Spatiotemporal distribution and controlling factors on ammonium in waters in the central Yangtze River Basin, China

High levels of ammonium in water can compromise the ecological environment and be harmful to human beings. It is of great significance to understand the source and controlling factors of ammonium in waters. However, the distribution and controlling factors on ammonium in the central Yangtze River Basin have been rarely reported. The results showed that 6.58% of the surface water (SW) exceeded the China national guideline of category III for NH4+-N (i.e., 1.0 mg/L) and 30.19% of the groundwater (GW) exceeded the China national guideline of category III for NH4+-N (i.e., 0.5 mg/L). Notably, the ammonium concentrations of the plain area in the middle were much higher, which reached to the highest value at the junction of the Yangtze River and Dongting Lake. Nitrogen in SW may originate from manure but more nitrogen sources in GW. The net anthropogenic nitrogen input (NANI) can provide enough organic nitrogen for the mineralization. NH4+-N in SW was more affected by fertilizer nitrogen and feed nitrogen input but more affected by agricultural nitrogen fixation in GW. Agricultural and industrial activities controlled NH4+-N in SW and GW by increasing nitrogen input and changing hydrological conditions. In general, this research exposed the controlling of different types of factors on ammonium in waters, providing a guidance for the water pollution prevention in study area.

Nitrogen Budget and Statistical Entropy Analysis of the Tiber River Catchment, a Highly Anthropized Environment

Modern farming causes a decline in the recycling of the soil’s inorganic matter due to losses by leaching, runoff, or infiltration into the groundwater. The Soil System Budget approach was applied to evaluate the net N budget at the catchment and sub-catchment levels of the Tiber River (central Italy) in order to establish the causes for different N budgets among the sub-catchments. Statistical Entropy Analysis (SEA) was used to evaluate the N efficiency of the Tiber River and its sub-catchments, providing information on the dispersion of different N forms in the environment. The total N inputs exceeded the total outputs, showing a low N retention (15.8%) at the catchment level, although some sub-catchments showed higher N retention values. The Utilized Agricultural Area was important in the determination of the N balance, as it was linked to zoo- and agricultural activities, although the Random Forest analysis showed that the importance ranking changed with the land use. The low N retention of the Tiber catchment was due to the soil characteristics (Cambisols and Leptosols), loads from atmospheric deposition, biological fixation, and the livestock industry. The SEA simulations showed a reduction of the N released into the atmosphere and groundwater compartments from 34% to 6% through a reduction of the N loads by 50%.

Long-Term Mississippi River Trends Expose Shifts in the River Load Response to Watershed Nutrient Balances Between 1975 and 2017

Excess nutrients transported by the Mississippi River (MR) contribute to hypoxia in the Gulf of Mexico. Nutrient balances are key drivers to river nutrient loads and represent inputs (fertilizer, manure, deposition, wastewater, N-fixation, and weathering) minus outputs (nutrient uptake and removal in harvest, and N emissions). Here, we quantified annual changes in nitrogen (N) and phosphorus (P) river loads and nutrient balances at the MR Outlet and documented that the river load response to watershed nutrient balances shifted between 1975 and 2017. Annual nutrient balances and river loads were positively correlated between 1975 and 1985, but after, a disconnect between both the N and P balances and river loads emerged, and the subsequent river load patterns were different for N versus P. We evaluated the relative impacts of legacy nutrients and other latent factors, for which data were not available, on river nutrient load trends. Our analysis showed that in the case of N, latent factors were potentially just as important in explaining changes in river nutrient loads over time as N balances, and in the case of P, they were even more important. We hypothesized that these factors included implementation of best management practices, changes in watershed buffering capacity, the effects of tile drainage, or increased precipitation. Our analytical approach shows promise for the investigation of drivers of water quality trends that are not well-represented in typical national scale geospatial datasets.

Net anthropogenic nitrogen and phosphorus inputs in Pearl River Delta region (2008-2016)

Excess inputs of nitrogen (N) and phosphorus (P) are the main contributors of aquatic environmental deterioration. Due to the agricultural and industrial activities in the rapidly urbanized basin, the anthropogenic N and P cycle are significantly different from other regions. In this study, we took the Pearl River Delta as an example and introduced the budget list of N and P in the five survey years, including the net anthropogenic N inputs (NANI) and net anthropogenic P inputs (NAPI). The results revealed that the intensities of NANI and NAPI in this area increased from 2008 to 2010 and then decreased after 2010. The peak values were 21001 kg N km^-2yr^-1 and 4515 kg P km^-2yr^-1 for the intensities of NNAI and NAPI, respectively, while the lowest values decreased to 19186 kg N km^-2yr^-1 and 4103 kg P km^-2yr^-1 in 2016. The most important contribution of NANI and NAPI sources in this area were net N and P inputs for human food and animal feed with an average contribution of 61.41% and 76.83%, which indicated that large amounts of N and P were introduced into the environment through the food system. This study expanded the knowledge on regional environmental management from human dietary consumption, human life consumption, animal consumption and fertilizer consumption. Its reuse will be put into practice by understanding the driving factors of N and P inputs in each region of the basin, combining the urbanization characteristics.

Regional estimation of net anthropogenic nitrogen inputs (NANI) and the relationships with socioeconomic factors

Human activities have strongly influenced nitrogen loads; thus, the accurate evaluation of net anthropogenic nitrogen input (NANI) is very important for developing countermeasures to control N pollution. The spatiotemporal distribution and main components of NANI at the city scale in Hubei Province in 2008-2018 were analyzed using the NANI model. Furthermore, the relationships between NANI and socioeconomic factors, namely, the gross industrial output value per unit area (GIOV), gross agricultural output value per unit area (GAOV), grain yield per unit area (GY), fertilizer consumption density (FCD), population density (PD), and cultivated land area per unit area (CLA), were further analyzed. The results show that NANI in Hubei tended to increase from 14,422.66 kg km^-2 year^-1 in 2008 to 16,779.39 kg km^-2 year^-1 in 2012 and then fell to 13,415.74 kg km^-2 year^-1 in 2018. In terms of the spatial distribution, the NANI values in the mid-east region of Hubei, i.e., Xiangyang, Jingmen, Jingzhou, Suizhou, Xiaogan, Wuhan, Ezhou, and Huanggang and counties directly under the jurisdiction of the province, were significantly higher than those in the west, i.e., Shiyan, Yichang, and Enshi autonomous prefecture. The largest 11-year annual NANI, 39,462.03 kg km^-2 year^-1, occurred in Ezhou, while Shiyan had the lowest 11-year annual NANI of 6592.32 kg km^-2 year^-1. N fertilizer use (N_fer), which accounted for 55.23% of the NANI was the largest N input source, followed by net N import in food and feed (N_im), atmospheric N deposition (N_dep), N fixation (N_fix), and seeding N (N_see). Pearson correlation analysis between the components of NANI and 6 socioeconomic factors revealed FCD as the primary factor responsible for NANI (r = 0.948), followed by GAOV (r = 0.607) and CLA (r = 0.558). The most direct driving factors of N_dep, N_fer, N_see, and N_im were GIOV (r = 0.727), FCD (r = 0.966), CLA (r = 0.813), and GAOV (r = 0.746), respectively. All factors had a significant negative impact on N_fix. Therefore, the most efficient strategy to decrease NANI is to control the fertilizer application amount and improve agricultural development. Additionally, it is necessary to replace traditional high-polluting industries with ecological industry to reduce industrial pollution. Graphical abstract.

Variation of net anthropogenic phosphorus inputs (NAPI) and riverine phosphorus fluxes in seven major river basins in China

The riverine phosphorus (P) import resulting from human activities is always a worldwide concern for environmental management due to the effect of eutrophication. In this study, we made modification of the NAPI method to make the results closer to the actual situation. We collected the data of seven major outflow rivers in China to have a comprehensive understanding of P export and P inputs and build the quantitative relationship between them. We estimated the net anthropogenic phosphorus inputs (NAPI), including fertilizer P (P_fert), net food and feed import P and non-food P using by human (P_im+nf), in seven major river basins in China and the corresponding riverine total phosphorus (TP) fluxes. The relationship between NAPI and riverine TP flux was also explored. NAPI in seven river basins presented an obvious uneven distribution. Huaihe River basin showed the highest NAPI of 4005.09 kg P km^-2 yr^-1 due to its highest intensities of human-activities, and the lowest NAPI was observed in Songhua River basin as 334.36 kg P km^-2 yr^-1. P_im+nf occupied a larger proportion of NAPI in the Pearl River and Liaohe River basins (> 65%), while P_fert contributed more to NAPI in the other basins (nearly 60%) with an exception of the Yangtze River basin (where P_im+nf and P_fert approximately contributed the same). Different contributions of NAPI components were mainly attributed to the different land uses. The total TP flux of all the seven rivers was 117.10 × 10³ t P yr^-1, with the highest flux in the Yangtze River (77.42 × 10³ t P yr^-1), contributed 72.88% to the total TP fluxes in China. Change in riverine TP flux could be well described by NAPI, river discharge, and percentage of lake area in the basin and this provided an effective way to predict TP fluxes in rivers.

Copper impact on enzymatic cascade and extracellular sequestration via distinctive pathways of nitrogen removal in green sorption media at varying stormwater field conditions

Nutrient removal efficiency in green sorption media such as biosorption activated media (BAM) for treating stormwater runoff can be heavily influenced either on a short- or long-term basis by varying field conditions of linear ditches due to the presence of copper in stormwater runoff. It is also noticeable that the linear ditch undergoes physical or mechanical impacts from the traffic compaction, chemical impact of carbon sources from the nearby farmland, and biological impact from potential animal activities (such as gopher tortoises, moles, and ants). In the nitrogen cycle, two denitrification pathways, the dissimilatory nitrate reduction to ammonia and common denitrification, are deemed critical for such assessment. A fixed-bed column study was set up to mimic different linear ditch field conditions for BAM applications and measure the effect of short-and long-term copper addition on microbial dynamics given the varying decomposition of dissolved organic nitrogen (DON). The findings confirm that, as the denitrifiers (in the second pathway) were the dominant species, their population continued to grow and maintain small-sized cells for extracellular sequestration under long-term copper impact. Furthermore, the study indicated that the ammonia oxidizer comammox was found in higher quantities than ammonia oxidizing bacteria or archaea. An enormous amount of DON was released during this process to bind the copper ion and reduce its toxicity as the enzymatic cascade effect appeared. In addition, the long-term copper exposure posed salient inhibitory effects on the microbial community regardless of varying field conditions in BAM. Short-term copper toxicity exerted an important but varying role in the enzymatic cascade effect over different linear ditch field conditions in BAM.

Temporal and spatial variations of net anthropogenic nitrogen inputs (NANI) in the Pearl River Basin of China from 1986 to 2015

Human activities have greatly influenced the natural nitrogen cycle, causing dramatic degradation of ecosystem function. Net anthropogenic nitrogen input (NANI) is an important factor contributing to the impact of human activities on the regional nitrogen cycle. Here, we analyzed the temporal and spatial variation of NANI in the Pearl River Basin of China between 1986 to 2015, and found that the total amount of NANI significantly increased from 3,362.25 kg N km^-2 yr^-1 to 8,071.15 kg N km^-2 yr^-1. Application of nitrogen fertilizers was the largest component of NANI in the Basin, accounting for 55.53% in the total NANI, followed by food/feed net nitrogen input (21.26%), atmospheric nitrogen deposition (12.95%), and crop nitrogen fixation (10.26%). Over the last three decades, nitrogen inputs from atmospheric nitrogen deposition have become the second largest source of NANI due to rapid industrialization and urbanization in the region. Regression analysis showed that the rapid growth of both GDP and population density were the main contributors to the increase of NANI. In addition, the increase in the number of red tides in the Pearl River Estuary was strongly correlated with NANI discharge (R² = 0.90, p<0.01), suggesting the NANI’s eutrophication effect. In total, this study provides a quantitative understanding of the temporal and spatial variations of NANI in the Pearl River Basin as well as the effects of NANI on estuarine waters, and offered key information for developing an integrated strategy for watershed nitrogen management.

A spatially explicit, empirical estimate of tree-based biological nitrogen fixation in forests of the United States

Quantifying human impacts on the N cycle and investigating natural ecosystem N cycling depend on the magnitude of inputs from natural biological nitrogen fixation (BNF). Here, we present two bottom-up approaches to quantify tree-based symbiotic BNF based on forest inventory data across the coterminous US plus SE Alaska. For all major N-fixing tree genera, we quantify BNF inputs using (1) ecosystem N accretion rates (kg N ha-1 yr-1) scaled with spatial data on tree abundance and (2) percent of N derived from fixation (%Ndfa) scaled with tree N demand (from tree growth rates and stoichiometry). We estimate that trees fix 0.30-0.88 Tg N yr-1 across the study area (1.4-3.4 kg N ha-1 yr-1). Tree-based N fixation displays distinct spatial variation that is dominated by two genera, Robinia (64% of tree-associated BNF) and Alnus (24%). The third most important genus, Prosopis, accounted for 5%. Compared to published estimates of other N fluxes, tree-associated BNF accounted for 0.59 Tg N yr-1, similar to asymbiotic (0.37 Tg N yr-1) and understory symbiotic BNF (0.48 Tg N yr-1), while N deposition contributed 1.68 Tg N yr-1 and rock weathering 0.37 Tg N yr-1. Overall, our results reveal previously uncharacterized spatial patterns in tree BNF that can inform large-scale N assessments and serve as a model for improving tree-based BNF estimates worldwide. This updated, lower BNF estimate indicates a greater ratio of anthropogenic to natural N inputs, suggesting an even greater human impact on the N cycle.

Source area management practices as remediation tool to address groundwater nitrate pollution in drinking supply wells

Nitrate in drinking water may cause serious health problems for consumers. Agricultural activities are known to be the main source of groundwater nitrate contaminating rural domestic and urban public water supply wells in farming regions. Management practices have been proposed to reduce the amount of nitrate in groundwater, including improved nutrient management practices and "pump and fertilize" with nitrate-affected irrigation wells. Here, we evaluate the feasibility and long-term impacts of agricultural managed aquifer recharge (Ag-MAR) in the source area of public water supply wells. A numerical model of nitrate fate and transport was developed for the Modesto basin, part of California's Central Valley aquifer system. The basin is representative of semi-arid agricultural regions around the world with a diversity of crop types, overlying an unconsolidated sedimentary aquifer system. A local public supply well in an economically disadvantaged community surrounded by farmland was the focus of this study. Model scenarios implemented include business as usual, alternative low-impact crops, and Ag-MAR in the source area of the public supply well. Alternative nutrient management and recharge practices act as remediation tools in the area between farmland and the public supply well. Improved agricultural source area management practices are shown to be an effective tool to maintain or even enhance groundwater quality in the targeted supply well while remediating ambient groundwater. Best results are obtained when lowering nitrate load while also increasing recharge in the source area simultaneously. This scenario reduced nitrate in the supply well's drinking water by 80% relative to the business as usual scenario. It also remediated ambient groundwater used by domestic wells between the source area farmlands and the supply well and showed 60% more reduction of nitrate after 60 years of application. Increasing recharge led to shorter initial response time (five years) and showed the most sustainable impact. Our analysis further suggests that Ag-MAR in a highly discontinuous, wide-spread pattern leads to slow water quality response and may not yield sufficient water quality improvements.

Importance of the vegetation-groundwater-stream continuum to understand transformation of biogenic carbon in aquatic systems - A case study based on a pine-maize comparison in a lowland sandy watershed (Landes de Gascogne, SW France)

During land-aquatic transfer, carbon (C) and inorganic nutrients (IN) are transformed in soils, groundwater, and at the groundwater-surface water interface as well as in stream channels and stream sediments. However, processes and factors controlling these transfers and transformations are not well constrained, particularly with respect to land use effect. We compared C and IN concentrations in shallow groundwater and first-order streams of a sandy lowland catchment dominated by two types of land use: pine forest and maize cropland. Contrary to forest groundwater, crop groundwater exhibited oxic conditions all-year round as a result of higher evapotranspiration and better lateral drainage that decreased the water table below the organic-rich soil horizon, prevented the leaching of soil-generated dissolved organic carbon (DOC) in groundwater, and thus limited consumption of dissolved oxygen (O₂). In crop groundwater, oxic conditions inhibited denitrification and methanogenesis resulting in high nitrate (NO₃^-; on average 1140 ± 485 μmol L^-1) and low methane (CH₄; 40 ± 25 nmol L^-1) concentrations. Conversely, anoxic conditions in forest groundwater led to lower NO₃^- (25 ± 40 μmol L^-1) and higher CH₄ (1770 ± 1830 nmol L^-1) concentrations. The partial pressure of carbon dioxide (pCO₂; 30,650 ± 11,590 ppmv) in crop groundwater was significantly lower than in forest groundwater (50,630 ± 26,070 ppmv), and was apparently caused by the deeper water table delaying downward diffusion of soil CO₂ to the water table. In contrast, pCO₂ was not significantly different in crop (4480 ± 2680 ppmv) and forest (4900 ± 4500 ppmv) streams, suggesting faster degassing in forest streams resulting from greater water turbulence. Although NO₃^-concentrations indicated that denitrification occurred in riparian-forest groundwater, crop streams nevertheless exhibited important signs of spring and summer eutrophication such as the development of macrophytes. Stream eutrophication favored development of anaerobic conditions in crop stream sediments, as evidenced by increased ammonia (NH₄⁺) and CH₄ in stream waters and concomitant decreased in NO₃^- concentrations as a result of sediment denitrification. In crop streams, dredging and erosion of streambed sediments during winter sustained high concentration of particulate organic C, NH₄⁺ and CH₄. In forest streams, dissolved iron (Fe²⁺), NH₄⁺ and CH₄ were negatively correlated with O₂ reflecting the gradual oxygenation of stream water and associated oxidations of Fe²⁺, NH₄⁺ and CH₄. The results overall showed that forest groundwater behaved as source of CO₂ and CH₄ to streams, the intensity depending on the hydrological connectivity among soils, groundwater, and streams. CH₄ production was prevented in cropland in soils and groundwater, however crop groundwater acted as a source of CO₂ to streams (but less so than forest groundwater). Conversely, in streams, pCO₂ was not significantly affected by land use while CH₄ production was enhanced by cropland. At the catchment scale, this study found substantial biogeochemical heterogeneity in C and IN concentrations between forest and crop waters, demonstrating the importance of including the full vegetation-groundwater-stream continuum when estimating land-water fluxes of C (and nitrogen) and attempting to understand their spatial and temporal dynamics.

Contrasting Nitrogen Fate in Watersheds Using Agricultural and Water Quality Information

Surplus nitrogen (N) estimates, principal component analysis (PCA), and end-member mixing analysis (EMMA) were used in a multisite comparison contrasting the fate of N in diverse agricultural watersheds. We applied PCA-EMMA in 10 watersheds located in Indiana, Iowa, Maryland, Nebraska, Mississippi, and Washington ranging in size from 5 to 1254 km with four nested watersheds. Watershed Surplus N was determined by subtracting estimates of crop uptake and volatilization from estimates of N input from atmospheric deposition, plant fixation, fertilizer, and manure for the period from 1987 to 2004. Watershed average Surplus N ranged from 11 to 52 kg N ha and from 9 to 32% of N input. Solute concentrations in streams, overland runoff, tile drainage, groundwater (GW), streambeds, and the unsaturated zone were used in the PCA-EMMA procedure to identify independent components contributing to observed stream concentration variability and the end-members contributing to streamflow and NO load. End-members included dilute runoff, agricultural runoff, benthic-processing, tile drainage, and oxic and anoxic GW. Surplus N was larger in watersheds with more permeable soils (Washington, Nebraska, and Maryland) that allowed greater infiltration, and oxic GW was the primary source of NO load. Subsurface transport of NO in these watersheds resulted in some removal of Surplus N by denitrification. In less permeable watersheds (Iowa, Indiana, and Mississippi), NO was rapidly transported to the stream by tile drainage and runoff with little removal. Evidence of streambed removal of NO by benthic diatoms was observed in the larger watersheds.

Further east: eutrophication as a major threat to the flora of Vladimir Oblast, Russia

Eutrophication remains a major threat to the flora of Western Europe despite measures to reduce nitrogen emissions. Although nutrient enrichment has been recorded for both inland waters and adjacent seas, there is almost no evidence from Russia for large-scale anthropogenic eutrophication of soils and its impact on terrestrial biota. I used the distribution grid data (337 grid squares, ca. 96 km(2)) on 1,384 vascular plants of Vladimir Oblast for two periods (1869-1999 vs. 2000-2012) to estimate the shifts in mean Ellenberg's indicator values for nitrogen and soil reaction. Decadal changes in the flora of acid sandy Meshchera Lowlands were observed directly during two grid surveys of 2002 and 2012 based on a coarser grid (50 squares, ca. 24 km(2)). Despite the spatial correlation of Ellenberg's indicator values for soil reaction and nitrogen, mean grid values for nitrogen are growing in areas with both acid and neutral soils. The changes in mean grid indicator values for nitrogen are caused by either local extinctions of species from nutrient-poor habitats or spread of nitrophilous plants. I found that oligotrophic habitats are declining rapidly within the eutrophic loamy landscapes. In contrast, changes in landscapes with acid sandy soils are caused by increasing number of records of nitrophilous species, both invasive and native. These two processes have different spatial patterns caused by varying levels of geochemical buffer capacity and should be considered separately. Fragmentary historical data on Vladimir Oblast flora agrees with the overall European picture of eutrophication in the twentieth century.

Daily nitrate losses: implication on long-term river quality in an intensive agricultural catchment of southwestern france

High nitrate concentrations in streams have become a widespread problem throughout Europe in recent decades, damaging surface water and groundwater quality. The European Nitrate Directive fixed a potability threshold of 50 mg L for European rivers. The performance of the Soil and Water Assessment Tool model was assessed in the 1110-km Save catchment in southwestern France for predicting water discharge and nitrate loads and concentrations at the catchment outlet, considering observed data set uncertainty. Simulated values were compared with intensive and extensive measurement data sets. Daily discharge fitted observations (Nash-Sutcliffe efficiency coefficient = 0.61, = 0.7, and PBIAS = -22%). Nitrate simulation (1998-2010) was within the observed range (PBIAS = 10-21%, considering observed data set uncertainty). Annual nitrate load at the catchment outlet was correlated to the annual water yield at the outlet ( = 0.63). Simulated annual catchment nitrate exportation ranged from 21 to 49 kg ha depending on annual hydrological conditions (average, 36 kg ha). Exportation rates ranged from 3 to 8% of nitrogen inputs. During floods, 34% of the nitrate load was exported, which represented 18% of the 1998-2010 period. Average daily nitrate concentration at the outlet was 29 mg L (1998-2010), ranging from 0 to 270 mg L. Nitrate concentration exceeded the European 50 mg L potability threshold during 244 d between 1998 and 2010. A 20% reduction of nitrogen input reduced crop yield by between 5 and 9% and reduced by 62% the days when the 50 mg L threshold was exceeded.

Typical urban gully nitrogen migration in Changchun City, China

In this study, Yitong River, which is located in Changchun, a representative city in northeastern China, was selected as the research area. Using position monitoring and field measurements, we quantitatively investigated the migration path and flux of nitrogen in a gully region in Changchun City undergoing rapid urbanization. The results showed that at the Yitong River subwatershed, the total nitrogen input flux was 188 kg/hm(2), the degree of which can be ranked in descending order as fertilizer input > biological immobilization > feed > atmospheric deposition. The total nitrogen output flux was 102.5 kg/hm(2), ranked in descending degree as products > waste output > denitrification > surface runoff. The net nitrogen storage was 85.5 kg/hm(2). The migration path and flux of nitrogen were markedly impacted by human activities, showing an imbalance between input and output, as well as a tendency toward nitrogen accumulation and pollution. The nitrogen budget for the Yitong River subwatershed suggested that more than 50 % of the net anthropogenic nitrogen input was lost to the environment, and about 14.5 % was discharged in rivers, indicating that agricultural and human activities in the basin substantially impact the river water quality and thus alter the nitrogen environmental geochemistry. Reducing the application and improving the efficiency of nitrogenous fertilizer use as well as reclaiming human life waste are efficient approaches to decreasing the nitrogen input flux and environmental accumulation and to promoting the balance between nitrogen input and output. These practices are also effective approaches to reducing non-point source pollution.

Estimating net anthropogenic nitrogen inputs to U.S. watersheds: comparison of methodologies

The net anthropogenic nitrogen input (NANI) approach is a simple quasi-mass-balance that estimates the human-induced nitrogen inputs to a watershed. Across a wide range of watersheds, NANI has been shown to be a good predictor of riverine nitrogen export. In this paper, we review various methodologies proposed for NANI estimation since its first introduction and evaluate alternative calculations suggested by previous literature. Our work is the first study in which a consistent NANI calculation method is applied across the U.S. watersheds and tested against available riverine N flux estimates. Among the tested methodologies, yield-based estimation of agricultural N fixation (instead of crop area-based) made the largest difference, especially in some Mississippi watersheds where the tile drainage was a significant factor reducing watershed N retention. Across the U.S. watersheds, NANI was particularly sensitive to farm N fertilizer application, cattle N consumption, N fixation by soybeans and alfalfa, and N yield by corn, soybeans, and pasture, although their relative importance varied among different regions.

Net anthropogenic phosphorus inputs (NAPI) index application in Mainland China

This study provides a new understanding on sources of P, which may serve as a foundation for further exploration of anthropogenic effects on P input. Estimation of net anthropogenic phosphorus input (NAPI) was based on an inventory of phosphorus (P) fertilizer use, consumption of human food and animal feed, seeding phosphorus and non-food phosphorus net flux. Across Mainland China, NAPI had an upward trend from 1981 to 2009, which reflects development trend of the population and economic. NAPI for years 1981, 1990, 2000 and 2009 are 190 kg P km(-2)yr(-1) (1.8 kg P per person yr(-1)), 295 kg P km(-2) yr(-1) (2.5 kg P per person yr(-1)), 415 kg P km(-2) yr(-1) (3.1 kg P per person yr(-1)) and 465 kg P km(-2) yr(-1) (3.4 kg P per person yr(-1)), respectively. On a geographical basis, NAPI per unit area is lower in northwest Mainland China than in southeast Mainland China with the largest NAPI of 3101 kg P km(-2) yr(-1) in Shanghai, while NAPI per person is in reverse with the largest NAPI 7.7 kg P per person yr(-1) in Tibet. P input of fertilizer is the largest source of NAPI, accounting for 57.35-83.73% (109-390 kg P km(-2) yr(-1)) of the total NAPI, followed by non-food P and P in human food and animal feed. Year 2000 was a critical point where P changed almost from net input to output. Grain production rate per unit mass of fertilizer showed an obvious downward trend. The primary factor in relation to the change in NAPI is total population.

Nitrogen deposition alters soil chemical properties and bacterial communities in the Inner Mongolia grassland

Nitrogen deposition has dramatically altered biodiversity and ecosystem functioning on the earth; however, its effects on soil bacterial community and the underlying mechanisms of these effects have not been thoroughly examined. Changes in ecosystems caused by nitrogen deposition have traditionally been attributed to increased nitrogen content. In fact, nitrogen deposition not only leads to increased soil total N content, but also changes in the NH4(+)-N content, NO3(-)-N content and pH, as well as changes in the heterogeneity of the four indexes. The soil indexes for these four factors, their heterogeneity and even the plant community might be routes through which nitrogen deposition alters the bacterial community. Here, we describe a 6-year nitrogen addition experiment conducted in a typical steppe ecosystem to investigate the ecological mechanism by which nitrogen deposition alters bacterial abundance, diversity and composition. We found that various characteristics of the bacterial community were explained by different environmental factors. Nitrogen deposition decreased bacterial abundance that is positively related to soil pH value. In addition, nitrogen addition decreased bacterial diversity, which is negatively related to soil total N content and positively related to soil NO3(-)-N heterogeneity. Finally, nitrogen.addition altered bacterial composition that is significantly related to soil NH4(+)-N content. Although nitrogen deposition significantly altered plant biomass, diversity and composition, these characteristics of plant community did not have a significant impact on processes of nitrogen deposition that led to alterations in bacterial abundance, diversity and composition. Therefore, more sensitive molecular technologies should be adopted to detect the subtle shifts of microbial community structure induced by the changes of plant community upon nitrogen deposition.

Effects of riparian buffers on nitrate concentrations in watershed discharges: new models and management implications

Watershed analyses of nutrient removal in riparian buffers have been limited by the geographic methods used to map buffers and by the statistical models used to test and quantify buffer effects on stream nutrient levels. We combined geographic methods that account for buffer prevalence along flow paths connecting croplands to streams with improved statistical models to test for buffer effects on stream nitrate concentrations from 321 tributary watersheds to the Chesapeake Bay, USA. We developed statistical models that predict stream nitrate concentration from watershed land cover and physiographic province. We used information theoretic methods (AIC(c)) to compare models with and without buffer terms, and we demonstrate that models accounting for riparian buffers better explain stream nitrate concentrations than models using only land cover proportions. We analyzed the buffer model parameters to quantify differences within and among physiographic provinces in the potentials for nitrate loss from croplands and nitrate removal in buffers. On average, buffers in Coastal Plain study watersheds had a higher relative nitrate removal potential (95% of the inputs from cropland) than Piedmont buffers (35% of inputs). Buffers in Appalachian Mountain study watersheds were intermediate (retaining 39% of cropland inputs), but that percentage was uncertain. The absolute potential to reduce nitrate concentration was highest in the Piedmont study watersheds because of higher nitrate inputs from cropland. Model predictions for the study watersheds provided estimates of nitrate removals achieved with the existing cropland and buffer distributions. Compared to expected nitrate concentrations if buffers were removed, current buffers reduced average nitrate concentrations by 0.73 mg N/L (50% of their inputs from cropland) in the Coastal Plain study watersheds, 0.40 mg N/L (11%) in the Piedmont, and 0.08 mg N/L (5%) in the Appalachian Mountains. Restoration to close all buffer gaps downhill from croplands would further reduce nitrate concentrations by 0.66 mg N/L, 0.83 mg N/L, and 0.51 mg N/L, respectively, in the Coastal Plain, Piedmont, and Appalachian Mountain study watersheds. Aggregate nitrate removal by riparian buffers was less than suggested by many studies of field-to-stream transects, but buffer nitrate removal is significant, and restoration could achieve substantial additional removal.

Net anthropogenic nitrogen accumulation in the Beijing metropolitan region

BACKGROUND, AIM, AND SCOPE

A rapid increase in anthropogenic nitrogen inputs has a strong impact on terrestrial and aquatic ecosystems. We have estimated net anthropogenic nitrogen accumulation (NANA) as an index of nitrogen (N) pollution potential in the Beijing metropolitan region, China. Our research provides a basis for understanding the potential impact of anthropogenic N inputs on environmental problems, such as nation-wide water quality degradation under the current rapid urban expansion in modern China.

METHODS

The NANA estimation is based on an inventory of atmospheric N deposition, N fertilizer use, consumption of human food and animal feed, N fixation, and riverine N import and export. We calculated N accumulation values for the years 1991, 1997, 2003, and 2007.

RESULTS AND DISCUSSION

The average NANA values for the urban and suburban areas from 1991 to 2007 were 24,038 and 13,090 kg N km(-2) year(-1), respectively. NANA is higher in eastern and southern areas than in northern and western areas, and higher in the urban area than in the suburban area. The overall average NANA in Beijing has a downward trend from 15,187 kg N km(-2) year(-1) in 1991 to 11,606 kg N km(-2) year(-1) in 2007, but is still two to five times as that of developed countries. N input from nitrogenous fertilizer is the largest source of NANA, accounting for 44.4% (6,764 kg N km(-2) year(-1)) of the total N input, followed by atmospheric N deposition and N in human food and animal feed. NANA is closely related to land use, on average 23,140 kg N km(-2) year(-1) in densely populated developed land, 17,904 kg N km(-2) year(-1) in agricultural land, and 10,445 kg N km(-2) year(-1) in forest land. Human population density is the best single predictor of NANA.

Past and future trends in nutrients export by rivers to the coastal waters of China

We analyzed the past and future trends in river export of dissolved and particulate nitrogen (N), phosphorus (P) and carbon (C) to the coastal waters of China, for sixteen rivers, as calculated by the Global NEWS models (Nutrient Export from WaterSheds). Between 1970 and 2000, the dissolved N and P export increased significantly, while export of other nutrients changed less. We analyzed the future trends (2000-2050) for the Millennium Ecosystem Assessment (MEA) scenarios. In general, the largest increases of dissolved nutrients export are projected for the Global Orchestration scenario, assuming a globalized world and a reactive approach toward environmental management. Future trends in river export of nutrients vary largely among basins, nutrient forms and scenarios. We calculate both increasing and decreasing trends between 2000 and 2050. We also identify the sources contributing to the nutrient export. For selected river basins we present results for alternative scenarios, which are based on the Global Orchestration scenario, but assume more environmental management. This illustrates how the NEWS models can be useful in regional analyses for decision making.

Differences in phosphorus and nitrogen delivery to the Gulf of Mexico from the Mississippi River Basin

Seasonal hypoxia in the northern Gulf of Mexico has been linked to increased nitrogen fluxes from the Mississippi and Atchafalaya River Basins, though recent evidence shows that phosphorus also influences productivity in the Gulf. We developed a spatially explicit and structurally detailed SPARROW water-quality model that reveals important differences in the sources and transport processes that control nitrogen (N) and phosphorus (P) delivery to the Gulf. Our model simulations indicate that agricultural sources in the watersheds contribute more than 70% of the delivered N and P. However, corn and soybean cultivation is the largest contributor of N (52%), followed by atmospheric deposition sources (16%); whereas P originates primarily from animal manure on pasture and rangelands (37%), followed by corn and soybeans (25%), other crops (18%), and urban sources (12%). The fraction of in-stream P and N load delivered to the Gulf increases with stream size, but reservoir trapping of P causes large local- and regional-scale differences in delivery. Our results indicate the diversity of management approaches required to achieve efficient control of nutrient loads to the Gulf. These include recognition of important differences in the agricultural sources of N and P, the role of atmospheric N, attention to P sources downstream from reservoirs, and better control of both N and P in close proximity to large rivers.

Estimating riverine discharge of nitrogen from the South Korea by the mass balance approach

The main objective of this research was to estimate the total mass of nitrogen discharged from various sources in Korea using the mass balance approach. Three different nitrogen mass balances were presented: (1) agricultural activities including raising crops and animal husbandry; (2) domestic activities, and (3) activities in forest and urban areas. These nitrogen balances were combined to estimate riverine discharge of nitrogen to the ocean in national scale. Nitrogen inputs include atmospheric deposition, biological nitrogen fixation, application of inorganic fertilizers/manures, animal feed/imported foodstuffs, and meat/fish. Nitrogen outputs include ammonia volatilization, denitrification, human/animal waste generation, crop/meat production, and riverine discharge to the ocean. The estimated total nitrogen input in Korea was 1,194.5 x 10(3) tons N/year. Nitrogen discharged into rivers was estimated as 408-422 x 10(3) tons N/year, of which 66-71% was diffuse in origin. The estimated diffuse discharges for land uses were estimated as 82 x 10(3) tons N/year from agricultural areas, 7 x 10(3) tons N/year from forestry and 75 x 10(3) tons N/year from urban and industrial areas.

Spring nitrate flux in the Mississippi River Basin: a landscape model with conservation applications

Nitrogen derived from fertilizer runoff in the Mississippi River Basin (MRB) is acknowledged as a primary cause of hypoxia in the Gulf of Mexico. To identify the location and magnitude of nitrate runoff hotspots, and thus determine where increased conservation efforts may best improve water quality, we modeled the relationship between nitrogen inputs and spring nitrate loading in watersheds of the MRB. Fertilizer runoff was found to account for 59% of loading, atmospheric nitrate deposition for 17%, animal waste for 13%, and municipal waste for 11%. A nonlinear relationship between nitrate flux and fertilizer N inputs leads the model to identify a small but intensively cropped portion of the MRB as responsible for most agricultural nitrate runoff. Watersheds of the MRB with the highest rates of fertilizer runoff had the lowest amount of land enrolled in federal conservation programs. Our analysis suggests that scaling conservation effort in proportion to fertilizer use intensity could reduce agricultural nitrogen inputs to the Gulf of Mexico, and that the cost of doing so would be well within historic levels of federal funding for agriculture.

Effects of land use and land cover, stream discharge, and interannual climate on the magnitude and timing of nitrogen, phosphorus, and organic carbon concentrations in three coastal plain watersheds

In-stream nitrogen, phosphorus, organic carbon, and suspended sediment concentrations were measured in 18 subbasins over 2 annual cycles to assess how land use and land cover (LULC) and stream discharge regulate water quality variables. The LULC was a primary driver of in-stream constituent concentrations and nutrient speciation owing to differences in dominant sources and input pathways associated with agricultural, urban, and forested land uses. Stream discharge was shown to be a major factor that dictated not only the magnitude of constituent concentrations, but also the chemical form. In high discharge agricultural subbasins, where nitrate was the dominant nitrogen form, there was a negative correlation between discharge and nitrate concentration indicating groundwater inputs as the dominant pathway. In urban settings, however, nitrate was positively correlated with discharge, and, in forested subwatersheds, where dissolved organic nitrogen (DON) was the dominant nitrogen form, there was a positive correlation between discharge and DON, indicating washoff from the watershed as the dominant input pathway. Similarly, phosphorus concentrations were strongly regulated by LULC, discharge, and seasonality. This comparative study highlights that different mechanisms regulate different forms of nitrogen, phosphorus, and carbon, and thus field programs or water quality models used for regulatory purposes must assess these nutrient forms to accurately apply management plans for nutrient reductions.

Regional scale nutrient modelling: exports to the Great Barrier Reef World Heritage Area

Clearing of native vegetation and replacement with cropping and grazing systems has increased nutrient exports to the Great Barrier Reef (GBR) to a level many times the natural rate. We present a technique for modelling nutrient transport, based on material budgets of river systems, and use it to identify the patterns and sources of nutrients exported. The outputs of the model can then be used to help prioritise catchment areas and land uses for management and assess various management options. Hillslope erosion is the largest source of particulate nutrients because of its dominance as a sediment source and the higher nutrient concentrations on surface soils. Dissolved nutrient fractions contribute 30% of total nitrogen and 15% of total phosphorus inputs. Spatial patterns show the elevated dissolved inorganic nitrogen export in the wetter catchments, and the dominance of particulate N and P from soil erosion in coastal areas. This study has identified catchments with high levels of contribution to exports and targeting these should be a priority.

Nutrient retention efficiency in streams receiving inputs from wastewater treatment plants

We tested the effect of nutrient inputs from wastewater treatment plants (WWTPs) on stream nutrient retention efficiency by examining the longitudinal patterns of ammonium, nitrate, and phosphate concentrations downstream of WWTP effluents in 15 streams throughout Catalonia (Spain). We hypothesized that large nutrient loadings would saturate stream communities, lowering nutrient retention efficiency (i.e., nutrient retention relative to nutrient flux) relative to less polluted streams. Longitudinal variation in ambient nutrient concentration reflected the net result of physical, chemical, or biological uptake and release processes. Therefore, gradual increases in nutrient concentration indicate that the stream acts as a net source of nutrients to downstream environments, whereas gradual declines indicate that the stream acts as a net sink. In those streams where gradual declines in nutrient concentration were observed, we calculated the nutrient uptake length as an indicator of the stream nutrient retention efficiency. No significant decline was found in dilution-corrected concentrations of dissolved inorganic nitrogen (DIN) and phosphate in 40 and 45% of streams, respectively. In the remaining streams, uptake length (estimated based on the decline of nutrient concentrations at ambient levels) ranged from 0.14 to 29 km (DIN), and from 0.14 to 14 km (phosphate). Overall, these values are longer (lower retention efficiency) than those from nonpolluted streams of similar size, supporting our hypothesis, and suggest that high nutrient loads affect fluvial ecosystem function. This study demonstrates that the efficiency of stream ecosystems to remove nutrients has limitations because it can be significantly altered by the quantity and quality of the receiving water.

Relating net nitrogen input in the Mississippi River basin to nitrate flux in the lower Mississippi River: a comparison of approaches

A quantitative understanding of the relationship between terrestrial N inputs and riverine N flux can help guide conservation, policy, and adaptive management efforts aimed at preserving or restoring water quality. The objective of this study was to compare recently published approaches for relating terrestrial N inputs to the Mississippi River basin (MRB) with measured nitrate flux in the lower Mississippi River. Nitrogen inputs to and outputs from the MRB (1951 to 1996) were estimated from state-level annual agricultural production statistics and NOy (inorganic oxides of N) deposition estimates for 20 states that comprise 90% of the MRB. A model with water yield and gross N inputs accounted for 85% of the variation in observed annual nitrate flux in the lower Mississippi River, from 1960 to 1998, but tended to underestimate high nitrate flux and overestimate low nitrate flux. A model that used water yield and net anthropogenic nitrogen inputs (NANI) accounted for 95% of the variation in riverine N flux. The NANI approach accounted for N harvested in crops and assumed that crop harvest in excess of the nutritional needs of the humans and livestock in the basin would be exported from the basin. The U.S. White House Committee on Natural Resources and Environment (CENR) developed a more comprehensive N budget that included estimates of ammonia volatilization, denitrification, and exchanges with soil organic matter. The residual N in the CENR budget was weakly and negatively correlated with observed riverine nitrate flux. The CENR estimates of soil N mineralization and immobilization suggested that there were large (2000 kg N ha-1) net losses of soil organic N between 1951 and 1996. When the CENR N budget was modified by assuming that soil organic N levels have been relatively constant after 1950, and ammonia volatilization losses are redeposited within the basin, the trend of residual N closely matched temporal variation in NANI and was positively correlated with riverine nitrate flux in the lower Mississippi River. Based on results from applying these three modeling approaches, we conclude that although the NANI approach does not address several processes that influence the N cycle, it appears to focus on the terms that can be estimated with reasonable certainty and that are correlated with riverine N flux.

Atmospheric nitrogen deposition to estuares in the mid-Atlantic and northeastern United States

The purpose of this work was to determine the contribution made by atmospheric nitrogen (N) deposition to the total N input to 10 estuaries on the east coast of the United States. We estimated the amount of N fixed by human activities in the watersheds (N fertilization, biotic N2fixation by legumes and pastures, atmospheric N deposition, and net food and feed import of N) of these 10 estuaries and used a land-use specific approach to estimate the N available for transport to the estuary from different watershed N sources (runoff from agriculture, urban areas and upland forests, point sources, and atmospheric deposition). Total atmospheric N inputs (watershed runoff plus direct deposition to the surface of estuary) accounted for 15-42% of the total N inputs to these 10 estuaries. Direct deposition to the surface of the estuary was an important atmospheric N source for four estuaries, accounting for 35-50% of the total atmospheric N inputs. Simulated reductions of atmospheric N deposition by 25% and 50% of current deposition rates reduced the contribution made by atmospheric N deposition to the total N loads by 1-6% and 2-11%, respectively. Largest reductions occurred in estuaries with direct atmospheric N deposition contributions >35% of the total atmospheric N input. Results from our simulated reductions suggest that considerable reductions (>25%) in atmospheric N deposition will be needed to significantly reduce the contribution made by atmospheric N deposition to the total N loads to our study estuaries. In addition, reductions in atmospheric N deposition will first be detected in estuaries with relatively high direct deposition inputs of atmospheric N deposition.

The need for comprehensive and consistent treatment of the nitrogen cycle in nitrogen cycling and mass balance studies: I. Terrestrial nitrogen cycle

A review of conceptual models that scientists use to characterize the nitrogen (N) cycle and to conduct N mass balance studies at global, regional and local scales is presented. Large uncertainties in processes and process rates make it difficult to conduct precise N mass balances and the dominant conceptual model has changed in recent decades. An earlier conceptual model recognized explicitly that human activities, especially agriculture, have both depleted terrestrial N and increased the fixation of atmospheric N in biologically available forms. The current conceptual model does not include adequate treatment of the depletion of the terrestrial N reservoir, the resulting transfer of N to the hydrosphere and atmosphere, or the cycling of terrestrial N below the plow layer. Thus, it delivers an unrealistically limited view of human influences on the N cycle. It is recommended that a comprehensive and consistent treatment of terrestrial N cycling be developed to better facilitate scientific explanation of historical N-related environmental changes and more closely balance N budgets on a range of geographical and temporal scales. Improved N-cycle models will provide an improved scientific basis for answering important resource management and policy questions.