optimizing irrigation schemes and land use types should be preferentially recommended for NPS pollution control over semiarid agricultural regions.

Analysis of non-point source nitrogen pollution in watersheds based on SWAT model

By simulating and estimating non-point source pollution in Luoyang, the sources and spatial distribution of nitrogen pollution sources in this region were revealed, which provided a decision-making basis for further promoting non-point source pollution control and governance. Based on the SWAT model and different scenarios, the non-point nitrogen pollution sources in Luoyang from 2009 to 2018 were quantitatively identified, and the reasons for their changes under different pollution sources and different land uses were analyzed. The results show: From the analysis of different pollution source types, the main sources of non-point source pollution total nitrogen (TN) are nitrogen fertilizer application, atmospheric deposition and soil nitrogen reservoirs, and the pollution brought by livestock and poultry breeding and rural life is relatively low; From the analysis of different land use types, the entry coefficient of each non-point source TN pollution in Luoyang region is ranked as follows: atmospheric deposition (18.1%) > nitrogen fertilizer application (14.7%) > rural life (12.5%) > livestock manure (2.9%) > soil nitrogen pool (0.2%); Different pollution sources have different critical control periods, the key period for controlling natural sources (soil nitrogen pools and atmospheric deposition) is the rainy season, while the focus for controlling anthropogenic sources (rural living, livestock and poultry farming and fertilizer application) is the period of pollution cast emissions. In terms of time, the key period for controlling natural sources (soil nitrogen pool and atmospheric deposition) is the rainy season, while the key period for controlling anthropogenic sources (rural life, livestock and poultry breeding and application of chemical fertilizers) is the period of pollution input and emission; The key pollution sources of different land uses are also different. The key pollution sources of non-point source pollution of garden land and human settlements are nitrogen fertilizer application and domestic sewage, while the three major pollution sources of paddy field and dry land are nitrogen fertilizer application, atmospheric deposition and soil nitrogen pool.

Management optimization of nonpoint source pollution considering the risk of exceeding criteria under uncertainty

Management of nonpoint source (NPS) pollution is highly important in watershed water environmental and ecological security. However, the many complexities and uncertainties that exist in the processes of export and management of NPS pollution exert substantial influences on the reliability of multiple management practices. This study developed an inexact multiobjective possibilistic mean-variance mixed-integer programming (IMPMMP) model for NPS pollution management through optimization of watershed land use pattern and livestock production structure. By coupling interval parameter programming, mixed-integer programming, multiobjective programming, and an export coefficient model within a general possibilistic mean-variance model framework, the IMPMMP model deals effectively with system uncertainties and complexities. Moreover, the risk of exceeding criteria (REC) in NPS pollution management systems can be considered. The proposed IMPMMP model was applied to a real-world case study in the Xinfengjiang Reservoir watershed in South China. Results showed that the preference of decision makers regarding land use adjustment plays a decisive role in determining model feasibility. The area provided for each land use type that could be adjusted has to reach a certain threshold to achieve the goals of reduced pollution load and REC control. The NPS pollution loads after optimization would be exported primarily from different land uses and the human population. Compared with NPS nitrogen pollution management, it is more difficult to reduce the NPS phosphorus load and to manage the corresponding REC through adjustment of the land use pattern and livestock production structure. Moreover, it is difficult to simultaneously reduce the NPS nitrogen and phosphorus pollution loads and REC in each subbasin. The model, which can provide policy makers with a series of schemes for optimization of land use pattern and livestock production structure, has satisfactory applicability and could be used for watershed NPS pollution management.

Modeling field scale nitrogen non-point source pollution (NPS) fate and transport: Influences from land management practices and climate

The use of nitrogen (N) fertilizer marked the start of modern agriculture that boosted food production to help alleviate food shortages across the globe but at the cost of severe environmental issues and critical stress to the agroecosystem. This paper was aimed at determining the fate and transport of nitrite and ammonia under future climate projections by adapting the recommended land management practices that are supposed to reduce nitrate N in surface water to state government target. To accomplish these objectives, a fully-distributed physical-based hydrologic model, MIKE SHE, and a hydrodynamic river model, MIKE 11, were coupled with MIKE ECO-Lab to simulate the fate and transport of different forms of N in the agro-ecosystem in the Upper Sangamon River Basin (USRB). Twelve (12) combinations of land management and climate projections were simulated to evaluate the N fate and transport in the USRB from 2020 to 2050. Under the current land management, the nitrate concentration in surface water was expected to exceed the EPA limit of 10 ppm up to 2.5% of the days in the simulation period. Regulating the fertilizer application rates to approximately 50% of the current rate will ensure this limit will not be exceeded in the future. Implementing cover cropping alone can potentially decrease nitrate N concentrations by 33% in surface water under dry climate and in the saturated zone under future projections. By combining the cover cropping and regulated application rate management, the nitrate N concentration in the saturated zone was expected to decrease by 67% compared with historic baseline. The modeling framework developed and used in this study can help evaluate the effectiveness of different management schemes aimed at reducing future nutrient load in our surface water and groundwater.

The effects of trace narasin on the biogeochemical N-cycle in a cultivated sandy loam.

Narasin is an antibiotic administered to broiler chickens to prevent coccidiosis. After storage, excreta containing parent narasin compounds is commonly spread as nitrogen fertilizer, yielding initial soil concentrations in the low μg·kg-1 range. In soil, antibiotics have been found to modify one or more pathways in the biogeochemical nitrogen cycle. The concentrations tested are often too high to be considered environmentally relevant, despite evidence that sub-therapeutic doses may also be disruptive. We conducted soil mesocosm experiments to determine the overall impact of trace narasin on major nitrogen pools and fluxes in soils treated with 0, 1, 10, 100, or 1000ng·kg-1 narasin. Water content in the mesocosms varied from 40% to 80% water-filled pore space (WFPS), simulating a range of different redox conditions. Under aerobic conditions (40% WFPS), exposure to narasin inhibited nitrification, yielding increases in soil ammonium by up to 76%, perhaps by targeting nitrifying fungi. Under the same conditions, narasin caused soil nitrate concentrations to decline 17-39%. When the soil was near saturation (80% WFPS), nitrate increased by an average of 30%. Mass balances and isotopic enrichment of N2O indicate that NAR may also affect anammox and the rate of nitrifier nitrification/denitrification. In aerobic soils, N2O flux increased with antibiotic dose and the rise in flux strongly correlates to the N2O:N2 product ratio from dentification. This relationship suggests that N2O flux may increase in soils exposed to narasin even when total denitrification is modestly inhibited. We conclude that trace concentrations of narasin can significantly modify biogeochemical activities in soil on short timescales. Our results indicate the potential for extremely low concentrations of antibiotics to impact agricultural productivity, terrestrial N2O flux, and non-point source nitrogen pollution.

Metagenomic analysis reveals distinct patterns of denitrification gene abundance across soil moisture, nitrate gradients.

This study coupled a landscape-scale metagenomic survey of denitrification gene abundance in soils with in situ denitrification measurements to show how environmental factors shape distinct denitrification communities that exhibit varying denitrification activity. Across a hydrologic gradient, the distribution of total denitrification genes (nap/nar + nirK/nirS + cNor/qNor + nosZ) inferred from metagenomic read abundance exhibited no consistent patterns. However, when genes were considered independently, nirS, cNor and nosZ read abundance was positively associated with areas of higher soil moisture, higher nitrate and higher annual denitrification rates, whereas nirK and qNor read abundance was negatively associated with these factors. These results suggest that environmental conditions, in particular soil moisture and nitrate, select for distinct denitrification communities that are characterized by differential abundance of genes encoding apparently functionally redundant proteins. In contrast, taxonomic analysis did not identify notable variability in denitrifying community composition across sites. While the capacity to denitrify was ubiquitous across sites, denitrification genes with higher energetic costs, such as nirS and cNor, appear to confer a selective advantage in microbial communities experiencing more frequent soil saturation and greater nitrate inputs. This study suggests metagenomics can help identify denitrification hotspots that could be protected or enhanced to treat non-point source nitrogen pollution.

[Dynamics and Runoff Losses of Nitrogen in Paddy Field Surface Water Under Combined Application of Biochar and Slow/Controlled-Release Fertilizer].

Field experiments were conducted to investigate the effects of four fertilization treatments, that is, controlled-release fertilizer (CRF), biochar combined with controlled-release fertilizer (BC+ CRF), biochar combined with stabilized fertilizer (BC+ SF), and biochar combined with controlled-release fertilizer and stabilized fertilizer (BC+ CRF+ SF), on the dynamics of the pH and nitrogen mass concentration and runoff losses of nitrogen in paddy field surface water in the Taihu Lake Basin. The results show that the average pH of the surface water decreases by 3.16%-4.48% for BC+ CRF+ SF and is in the range of 5.64-8.15. The average total nitrogen (TN) mass concentration of surface water ranges from 19.05 to 25.23 mg·L-1. A significant decrease of 4.75%-6.58% in the TN mass concentration of surface water was observed for BC+ CRF+ SF. The average ammonium (NH4+-N) and nitrate (NO3--N) mass concentration of the surface water vary from 0.01-17.26 mg·L-1 and from 0.24-3.11 mg·L-1, respectively. Inorganic nitrogen is dominated by NH4+-N in surface water. Compared with individual CRF, other treatments significantly reduce the NH4+-N mass concentration of surface water by 35.89%-48.78% and the NO3--N mass concentration of surface water by 20.54%-37.01%. The BC+ SF combination shows a significant reduction in the NH4+-N and NO3--N mass concentration of the surface water, which greatly lowers the risk of inorganic nitrogen loss via runoff. The runoff losses of TN, NH4+-N, and NO3--N are in the range of 16.24-18.09, 1.76-2.22 and 0.76-1.38 kg·hm-2, respectively. Compared with the individual CRF, the runoff loss of TN, NH4+-N, and NO3--N of other treatments is reduced. The BC+ CRF+ SF combination shows a significant reduction in the runoff losses of nitrogen, which greatly lowers the risk of nitrogen nonpoint source pollution from paddy fields.

Fate of nitrogen in overlying water with biochar addition to sediment in planted ditches.

Best management practice (BMP) in drainage ditch systems is of high concern due to its importance in intercepting and removing pollution. Biochar has recently become an interesting option for agricultural pollution elimination, but knowledge is still limited on the fate of nitrogen (N) in drainage ditch systems with biochar added into the sediment. This isotope tracer study examined the effects of biochar addition into the sediment on the fate of inorganic N (15N-NO3- and 15N-NH4+) in sediment microcosms through a 93 day incubation experiment. Four types of sediment microcosm (sediment only, sediment + biochar, sediment + plant, sediment + biochar + plant) were set to investigate N migration and transformation using 15N pool dilution. The results indicated that 0.75% biochar addition significantly increased sediment pH and water holding capacity by 0.29 and 2.6%, respectively. Biochar addition suppressed sediment 15N-N2O emissions and decreased 15N uptake by Calamagrostis angustifolia. From the 15N distribution in different sediment layers, it was found that biochar may increase the risk of N leaching and speed up its downward mobility. Meanwhile, 15N fate was affected by the planting of Calamagrostis angustifolia, exhibiting reduced leakage and residue in sediment. Vegetated microcosms suppressed N2O emissions, as the Calamagrostis angustifolia effectively competed with microbes and utilized N for its own growth and development. Therefore, Calamagrostis angustifolia could be beneficial in the removal of agricultural non-point source nitrogen pollution as a potential option for BMP.

Assessment of Nitrogen Inputs into Hunt River by Onsite Wastewater Treatment Systems via SWAT Simulation

Nonpoint source nitrogen pollution is difficult to effectively model in groundwater systems. This study aims to elucidate anthropogenic nonpoint source pollution discharging into Potowomut Pond and ultimately Narragansett Bay. Hydrologic modeling with Soil and Water Assessment Tool (SWAT) and SWAT Calibration and Uncertainty Program (SWAT-CUP) was used to simulate streamflow and nitrogen levels in the Hunt River with and without onsite wastewater treatment systems (OWTS). The objective of this study was to determine how input of OWTS data impacts nitrogen loading into the Hunt River Watershed in Rhode Island, USA. The model was simulated from 2006 to 2014, calibrated from 2007 to 2011 and validated from 2012 to 2014. Observed streamflow data was sourced from a US Geological Survey gauge and nitrogen loading data from University of Rhode Island Watershed Watch (URIWW). From the results, adding OWTS data to the SWAT simulation produced a better calibration and validation fit for total fit (Nash–Sutcliffe Efficiency (NSE) = 0.50 calibration, 0.78 validation) when compared with SWAT simulation without OWTS data (NSE = −1.3 calibration, −6.95) validation.

Quantitative identification of nitrate pollution sources and uncertainty analysis based on dual isotope approach in an agricultural watershed

Quantitative identification of nitrate (NO3−-N) sources is critical to the control of nonpoint source nitrogen pollution in an agricultural watershed. Combined with water quality monitoring, we adopted the environmental isotope (δD-H2O, δ18O-H2O, δ15N-NO3−, and δ18O-NO3−) analysis and the Markov Chain Monte Carlo (MCMC) mixing model to determine the proportions of riverine NO3−-N inputs from four potential NO3−-N sources, namely, atmospheric deposition (AD), chemical nitrogen fertilizer (NF), soil nitrogen (SN), and manure and sewage (M&S), in the ChangLe River watershed of eastern China. Results showed that NO3−-N was the main form of nitrogen in this watershed, accounting for approximately 74% of the total nitrogen concentration. A strong hydraulic interaction existed between the surface and groundwater for NO3−-N pollution. The variations of the isotopic composition in NO3−-N suggested that microbial nitrification was the dominant nitrogen transformation process in surface water, whereas significant denitrification was observed in groundwater. MCMC mixing model outputs revealed that M&S was the predominant contributor to riverine NO3−-N pollution (contributing 41.8% on average), followed by SN (34.0%), NF (21.9%), and AD (2.3%) sources. Finally, we constructed an uncertainty index, UI90, to quantitatively characterize the uncertainties inherent in NO3−-N source apportionment and discussed the reasons behind the uncertainties.

Linking Nitrogen Export to Landscape Heterogeneity: The Role of Infrastructure and Storm Flows in a Mediterranean Urban System

AbstractUrban ecosystems are often sources of nonpoint source (NPS) nitrogen (N) pollution to aquatic ecosystems. However, N export from urban watersheds is highly variable. Examples of densely urbanized watersheds are not well studied, and these may have comparatively low export rates. Commonly used metrics of landscape heterogeneity may obscure our ability to discern relationships among landscape characteristics that can explain these lower export rates. We expected that differences not often captured by these metrics in the relative cover of vegetation, structures, and impervious surfaces would better explain observed variation in N export. We examined these relationships during storms in residential watersheds. Contrary to expectations, land cover did not directly predict variation in N or water export. Instead, N export was strongly linked to drainage infrastructure density. Our research highlights the role of fine‐scaled landscape attributes, mainly infrastructure, in explaining patterns of N export from densely urbanized watersheds. Changes to hydrologic flow paths by infrastructure explained more variation in N export than land cover. Our findings support further development of landscape ecological models of urban N export that focus on hydrologic modification by infrastructure rather than traditional landscape measures such as land use, as indicators for evaluating patterns of NPS nitrogen pollution in densely urbanized watersheds.