Uncovering the spatial characteristics of global net anthropogenic nitrogen input at high resolution and across 1.42 million lake basins.

Global Net Anthropogenic Nitrogen Input (NANI) at high resolution is crucial for assessing the impact of human activities on aquatic environments. Insufficient global high-resolution data sources and methods have hindered the effective examination of the global characteristics and driving forces of NANI. This study presents a general framework for calculating global NANI, providing estimates at a 5-arc-minute resolution and over 1.42 million lake basins in 2015. The results highlight the region near the Tropic of Cancer as a concentration area for high NANI and an inflection point for latitude-based accumulation variation. It also emphasizes the uneven distribution of NANI among continents, with Asia and Africa having the highest proportions, yet their high and low values are notably lower than those of Europe and South America. A similar pattern is observed in global lakes, where Asia has the smallest quantity and volume, but the highest NANI intensity. In contrast, North America and Europe have larger quantities and volumes but the lowest NANI intensity. The global distribution characteristics reveal a clustering pattern in high and low values, with 1.25 % of the area having a sum of NANI exceeding 20 %. The uncertainty analysis regarding model parameters indicates that continents with the highest NANI do not always exhibit the highest uncertainty. These results bridge the gap between global nitrogen sustainable management and anthropogenic nitrogen input. They support research on spatiotemporal changes and controlling factors of global river nutrient loads, as well as the impact of climatic factors on basin nitrogen loss and its variability.

Anthropogenic nitrogen pollution impacts saltmarsh resilience with inhibition of seedling establishment and population dispersal.

Saltmarsh, a prominent buffer ecosystem, has been identified as an important sink for nitrogen (N) pollutants from marine- and land-based anthropogenic activities. However, how the enriched anthropogenic N impacts saltmarsh sustainability has been neglected due to limited understanding of marsh resilience based on seedling establishment and population dispersal under anthropogenic N inputs. This study combined mesocosm experiments and model simulations to quantify the effects of increased anthropogenic N on the seedling-based vegetation expansion of Spartina alterniflora. The results indicated that seedling survivals, growth rates, and morphological indicators were inhibited by 20.08 %, 37.14 %, and > 35.56 %, respectively, under 1.5 gN/kg anthropogenic N. The sensitivity rate of vegetation expansion was increased by 70 % with 1 gN/kg increased N concentration under the scenario of low seedling density (< 15 m/yr). These findings revealed an important unidentified weakness of the marsh development process to anthropogenic N inputs. Finally, we highlighted the importance of appropriate protection measures to control nutrient pollution in salt marshes. Our study provides new insights for enhancing the resilience and sustainability of saltmarsh ecosystems.

Comparison of inorganic nitrogen concentrations in airborne particles at inshore and offshore sites in the Yellow Sea (2017-2019): Long-range transport and potential impact on marine productivity.

Atmospheric deposition of nitrogen is one of the most important external nutrient sources. We investigated the concentrations of NO3- and NH4+ in airborne particles at both an offshore and an inshore site in the Yellow Sea. At the offshore site, devoid of local sources and located downwind from the highly developed areas of Korea and China, the concentrations of atmospheric particulate NO3- and NH4+ were ∼88 ± 101 nmol m-3 and ∼102 ± 102 nmol m-3, respectively, likely due to the transboundary long-range transport of pollutants. The inshore site showed a concentration ∼2 times higher than the offshore site. Considering not only dry inorganic nitrogen deposition but also wet and organic material deposition, the total atmospheric nitrogen deposition was estimated to contribute roughly 10 % to the new production in the Yellow Sea.

Estimating Yangtze River basin's riverine N2O emissions through hybrid modeling of land-river-atmosphere nitrogen flows.

Riverine ecosystems are a significant source of nitrous oxide (N2O) worldwide, but how they respond to human and natural changes remains unknown. In this study, we developed a compound model chain that integrates mechanism-based modeling and machine learning to understand N2O transfer patterns within land, rivers, and the atmosphere. The findings reveal a decrease in N2O emissions in the Yangtze River basin from 4.7 Gg yr-1 in 2000 to 2.8 Gg yr-1 in 2019, with riverine emissions accounting for 0.28% of anthropogenic nitrogen discharges from land. This unexpected reduction is primarily attributed to improved water quality from human-driven nitrogen control, while natural factors contributed to a 0.23 Gg yr-1 increase. Notably, urban rivers exhibited a more rapid N2O efflux ( [Formula: see text] ), with upstream levels nearly 3.1 times higher than rural areas. We also observed nonlinear increases in [Formula: see text] with nitrogen discharge intensity, with urban areas showing a gradual and broader range of increase compared to rural areas, which exhibited a sharper but narrower increase. These nonlinearities imply that nitrogen control measures in urban areas lead to stable reductions in N2O emissions, while rural areas require innovative nitrogen source management solutions for greater benefits. Our assessment offers fresh insights into interpreting riverine N2O emissions and the potential for driving regionally differentiated emission reductions.

Nitrogen addition affects floral and vegetative traits, reproduction, and pollinator performance in Capsicum annuum L.

BACKGROUND AND AIMS: It has been demonstrated that nitrogen (N) addition alters flower morphology, floral rewards and pollinator performance. However, little is known about the effects of N addition on plant reproduction, including fruit set and seed set during selfing and outcrossing, floral and vegetative traits, and pollinator performance. We hypothesized that N addition would influence fruit set, seed set in selfed and outcrossed flowers, the relationship between vegetative and flower traits, and pollinator performance. METHODS: A 2-year pot experiment was conducted in which Capsicum annuum was exposed to three levels of relatively short-term N supply, i.e. 0 g m-2 (no N addition, as a control), 4 g m-2 (4N) and 16 g m-2 (16N), which are equivalent to about 0-, 1- and 4-fold of the peak local N deposition. We measured flower rewards, flower morphology, flowering phenology, as well as pollinator visitation rate, fruit set and seed set by self- and outcross-fertilization of C. annuum. RESULTS: The four levels of N addition increased plant biomass, biomass allocation to flowers, flower size, stigma-anther separation, nectar production and pollen production, resulting in an increase in pollinator visitation and fruit set. Nevertheless, the control and 16 levels of N addition reduced plant biomass, biomass allocation to flowers, flower size and stigma-anther separation, and nectar and pollen production, and consequently decreased pollinator visitation and fruit set. Exclusion of pollinators and hand-pollination experiments revealed that low levels of N addition were associated with high seed set in outcrossed flowers; however, this trend was reversed in flowers grown in the control and 16N treatments. CONCLUSION: Our results suggest that an optimal level of 4N can enhance the correlation between flower traits, pollinator performance and plant reproduction. Our findings cast new light on the underlying mechanisms of plant-pollinator interactions and plant adaptation to nitrogen deposition.

Nitrogen addition increases mass loss of gymnosperm but not of angiosperm deadwood without changing microbial communities.

Enhanced nitrogen (N) deposition due to combustion of fossil fuels and agricultural fertilization is a global phenomenon which has severely altered carbon (C) and N cycling in temperate forest ecosystems in the northern hemisphere. Although deadwood holds a substantial amount of C in forest ecosystems and thus plays a crucial role in nutrient cycling, the effect of increased N deposition on microbial processes and communities, wood chemical traits and deadwood mass loss remains unclear. Here, we simulated high N deposition rates by adding reactive N in form of ammonium-nitrate (40 kg N ha-1 yr-1) to deadwood of 13 temperate tree species over nine years in a field experiment in Germany. Non-treated deadwood from the same logs served as control with background N deposition. Our results show that chronically elevated N levels alters deadwood mass loss alongside respiration, enzymatic activities and wood chemistry depending on tree clade and species. In gymnosperm deadwood, elevated N increased mass loss by +38 %, respiration by +37 % and increased laccase activity 12-fold alongside increases of white-rot fungal abundance +89 % (p = 0.03). Furthermore, we observed marginally significant (p = 0.06) shifts of bacterial communities in gymnosperm deadwood. In angiosperm deadwood, we did not detect consistent effects on mass loss, physico-chemical properties, extracellular enzymatic activity or changes in microbial communities except for changes in abundance of 10 fungal OTUs in seven tree species and 28 bacterial OTUs in 10 tree species. We conclude that N deposition alters decomposition processes exclusively in N limited gymnosperm deadwood in the long term by enhancing fungal activity as expressed by increases in respiration rate and extracellular enzyme activity with minor shifts in decomposing microbial communities. By contrast, deadwood of angiosperm tree species had higher N concentrations and mass loss as well as community composition did not respond to N addition.

Distribution pattern of N-damo bacteria along an anthropogenic nitrogen input gradient from the coastal mangrove wetland to the South China sea sediments.

Microbial nitrite-dependent anaerobic methane oxidation (n-damo) process is important for mitigating methane emission and anthropogenic nitrogen inputs in the marine environment. However, the distribution pattern of n-damo bacteria along an anthropogenic N-input gradient from the coastal wetland to the pristine South China Sea is poorly understood. This study investigated the diversity and abundance of n-damo bacteria in samples collected along a N-input gradient from Mai Po (MP) mangrove wetland sediments of the Pearl River Estuary (PRE) to the deep ocean sediments of the South China Sea (SCS). Retrieved 16S rDNA sequences showed a shift of n-damo community composition of complex structures with both freshwater and marine n-damo lineages in MP intertidal sediments to marine dominated characteristic in SCS sediments. The observed variation of Shannon and Chao1 indexes of n-damo bacteria shared a similar trend of a decrease at first followed by an increase along the targeting gradient with previously investigated methanogens, anaerobic methanotrophic archaea, ammonia-oxidizing archaea and ammonia-oxidizing bacteria, but had a reverse pattern with anammox bacteria. The community structure of pmoA gene sequences contained freshwater lineages only in SCS continental shelf sediments closer to the PRE, and turned to group with other marine samples in deeper and pristine sediments. Results suggested that n-damo bacteria might be a major contributor to anaerobic denitrification in the SCS sediments because their abundances were much higher than previously studied anammox bacteria in the same sample set. The distribution pattern of n-damo bacterial diversity, richness and abundance along the anthropogenic N-input gradient implies that they could be used as a bio-indicator for monitoring the anthropogenic/terrestrial inputs in marine environments.

Effects of watershed land use on coastal marine environments: A multiscale exploratory analysis with multiple biogeochemical indicators in fringing coral reefs of Okinawa Island.

The analytical spatial scale and selection of biogeochemical indicators affect interpretations of land-use impacts on coastal marine environments. In this study, nine biogeochemical indicators were sampled from 36 locations of coral reefs fringing a subtropical island, and their relationships with watershed land use were assessed by spatial autoregressive models with spatial weight matrixes based on distance thresholds of a few to 30 km. POM-relevant indicators were associated with agricultural and urban lands of watersheds within relatively small ranges (6-14 km), while the concentrations of inorganic nutrients were associated with watersheds within 20 km or more. The macroalgal δ15N showed a strong relationship with agricultural lands of watersheds within 7 km and urban/forest lands of watersheds within 24 km. These results demonstrate significant effects of land use on the coral reef ecosystems of the island, and the importance of appropriate combinations of analytical scales and biogeochemical indicators.

Spatio-temporal variation of net anthropogenic nitrogen inputs (NANI) from 1991 to 2019 and its impacts analysis from parameters in Northwest China.

At present, excessive nutrient inputs caused by human activities have resulted in environmental problems such as agricultural non-point source pollution and water eutrophication. The Net Anthropogenic Nitrogen Inputs (NANI) model can be used to estimate the nitrogen (N) inputs to a region that are related to human activities. To explore the net nitrogen input of human activities in the main grain-producing areas of Northwestern China, the county-level statistical data for the Ningxia province and NANI model parameters were collected, the spatio-temporal distribution characteristics of NANI were analyzed and the uncertainty and sensitivity of the parameters for each component of NANI were quantitatively studied. The results showed that: (1) The average value of NANI in Ningxia from 1991 to 2019 was 7752 kg N km-2 yr-1. Over the study period, the inputs first showed an overall increase, followed by a decrease, and then tended to stabilize. Fertilizer N application was the main contributing factor, accounting for 55.6%. The high value of NANI in Ningxia was mainly concentrated in the Yellow River Diversion Irrigation Area. (2) The 95% confidence interval of NANI obtained by the Monte Carlo approach was compared with the results from common parameters in existing literature. The simulation results varied from -6.4% to 27.4% under the influence of the changing parameters. Net food and animal feed imports were the most uncertain input components affected by parameters, the variation range was -20.7%-77%. (3) The parameters of inputs that accounted for higher proportions of the NANI were more sensitive than the inputs with lower contributions. The sensitivity indexes of the parameters contained in the fertilizer N applications were higher than those of net food and animal feed imports and agricultural N-fixation. This study quantified the uncertainty and sensitivity of parameters in the process of NANI simulation and provides a reference for global peers in the application and selection of parameters to obtain more accurate simulation results.

Management implications of spatial-temporal variations of net anthropogenic nitrogen inputs (NANI) in the Yellow River Basin.

It is an important content of environment management to accurately identify the time change and spatial distribution of net anthropogenic nitrogen inputs (NANI) in the river basin. In order to develop a unified management and diverse control strategy that fits the characteristics of the basin, this study establishes the NANI-S model combining the NANI model with the spatial autocorrelation analysis method, which is a quantification-analysis-control process, and takes the 70 prefecture-cities in the Yellow River Basin (YRB) as the study area. The result shows that (1) the NANI of YRB increased first and then decreased with an average NANI value of 6787.59 kg/(km2·a), showing that the overall N pollution situation of the YRB shows a trend of improvement in nitrogen (N) fertilizer input as the main source, and the average contribution rate was 47.45%. (2) There were obvious spatial differences in the NANI in the YRB because the global Moran's I fluctuated between 0.67 and 0.78. Cities with high NANI clustered in the middle and lower reaches, while low NANI clustered in the upper reaches. (3) Improving fertilizer utilization rate and industrial and domestic sewage treatment capacity was the key point of N control. Based on the results, practical policy recommendations for water pollution management were constructed, which provides a scientific basis for pollution prevention and high-quality development in the basin. In addition, this analysis method can also be applied to other basin N management studies.

Nitrogen legacies in anthropogenic landscapes: a case study in the Mondego Basin in Portugal.

Nitrogen (N) legacies have built up in anthropogenic landscapes over decades of agricultural intensification, and these legacies lead to time lags in water quality change measurable even beyond the moment of application of N. It is important to understand these legacies to quantify the relationship between N inputs and N concentrations in streams and implement best management practices for water quality improvement; however, little is known about the magnitude of legacies in various landscape elements like soils and groundwater. Here, we have used the ELEMeNT (Exploration of Long-tErM Nutrient Trajectories) model to explore the buildup and depletion of N legacies over a 216-year period, across the Mondego River Basin, a 6645-km2 watershed in Portugal, where human interventions have considerably changed the characteristics of the basin to prevent floods and improve farming conditions in recent decades. The results show that the increase in the amount of inorganic fertilizer applied was the main driver for the anthropogenic N loads in the watershed from 1950 until the beginning of the 1990s. The N inputs have been decreasing since then, but N loads in the river did not document any decrease till the 1990s; after which there was a decline. This time lag between the N inputs to the watershed and the N loads in the river (about two decades) is a function of accumulation of N legacy.

Trends of the response-relationship between net anthropogenic nitrogen and phosphorus inputs (NANI/NAPI) and TN/TP export fluxes in Raohe basin, China.

The intensification of anthropogenic nitrogen (N) and phosphorus (P) inputs profoundly affects water environmental quality. Hence it is pivotal to clarify the response relationship between riverine TN/TP export and anthropogenic N/P inputs to provide strategies guidance in N/P management. Based on the variation of net anthropogenic N and P inputs (NANI/NAPI) in the Raohe basin from 1990 to 2018, we constructed the response relationship between NANI/NAPI and total nitrogen and phosphorus (TN/TP) export fluxes in the riverine, which successfully predicted N and P export at the basin scale management. We found N export ratio (ratio of TN export to NANI) increased with slight fluctuation and was mainly affected by the combined effects of Nfer (fertilizer N inputs) and Ndep (atmospheric N deposition) etc., while the decrease of P export ratio (ratio of TP export to NAPI) was mainly due to intensive retention effect of the soil and sediment induced by anthropogenic influence to P transportation process. These results indicate that the downstream aquatic systems take a high risk of increasing N load pressure and the basin systems suffer a danger from rising P load pressure. Therefore, it is recommended to concentrate more on downstream aquatic systems during the N management strategy implementation and pay closer attention to the whereabouts of P in the basin system.

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.

Temporal-spatial dynamics of anthropogenic nitrogen inputs and hotspots in a large river basin.

Environmental pollution caused by human activities in the Yangtze River Basin (YRB), especially nitrogen pollution, has always been a hot topic. High-intensity anthropogenic nitrogen (AN) inputs have undergone some changes on account of environmental management practices in the YRB. We used the latest statistical data (2000-2017) to estimate spatiotemporal heterogeneity of AN inputs across the YRB, characterize hotspots of AN inputs, and predict the future trend, which is critical to meet nitrogen management challenges. We found agricultural sources were major contributors to nitrogen inputs (more than 70%) in the YRB. Due to the reduction in agricultural fertilizers use in China, AN inputs had gradually decreased from a peak of 19.0 Tg/yr in 2014 after a rapid growth period. Additionally, the nitrogen flux in sub-catchments and from various sources indicated an increasing distribution characteristic from the upper reaches to the lower reaches. Hotspots of AN inputs were mainly concentrated in the Sichuan Basin and the Middle-Lower Yangtze Plain (more than 50 tons/km2), however, growth rates were relatively low or even negative. STIRPAT model showed population size was the most important factor affecting AN loads. Although the growth rate would slow down in the future, AN loads would be maintained at a high level. Besides, aquaculture had become an important source of potential nitrogen growth in the whole basin, although the contribution was relatively small at present. Controlling nitrogen loads in hotspots and avoiding high inputs of new nitrogen sources should be the focus of future nitrogen environmental management.

Pond aquaculture effluents feed an anthropogenic nitrogen loop in a SE Asian estuary.

Coastal aquaculture expansion resulted in mangrove area loss and ecosystem degradation in the past decades, mainly in tropical Asia. Despite increasing environmental concerns regarding nutrient and organic matter-rich effluents, little is known on the effects on adjacent estuarine and coastal food webs. To assess the impact and fate of anthropogenic nitrogen released from aquaculture facilities, we studied water quality and nitrogen (N) flow across an estuarine food web in an estuary in Hainan, China, using nitrogen stable isotopes (δ15N). We found higher δ15N values of ammonium, nitrate and suspended matter in the pond-covered inner estuary than further upstream, suggesting a strong influence of untreated pond effluents, which had a high δ15N (ammonium: ~16‰, nitrate: ~7‰, suspended matter: ~8‰). Fish and benthic invertebrates of the inner estuary had a higher δ15N than consumers further upstream and in similar aquaculture-free estuaries elsewhere, most likely due to direct or indirect uptake of 15N-enriched aquaculture effluents by phytoplankton and benthic algae. A major part of the artisanal catches from the estuary consists of small-size fish which is used as feed in the local aquaculture. Thus, estuarine fish incorporating aquaculture-effluent based food web signals are harvested and recycled as feed in aquaculture facilities, whose effluents sustain this local food web. The δ15N being at the high end of the global range on all trophic levels indicates an anthropogenic nitrogen loop in which some portion of the reactive nitrogen initially introduced into aquaculture ponds is continuously recycled and affects the estuarine food web. This recycling also indicates a shortcut in the otherwise inefficient nitrogen sink function of estuaries. Therefore, in areas with large-scale coastal aquaculture like in China and SE Asia the effect of reactive nitrogen from aquaculture sources on the performance of coastal ecosystems may be larger than previously thought.

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 (Nfer), which accounted for 55.23% of the NANI was the largest N input source, followed by net N import in food and feed (Nim), atmospheric N deposition (Ndep), N fixation (Nfix), and seeding N (Nsee). 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 Ndep, Nfer, Nsee, and Nim 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 Nfix. 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.

Different responses of nitrite- and nitrate-dependent anaerobic methanotrophs to increasing nitrogen loading in a freshwater reservoir.

Nitrite (NO2-)- and nitrate (NO3-)-dependent anaerobic oxidation of methane (AOM) are two new additions in microbial methane cycle, which potentially act as important methane sinks in freshwater aquatic systems. Here, we investigated spatial variations of community composition, abundance and potential activity of NO2-- and NO3--dependent anaerobic methanotrophs in the sediment of Jiulonghu Reservoir (Zhejiang Province, China), a freshwater reservoir having a gradient of increasing nitrogen loading from upstream to downstream regions. High-throughput sequencing of total bacterial and archaeal 16S rRNA genes showed the cooccurrence of Candidatus Methylomirabilis oxyfera (M. oxyfera)-like and Candidatus Methanoperedens nitroreducens (M. nitroreducens)-like anaerobic methanotrophs in the examined reservoir sediments. The community structures of these methanotrophs differed substantially between the sediments of upstream and downstream regions. Quantitative PCR suggested higher M. oxyfera-like bacterial abundance in the downstream (8.6 × 107 to 2.8 × 108 copies g-1 dry sediment) than upstream sediments (2.4 × 107 to 3.5 × 107 copies g-1 dry sediment), but there was no obvious difference in M. nitroreducens-like archaeal abundance between these sediments (3.7 × 105 to 4.8 × 105 copies g-1 dry sediment). The 13CH4 tracer experiments suggested the occurrence of NO2-- and NO3--dependent AOM activities, and their rates were 4.7-14.1 and 0.8-2.6 nmol CO2 g-1 (dry sediment) d-1, respectively. Further, the rates of NO2--dependent AOM in downstream sediment were significantly higher than those in upstream sediment. The NO3- concentration was the key factor affecting the spatial variations of abundance and activity of NO2--dependent anaerobic methanotrophs. Overall, our results showed different responses of NO2-- and NO3--dependent anaerobic methanotrophs to increasing nitrogen loading in a freshwater reservoir.

A new approach to the biological monitoring of freshwater systems: Mapping nutrient loading in two South African rivers, a case study.

Excessive addition of nitrogen (N) has threatened aquatic ecosystems for decades. Traditional water quality and biological monitoring assessment tools are widely used for monitoring nutrient loads and ecosystem health, but most of these methods cannot distinguish between different types and sources of pollution. This is a challenge, particularly when dealing with non-point sources of anthropogenic nitrogen inputs into freshwater systems. Recent laboratory studies using stable isotopic ratios (δ15N and C/N) of aquatic macrophytes (duckweed: Spirodela spp.) have shown successful differentiation and mapping between different N-sources and further, showed abilities to act as early warning indicators for environmental N-loading. Therefore, the aim of this study was to field test the potential of stable isotopic values of transplanted Spirodela spp. to map temporal and spatial N-loading variation and determine the main sources of N-loading in two river systems in the Eastern Cape Province of South Africa, using previously grown, isotopically calibrated and transplanted Spirodela plants, collected over a 13-month sampling period. Nitrogen isotopic values (δ15N) of Spirodela plants traced environmental N-loading and identified pollution hotspots and sources through time and space over a wide range of nutrient gradients. δ15N isotopic values of Spirodela spp. provided detailed dynamics on N-loading, therefore supporting its utilisation in the biological monitoring of ecosystem health and the early detection of eutrophication in freshwater systems.

Spatio-temporal dynamics of nitrogen and phosphorus input budgets in a global hotspot of anthropogenic inputs.

The increased input of anthropogenic nitrogen (N) and phosphorus (P) to watershed ecosystems has been cited as among the most important reasons for widespread water pollution. Revealing spatio-temporal patterns of N and P input budgets in regions with intensified human activity can facilitate a better understanding of human-induced N and P cycles. Here, we present budget inventories including both anthropogenic non-point and point N and P inputs into the Huai River Basin, which has been identified as one of the hotspots of anthropogenic inputs across the world. On average, total anthropogenic N and P inputs in the year 2010 reached 28,000 kg N km-2 yr-1 and 2800 kg P km-2 yr-1, showing a 50% and 42% increases in comparison with 1990, respectively. Both non-point-source and point-source N & P inputs have exhibited a rapid increase from 1990 to 2010, which has been related to the increasing human population and socio-economic development. The intensive farming implemented to meet the growing food demand was responsible for continuous growth in non-point-source inputs. Meanwhile, rapid urbanization with lagged environmental management was the major reason for the increased point-source inputs. Spatial patterns of N & P inputs were similar across different periods, showing that the hotspots generally centralized in a few northern counties. By further interpreting the critical sources and their drivers of inputs to each region through time, our work provides insights for targeted management. Future mitigation strategies such as optimizing the farming methods, improving manure management and enhancing sewage treatment are necessary to address the environmental concerns of excessive inputs.

[Impact of Human Activities on Net Anthropogenic Nitrogen Inputs (NANI) at Township Scale in Erhai Lake Basin].

Excessive nitrogen inputs from human activities have become the main cause of water eutrophication and related ecological hazards. In order to study the impact of human activities on nitrogen in the basin, and based on statistical data of administrative units in 16 towns and villages, this study used the NANI model to calculate net anthropogenic nitrogen inputs (NANI) at township scale in Erhai Lake basin. Results show that the total amount of NANI in Erhai Lake basin was 29.81×103 t in 2014, and nitrogen input intensity per unit area was 10986 kg·(km2·a)-1, significantly higher than the national average. The input of nitrogen from food by the local tourist population was 0.26×103 t, accounting for 8% of local food nitrogen input. Nitrogen input from chemical fertilizer is the largest NANI input source, accounting for 47% of net nitrogen input in the basin, followed by net nitrogen input of food and feed. The spatial distribution of NANI at township scale shows evident regionalization, with higher values in the north and lower values in the south of the basin. The intensity of NANI in towns with cropland or population is high. The corresponding risk of nitrogen pollution in Erhai Lake basin is therefore a primary concern, and will remain so in the near future.