A hybrid method for quantifying China's nitrogen footprint during urbanisation from 1990 to 2009

Scenario Analysis of Food Phosphorus Footprint in Kisumu, a Lakeside East African City in Lake Victoria (Kenya)

Increased food production and consumption patterns have resulted in higher urban food phosphorus footprints, leading to a series of resource and environmental problems worldwide. We quantified the food phosphorus footprint of the African city of Kisumu using substance flow analysis. Our aim was to develop Kisumu's sustainable phosphorus management framework so that the city would reduce phosphorus losses into the food system. Our results show that in the year 2023, the import and export of food phosphorus in the Kisumu food system was 2730.26 ± 2.7% t P yr-1 and 3297.05 ± 2.4% t P yr-1, respectively. There was -566.79 ± -18% t P yr-1 food phosphorus deficit in the Kisumu food system. Crop planting subsystem runoff/leaching/erosion loss, household consumption subsystem waste loss, and pit latrine subsystem blackwater loss are the major pathways of phosphorus losses into the environment and the main contributors to the food phosphorus footprint in the city. The 2030 scenario analysis shows that implementing a comprehensive scenario scheme throughout the entire lifecycle process from phosphorus input to waste disposal is the best choice for reducing phosphorus losses and suppressing the growth of food phosphorus footprint in the future. Our study shows that the food phosphorus footprint in the Kisumu food system was 0.67 kg P cap-1yr-1 in 2023, which is still at a low level but may enter a continuous upward trend with the improvement of socio-economic development of the city. In our framework, we have proposed a few essential measures that include urine separation, installation of septic tank, adjustment of dietary structure, flexible layout of sanitary disposal facilities, and separation of organic waste streams to reduce food phosphorus footprints in Kisumu. Given the similarity of cities along the shores of Lake Victoria, our calculation methods and management strategies can be applied to other cities in the region.

Historical development, impact mechanism and future trends of nitrogen footprint in Wuxi City, China

Human activities have changed the biogeochemical cycle of nitrogen, leading to a large amount of reactive nitrogen (Nr) into the environment, aggravating a series of environmental problems, affecting human and ecosystem health. Cities are the core areas driving nitrogen cycling in terrestrial ecosystems, however, there are numerous influencing factors and their contributions are unclear. The nitrogen footprint is an important index to understand the impact of human activities on the environment, however, the calculation of urban nitrogen footprint needs a simplified and accurate system method. Here we use a nitrogen footprint calculation model at the urban system level based on system nitrogen balance, and a multi-factor extended STIRPAT (stochastic impact by regression on population, affluence, and technology) model suitable for analyzing the impact mechanism of nitrogen footprint to estimate nitrogen footprint of Wuxi City during 1990-2050. We find that: (1) from 1990 to 2020, the total nitrogen footprint of Wuxi City was in an increasing trend, but the per capita nitrogen footprint was in a decreasing trend. The per capita nitrogen footprint of 22.36 kg capita-1 in 2020 was at a lower level globally. (2) Nr discharge from fossil fuel combustion and Haber-Bosch nitrogen fixation accounted for the main proportion of nitrogen footprint. (3) Dietary choice (Ad), GDP per capita (Ag), urbanization rate (Au), population (P), and fossil energy productivity (Te) were the key factors contributing to the increase of the nitrogen footprint, which resulted in an annual increase of 1.39 %. While nitrogen footprint productivity (Tn), nitrogen use efficiency in crop farming (Tc), and nitrogen use efficiency in animal breeding (Ta) were the key inhibit factors that inhibit the increase of nitrogen footprint, and these factors slow down the annual growth rate of nitrogen footprint by 0.39 %. (4) The continuous growth of nitrogen footprint in the baseline and population growth scenarios will bring more environmental problems and greater environmental governance pressure to Wuxi City, while the sustainable scenario that includes comprehensive means such as economic adaptation and technological improvement is more in line with the requirements of high-quality development in China. Several mitigation measures are then proposed by considering Wuxi's realities from both key impact factors and potential for nitrogen footprint reduction in different scenarios, which can provide valuable policy insights to other cities, especially lakeside cities to mitigate nitrogen footprint.

Uneven agricultural contraction within fast-urbanizing urban agglomeration decreases the nitrogen use efficiency of crop production

Diverse development paths among cities within an urban agglomeration can lead to uneven changes in their agricultural production scale, which reshape the inter-city food supply patterns and the spatiotemporal characteristics of nitrogen (N) pollution from the food system. Here, using Guangdong-Hong Kong-Macao Greater Bay Area of China as a case, we found a substantial decrease in N use efficiency of crop production from 45.2% to 29.3% during 1989-2007, along with a growing level of concentration of food N production in less-urbanized cities. From 1989 to 2018, 12.3% to 42.2% of total N pollution in food production became embedded in inter-city trade, leading to aggregation of N pollution in peripheral cities with relatively low levels of economic development. We suggest that protection and intensification of cropland from urban encroachment, as well as enhancing the economic and technical synergies among cities, can serve the sustainable transition of the food system with coordinated N pollution mitigation.

Potential synergy of microplastics and nitrogen enrichment on plant holobionts in wetland ecosystems

Wetland ecosystems are global hotspots for environmental contaminants, including microplastics (MPs) and nutrients such as nitrogen (N) and phosphorus (P). While MP and nutrient effects on host plants and their associated microbial communities at the individual level have been studied, their synergistic effects on a plant holobiont (i.e., a plant host plus its microbiota, such as bacteria and fungi) in wetland ecosystems are nearly unknown. As an ecological entity, plant holobionts play pivotal roles in biological nitrogen fixation, promote plant resilience and defense chemistry against pathogens, and enhance biogeochemical processes. We summarize evidence based on recent literature to elaborate on the potential synergy of MPs and nutrient enrichment on plant holobionts in wetland ecosystems. We provide a conceptual framework to explain the interplay of MPs, nutrients, and plant holobionts and discuss major pathways of MPs and nutrients into the wetland milieu. Moreover, we highlight the ecological consequences of loss of plant holobionts in wetland ecosystems and conclude with recommendations for pending questions that warrant urgent research. We found that nutrient enrichment promotes the recruitment of MPs-degraded microorganisms and accelerates microbially mediated degradation of MPs, modifying their distribution and toxicity impacts on plant holobionts in wetland ecosystems. Moreover, a loss of wetland plant holobionts via long-term MP-nutrient interactions may likely exacerbate the disruption of wetland ecosystems' capacity to offer nature-based solutions for climate change mitigation through soil organic C sequestration. In conclusion, MP and nutrient enrichment interactions represent a severe ecological risk that can disorganize plant holobionts and their taxonomic roles, leading to dysbiosis (i.e., the disintegration of a stable plant microbiome) and diminishing wetland ecosystems' integrity and multifunctionality.

Spatial variation in actual and required nitrogen use efficiency and the potential to close the gap by management practices

To boost crop production, China uses almost a third of the world's nitrogen (N) fertilizer. However, N losses due to enhanced application of N fertilizers has led to surface water and groundwater pollution. A reduction in N losses without reducing crop yields is possible by increasing nitrogen use efficiency (NUE), which is important for the effective management of local crop production and water quality. This study used two representative agricultural counties in China (Quzhou and Qiyang) to assess if it is possible to achieve N loss thresholds in surface and groundwater by optimizing N management measures while maintaining actual crop production. We used a spatially explicit N balance model to assess the spatial variation in actual N inputs to soil and N losses to water, and in critical N losses and associated agricultural N inputs. We also used this model to calculate the spatial variation in actual NUEs and the required NUE to align actual crop production with N thresholds. We then assessed the feasibility of achieving the necessary NUE changes through optimizing agricultural N management strategies. It was found that actual N input exceeded critical N input in 95 and 83 % of the agricultural area in Quzhou and Qiyang, respectively. To meet actual crop production without exceeding N loss thresholds, the NUE needs to increase with 11 to 15 % whereas the total N input needs to be reduced by 37 %. NUE gaps can be closed by reducing N rates, enhancing organic manure recycling, and using efficiency-enhancing fertilizers, with optimal combinations being dependent on site conditions.

Optimization of the N footprint model and analysis of nitrogen pollution in irrigation areas: A case study of Ningxia Hui Autonomous Region, China

Water diverted from rivers for irrigation areas often contains large amounts of nitrogen (N), which is frequently overlooked and its role in contributing to N pollution is unknown. To investigate the influence of water diversion on N in different systems within irrigation areas, we developed and optimized the N footprint model, taking into account the N carried by irrigation water diversion and drainage in irrigated areas. This optimized model can serve as a reference for evaluating N pollution in other irrigated areas. By analyzing 29 years (1991-2019) of statistical data from a diverted irrigation area in Ningxia Hui Autonomous Region (Ningxia), China, the study assessed the contribution of water diversion to N in agriculture, animal husbandry, and human domestic activities. The results demonstrated that water diversion and drainage accounted for 10.3% and 13.8% in whole system, of the total N input and output in Ningxia, highlighting the potential N pollution risks associated with these activities. Additionally, the use of fertilizers in the plant subsystem, feed in the animal subsystem, and sanitary sewage in the human subsystem represented the main sources of N pollution in each subsystem. On a temporal scale, the study found that N loss increased year by year before reaching a stable level, indicating that N loss had reached its peak in Ningxia. The correlation analysis suggested that rainfall could regulate N input and output in irrigated areas by showing a negative correlation with water diversion, agricultural water consumption, and N from irrigated areas. Moreover, the study revealed that the amount of N brought by water diverted from rivers for irrigation should be taken into account when calculating the amount of fertilizer N required in the irrigation area.

Food Nitrogen Footprint Increased by 35% on the Third Pole During 1998-2018

The N footprint is considered as an indicator of potential environmental damage from N. Quantitative analysis of N footprint distribution, sources and drivers can help mitigate its negative impacts and promote sustainable N management. In this study, we constructed a city-scale food N footprint (FNF) framework for the Qinghai-Tibet Plateau (QTP) using a N mass balance approach. We quantitatively analyzed the FNF during food production and consumption on the QTP from 1998 to 2018. We used the logarithmic mean Divisa index decomposition method to analyze the driving forces of the FNF, and the decoupling of the FNF. The results showed that the per capita FNF of the QTP increased from 24.92 kg N cap^-1 in 1998 to 27.70 kg N cap^-1 in 2018, and the total FNF increased by 35.11% from 1998 to 2018. The spatial distribution of the FNF was uneven, with N losses from crop production and animal production being the leading contributing source to the FNF (86%). Economic development and urbanization were the main driving forces behind the FNF increase, while N consumption intensity inhibited the growth of the FNF. With the rapid growth of GDP, the FNF in the eastern part of the QTP grew relatively slowly, indicating a gradual decoupling of the FNF from economic development. To reconcile the relationship between socioeconomic drivers and the FNF, it is necessary to focus on coupling relationships between subsystems within the food production and consumption system to promote N recycling.

Aggravation of nitrogen losses driven by agriculture and livestock farming development on the Qinghai-Tibet Plateau

Nitrogen (N) losses from crop-livestock production is a major threat to the environment and human health at regional, national and global scales. A comprehensive understanding of the sources, spatiotemporal distribution and drivers of N losses is of great significance for mitigating its negative impacts and promoting N sustainable management. Here, we used the county-scale N flow model to quantitatively analyze the N losses and their driving forces of crop-livestock production on the Qinghai-Tibet Plateau (QTP). Between 2000 and 2018, the total N losses increased for more than 79% of counties on the QTP. The hotspot areas accounted for over 80% of total N losses, expanding from the east and south to the north and west of the QTP. NH₃ was the main source of atmospheric N losses (over 80%) while the direct discharge of manure was the main source of water N losses. Structural equation modeling (SEM) showed that chemical fertilizer caused the largest driving effect on atmospheric N losses, and the total output value of agriculture and forestry was the main driver of water N losses. Uneven distribution of crop production and livestock contributed to the aggravation of N losses. Over 70% of counties had grater manure N excretion than crops could take up, and large proportion of manure could not be returned to the field. More than 90% of the counties used grater amount of chemical fertilizer N than crops could take up, indicating that livestock manure has not yet fully replaced chemical fertilizer N. The results provide effective guidance and support for N utilization and management of livestock in agricultural and pastoral areas.

Scenarios and sustainability of the economy-nitrogen-resource-environment system using a system dynamic model on the Qinghai-Tibet Plateau

Nitrogen (N) plays a vital role in the development of crop production and animal husbandry in agricultural and pastoral areas. However, the irrational utilization of N resources and subsequent environmental issues with rapid economic development has attracted wide public attention. Coordinating the economy-N-resource-environment (ENRE) system is of great importance for regional sustainable development. In this study, the dynamics of the ENRE system of a typical agricultural and pastoral area on the Qinghai-Tibet Plateau (QTP) were simulated using the VENSIM software from 1998 to 2018. Four typical scenarios (current development scenario, economic development scenario, environment protection scenario and resource optimization scenario) are established to assess the sustainability level and the coupling coordination degrees (CCDs) of the three subsystems, i.e., the economy, N-resource and environment subsystems from 2019 to 2030. Our study indicates that the N flow-based system dynamics (SD) model connects the different subsystems of the ENRE system together well and allows different scenario simulations. From 2019 to 2030, the ENRE system is at a weak sustainability level during the simulation period, and the three subsystems are at slightly unbalanced stages of development in terms of CCD level. The sustainability and CCD levels of the four examined scenarios are as follows: resource optimization scenario > economic development scenario > environment protection scenario >current development scenario, with average values of 0.45, 0.37; 0.42, 0.36; 0.41, 0.35; and 0.39, 0.34, respectively. Under the resource optimization scenario, reducing N inputs to food production and consumption and reducing the planting area of cash crops can effectively improve the N use efficiency of the food chain in the N-resource subsystem (15.34% from 2019 to 2030 on average). Our results provide a reference for promoting sustainable development and formulating policies in agricultural and pastoral regions.

Nitrogen flow in the food production and consumption system within the Yangtze River Delta city cluster: Influences of cropland and urbanization

Intensive anthropogenic activities associated with the food production and consumption system (FPC) drive massive reactive nitrogen inputs to city clusters resulting in serious nitrogen (N) pollution. We conducted a substance flow analysis to examine N flows in the FPC within the Yangtze River Delta city cluster from 2011 to 2019. The total N input and output showed parabolic downward trends, with decreases from 4008.27 to 3472.57 Gg N yr^-1 and 3518.65 to 3061.29 Gg N yr^-1, respectively; chemical fertilizer (54.7%-57.3%) and N loss (87.1%-90.9%) were the primary components of N input and output, respectively. The decreased total N input was related to reductions in chemical fertilizers and livestock numbers. However, a notable increase in N input to the human subsystem was observed, and urbanization was associated with increased N inputs within the human subsystem via higher amounts of food N consumed per capita and proportions of animal-based food N consumed. Total N loss initially increased then decreased; Nantong, Jiaxing, Shanghai, Yancheng, Taizhou, and Yangzhou were the top six cities in N loss intensity. The proportion of cultivated land area, livestock numbers per unit area, and population density were important factors influencing the spatial heterogeneity of N loss intensity. Twenty-six cities were divided into six groups based on their N loss composition, and various N management strategies were proposed. This study highlights the strong influences of cropland and urbanization on N flows within the FPC, which can be used as a reference for N management at a city cluster scale.

A stepwise emission clustering analysis method for analyzing the effects of heavy metal emissions from multiple income groups

Emissions of various heavy metals (i.e. Hg, As, Cd, Cr and Pb) have emerged as one of the most tremendous pressures on the socio-economic and environmental systems. The pressures caused by such heavy metals would be getting intensified if no adequate and timely solution is implemented, especially because their emissions are associated with economic activities (i.e. regional trade and household consumption). In this study, emissions of various heavy metals derived from regional trade as well as rural and urban household consumption are quantified to identify the critical regions and significant household consumption. In order to detail the impacts of household consumption on such emissions, rural and urban household are divided respectively into five and seven groups with hierarchic income from poorest to richest. The Hg, As, Cd, Cr and Pb emissions caused by intermediate commodity consumption are 1172.86, 2607.57, 23.28, 57.49 and 85.16 tons, respectively. Among them, such emissions induced by self-consumption are 662.95, 1539.34, 12.28, 33.10 and 43.49 tons. Shandong, Guangdong and Jiangsu are identified for high self-emissions due to their advanced economy and rare resources. On the contrast, Hebei and Shanxi with abundant resources are the critical regions for the high transfer-out emissions. Moreover, emissions of multiple heavy metals are inequal due to the variations of rural and urban income groups. Emissions caused by R5 are identified through stepwise cluster analysis for its significant difference in compare with other income groups. It is verified that the main difference of emissions in economically developed regions are caused by rural income groups, while urban income groups are the critical reason for the disaggregation of emissions in less developed regions. Policies should be further implemented based on the regional similarity and income-group inequality.

Spatial effects and impact factors of food nitrogen footprint in China based on spatial durbin panel model

Reactive nitrogen (Nr) has far-reaching advantages and disadvantages on human beings. Nitrogen footprint (NF) is a tool to quantify the use of Nr in the environment. Food nitrogen footprint (FNF) accounts for the largest proportion of the total NF, and the differences between provinces in China exist objectively. In order to explore the spatial correlation and socio-economic driving factors of China's FNF, this paper uses N-calculator tool to calculate the FNF of 30 provinces in China from 2000 to 2018, and uses exploratory spatial data to analyze the spatial correlation and changes of provincial FNF, The driving factors and spatial effects of FNF change in the province were analyzed by using spatial Durbin panel model and spatial regression partial differential method. The results showed that: (1) There is a significant and stable positive spatial dependence and heterogeneity in the FNF among provinces; (2) The direct effect factors of promoting the growth of FNF in the province are urban household Engel coefficient, per capita disposable income of rural residents and rural household Engel coefficient. The main factors of restraining the growth of FNF in the province are wastewater discharge per unit GDP and per capita GDP; (3) the spillover effect is mainly manifested as the negative effect of the increase of urban household Engel coefficient on neighboring provinces, and the spillover effect of per capita disposable income of urban residents and nitrogen fertilizer application rate per unit grain yield on the growth of FNF of neighboring provinces is significant. From the policy level, it is necessary to guide healthy and scientific eating habits, reduce the proportion of meat and fish in the diet structure, reduce the nitrogen fertilizer application per unit grain yield, and improve the efficiency of chemical fertilizer utilization. When formulating relevant policies, government departments should give consideration to the cooperation between provincial and regional governments.

Nitrogen cascade in the agriculture-food-environment system of the Yangtze Delta, 1998-2018

The nitrogen (N) cascade in rural areas of Changshu County should be measured and evaluated due to the large increase in anthropogenic disturbances in China's Yangtze Delta. Here, we developed a village-scale N flow model using Changshu County and its towns as a case study. The model included four subsystems and was used to describe the driving forces behind the N cascade from agricultural food production and household consumption to the environment (agriculture-food-environment) system. It was found that from 1998 to 2018 the N input increased from 274.63 to 848.65 kg N ha^-1. The cropland N use efficiency (NUEc) decreased by 10.35%, whereas the livestock feed N use efficiency (NUEa) increased by 51.84%. A relatively lower NUE, with a higher N input, was found in Shajiabang Town, which was attributed to hairy crab farming. Changes in dietary patterns led to the food N cost (FNC) being in the range of 4.59-7.74 kg kg^-1. Over the past two decades, the N losses from the agriculture-food-environment system decreased by 45.40% from 12,436.60 t N yr^-1 (1998). The contribution of the croplands, livestock-breeding, and household consumption to the N losses were 32.44%, 37.78%, and 29.78%, respectively. About 62.83% of the total N losses entered the water environment. Nitrogen emissions from the croplands accounted for 63.21% of the N losses into the atmosphere. Nitrogen oxide (NO_x) emissions accounted for 38.50% of the gas emissions, followed by NH₃ (28.36%) and N₂O (2.81%). The total N losses decreased annually but losses to the water environment increased by 5.10% from 60.16% (1998). The contribution of food production to the total N loss displayed a decreasing trend, while that of food consumption exhibited an increasing trend. Population growth and increased volumes of domestic waste in the Changsu area were the main driving forces for the increased contribution of household food consumption. The significant decline in cropland area and increase in built-up and heavily trafficked areas indicated an overall increase in anthropogenic disturbances, stimulating the N cascade in the Yangtze Delta from 1998 to 2018.

Linking Dietary Patterns to Environmental Degradation: The Spatiotemporal Analysis of Rural Food Nitrogen Footprints in China

Background: China has a large emerging economy that illustrates how dietary patterns can affect food-source nitrogen (N) cycling. The indicator of food nitrogen footprint (NF) reflects the amount of reactive nitrogen (Nr) emissions and impacts of these emissions on the environment. It is a result of food production and consumption to satisfy basic dietary demands of a given population. Different from urban food consumption with improved waste treatment, rural food consumption significantly affects the environment from food production to waste disposal. We therefore, performed a nationwide case study to link dietary patterns to environmental degradation based on rural food NF accounting. Methods: The N-Calculator model was adopted to reveal the spatiotemporal characteristics of food NFs per capita, and regional food NFs related to rural diets in China from 2000 to 2019. Then, food-source Nr emissions to regional environment were quantified based on food NF accounting and relevant inventory of regional Nr emissions. Results: (i) The average annual food NF per-capita in rural regions was lower than that of its national counterpart, but exhibited regional differences, mainly attributed to the dietary role of cereals. (ii) There existed significant spatiotemporal characteristics among regional food NFs that were mainly contributed by plant-derived food consumptions (73%). Sichuan, Henan, Shandong, and Hunan exhibited larger regional food NFs, and Beijing, Shanghai, and Tibet showed a growth in NFs, wherein rural diets were dominated by animal-derived food. (iii) Rural diets affected the environment by the pathways of ammonia and nitrous oxide volatilization processes, as well as Nr loss to water, accounting for a 33, 5, and 62% average of food NFs across regions. (iv) Although current rural dietary patterns suggest reliance on cereal and vegetable consumptions, more animal-derived types of food would be consumed as urbanization continues, especially in developed regions, creating a barrier for further reduction in national food NF. Conclusion: The findings of this study highlight the importance of changing dietary patterns to the human health-environment dilemma. Strategies that include improvements in N recycling rates, adjustments in dietary patterns, and reductions in food wastes could mitigate regional N pollution with rural dietary shifts.

Environmental impacts of animal-based food supply chains with market characteristics

Animal-based food supply chains lead to significant environmental impacts, which can be influenced by production systems, distribution networks and consumption patterns. To develop strategy aimed at reducing the environmental impact of animal-based food supply chains, the common environmental hotspots among different types of food, the role of transport logistics and the consequence of end market need to be better understood. Life cycle assessment was adopted to model three types of animal-based food chains (beef, butter and salmon), with specific technologies, high spatial-resolution logistics and typical consumption patterns for three markets: local, regional (intra-European) and international. The results confirmed that the farm production stage usually had the greatest environmental impact, except when air transport was used for distribution. Potentially, the role of end market also can significantly influence the environmental impacts. To understand more, three improvement options were examined in detail with regard to hotspots for climate change: novel feed ingredients (farm production stage), sustainable aviation fuel (transport and logistics stage) and reduction of wasted food (consumption and end of life stage). Significant reduction was achieved in the salmon system by sustainable aviation fuel (64%) and novel feed (15%). Minimizing food waste drove the greatest reduction in the beef supply chain (23%) and the international butter supply chain can reduce 50% of GHG mission by adopting sustainable aviation fuel. Combined interventions could reduce GHG emission of animal-based food supply chains by 15% to 82%, depending on market, transport and food waste behaviour. The results show that eco-efficiency information of animal-based foods should include the full supply chain. The effective mitigation strategy to achieve the greatest reduction should not only consider the impacts on-farm, but also detail of the downstream impacts, such as food distribution network and consumption patterns.

Nitrogen flows associated with food production and consumption system of Shanghai

The release of reactive nitrogen (N_r) from food production and consumption constitute the primary source of nitrogen pollution. However, nitrogen flows and the driving factors of food chain of Shanghai, China have not been previously studied. Here, we used a substance flow analysis model to analyze the changes in N_r inputs and outputs in agricultural production, livestock and poultry farming, and food consumption related to the Shanghai food chain between 2000 and 2018. The driving forces of N_r inputs, N_r use efficiency, and N_r surpluses/deficits in the food production and consumption system were also investigated. The results indicated that the main sources of N_r input in the food production and consumption system were nitrogen fertilizers, livestock and poultry feed from external sources, and plant-based foods, which accounted for 36.28-59.45% of N_r input in agricultural production, 37.32-76.57% of N_r input in livestock and poultry farming, and 35.38-59.37% of N_r input in food consumption, respectively. The main forms of N_r outputs were surplus nitrogen in the soil, excretal nitrogen from livestock and poultry animals, and excretal nitrogen from humans, which accounted for 38.2-48.89% of N_r output in agricultural production, 36.78-55.18% of N_r output in livestock and poultry farming, and 85.36% of N_r output in food consumption, respectively. From 2000 to 2018, the N_r inputs per unit area from agricultural production decreased at a rate of 20.42% before 2012, and then increased at a rate of 5.72%. Moreover, the N_r use efficiency of agricultural production component of Shanghai was at a low level, only 18.43-27.6%. Cultivation area of crops was the main driving forces of the N_r input to food production and consumption system. These results provide essential data for controlling nitrogen pollution caused by Shanghai food production and consumption, which can serve as a reference for administrative agencies in formulating policies.

The Effect of Religious Dietary Cultures on Food Nitrogen and Phosphorus Footprints: A Case Study of India

The excessive consumption of nitrogen (N) and phosphorus (P), two vital nutrients for living organisms, is associated with negative environmental and health impacts. While food production contributes to a large amount of N and P loss to the environment, very little N and P is consumed as food. Food habits are affected by multiple regulations, including the dietary restrictions and dictates of various religions. In this study, religion-sensitive N-Calculator and P-Calculator approaches were used to determine the impact of religious dietary culture on the food N and P footprints of India in the major religious communities. Using 2013 data, the food N footprint of Hindus, Muslims, Christians, and Buddhists was 10.70, 11.45, 11.47, and 7.39 kg-N capita-1 year-1 (10.82 kg-N capita-1 year-1 was the national average), and the food P footprint was 1.46, 1.58, 1.04. and 1.58 kg-P capita-1 year-1 (1.48 kg-P capita-1 year-1 was the national average). The findings highlight the impact of individual choice on the N and P food footprints, and the importance of encouraging the followers of religion to follow a diet consistent with the food culture of that religion. The results of this study are a clear indication of the requirement for religion-sensitive analyses in the collecting of data pertinent to a particular country for use in making government policies designed to improve the recycling of food waste and the treatment of wastewater.

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.

Transboundary Environmental Footprints of the Urban Food Supply Chain and Mitigation Strategies

Food supply has been the central issue of human development for millennia and has become increasingly critical in an urbanizing world. However, the environmental footprints and associated mitigation strategies of food consumption have rarely been comprehensively characterized at urban or regional scales. Here, we analyze the water, carbon, reactive nitrogen, and phosphorus footprints of food consumption in Chinese urban regions and demonstrate how such information can help to formulate tailored mitigation strategies. The results show that in three of the largest urban regions of China, 44-93% of the four footprints are embodied in transboundary food supply. The size of the footprints and the effectiveness of mitigation measures in food supply chain vary across the environmental footprints and urban regions. However, targeting agriculture and food processing sectors in Hebei, Shandong, and Henan provinces can reduce these footprints by up to 47%. Our findings show that the analysis of the environmental footprints along the transboundary food supply chains could inform individualized and effective mitigation targets and strategies.

Spatio-temporal dynamics of nitrogen use efficiencies in the Chinese food system, 1990-2017

Understanding the influence factors of nitrogen (N) use efficiencies (NUEs) in different stages of the food system at the provincial scale is critical to achieving cleaner food production while ensuring food security. Nevertheless, they are not well understood. Here we comprehensively analyzed NUE and its influence factors at different stages of the provincial food system. The results showed that per unit agricultural land N input increased by 5-92% in 27 provinces, during 1990-2010, resulting in a low NUE for the crop system when N input per unit agricultural land exceeded about 400 kg N ha^-1. This situation has brought some positive changes, as N input decreased by 3-271 kg N ha^-1 in 77% of the provinces in 2017, relative to that of 2010, but 10 provinces were still over 450 kg N ha^-1 in 2017. Animal food production is expected to continue to expand because 35% and 68% of provinces' urban and rural households, respectively, were still below the recommended minimum animal food N consumption recommendation in 2017, posing great challenges for reducing environmental N pollution. An exciting result is that the NUE of the animal system can be improved by increasing the share of animal food contributed by poultry, eggs, milk and fish, to align with the diets recommended by the Chinese Nutrition Society. NUEs of the provincial food systems excluding Inner Mongolia, Xinjiang, Qinghai and Tibet, would increase by 13% if the net imported food N increased by 1 kg capita^-1. Nevertheless, virtual NUE-including N input for imported food in the calculation of NUE-should be considered for accurate comparison of the NUEs of the provincial food systems, especially in highly urbanized areas, while N input for non-food animals should be excluded for accurate evaluation of the NUE in pastoral areas, considering their special production systems and feeding structures.

Transforming nitrogen management of the urban food system in a food-sink city

Nitrogen flows in urban food systems are attracting increasing concern. However, characteristics of nitrogen flow and systematic measures to reduce reactive nitrogen losses in the food systems of consumption-oriented cities in developing countries have not been well understood, especially in a quantitative way. This study empirically investigates the transforming nitrogen flows of an urban food system in a food-sink city in China, with a nitrogen metabolism model. Three types of nitrogen loads transfer are identified: from production to consumption side, between different environmental media, and from areas within to areas beyond the city boundary. By integrating sensitivity analysis into the metabolism model, increases in the sewage treatment rate, the sewage nitrogen removal rate, and the ratio of animal excreta returned to field are found to contribute the most to the water nitrogen load reduction, and reducing food waste at the consumer level is the most influential measure for lowering soil nitrogen loads, under the existing nitrogen flow regime. Additionally, a three-tier template framework is proposed to streamline city strategies (prevention, abatement, recycling, regional cooperation, etc.) for reducing the N loads of urban food systems, providing references for sustainable nutrient management in urban ecosystems.

Driving forces of nitrogen flows and nitrogen use efficiency of food systems in seven Chinese cities, 1990 to 2015

The effects of population growth (PG), dietary changes (DC), native rural-to-urban migration (NM), migration from regions distant from the cities (M), and agricultural patterns and practices (AP) on N use in food systems and the food trade, and on apparent and virtual nitrogen (N) and N use efficiencies (NUE), at the city scale, are not well understood. Here we selected seven Chinese cities as the study subjects, analyzed the food trade effects on apparent and virtual N inputs and NUE, and quantified the relative magnitudes of these factors on N inputs to cities' food systems during 1990-2015, by designing several scenarios. Our results show that food-sink cities are relying more and more on external food and feed, but in 2015 they transferred 33.8-74.9% of their N input for food or feed productions to areas outside their boundaries, and the food trade showed different effects on the virtual N cost of food N consumption. Apparent NUEs of food systems were 33.1-74.9% higher than those calculated from virtual N costs in Beijing, Tianjin, Shanghai, Lanzhou and Xiamen in 2015. But in cities that export large amounts of food and feed-for example, Chongqing and Changchun-apparent NUE was underestimated by 4.0-46.4% relative to virtual NUE. Native PG, DC, NM, M, and AP accounted for 1.2-14.1%, -6.6-30.0%, 0.6-8.2%, -7.7-131.0%, and -43.8-12.8%, respectively, of the increase in virtual N inputs associated with cities' food systems in 2015, compared to 1990. Our study concludes that M, DC, and AP changes should be considered for mitigating N input in these Chinese cities, and virtual N exports induced by the food trade should also be included if the city is a net food exporter. Selective food trade could help improve the NUE of cities' food systems, and virtual NUE should be used as an indicator, rather than apparent NUE.

Is the nitrogen footprint fit for purpose? An assessment of models and proposed uses

The nitrogen footprint has been proposed as an environmental indicator to quantify and highlight how individuals, organizations, or countries contribute to nitrogen pollution. While some footprint indicators have been successful in raising awareness of environmental pressures among the public and policy-makers, they have also attracted criticism from members of the life cycle assessment (LCA) community who find some footprints confusing and misleading as they measure substance and energy flows without considering their environmental impacts. However, there are also legitimate reasons to defend footprints as a useful class of indicators despite their incompatibility with LCA principles. Here, in light of this previous research and debate, we critically assess models and proposed uses for the nitrogen footprint, and explore options for further development. As the nitrogen footprint merely quantifies gross nitrogen emissions irrespective of time, location, and chemical form, it is a crude proxy of environmental and health impacts compared to other, more sophisticated environmental impact indicators. However, developing the nitrogen footprint toward LCA-compatible impact assessment would imply more uncertainty, more complexity, and more work. Furthermore, we emphasize that impact assessment has an unavoidable subjective dimension that should be recognized in any development toward impact assessment. We argue that the nitrogen footprint in its present form is already fit for some purposes, and therefore further development towards impact assessment may be unnecessary or even undesirable. For some uses it seems more important that the footprint has a clear physical meaning. We conclude that the best way forward for the nitrogen footprint depends crucially on what story it is used to tell.

Recent patterns of anthropogenic reactive nitrogen emissions with urbanization in China: Dynamics, major problems, and potential solutions

Anthropogenic emissions of reactive nitrogen (Nr) result in adverse impacts on the ecosystems. Nowhere has that threat been more challenging than in rapidly urbanized China, the world's largest anthropogenic Nr producer. The Nanjing Declaration in 2004 called for global reductions in Nr emissions. To assess China's progress, multisource Nr emissions were evaluated with a quantitative method from 2004 to 2014. The results showed that national Nr emissions had increased with fluctuation over this period, 55-59% of the total Nr emissions were emitted to the atmosphere, and that agricultural production still was the biggest contributor (62-69%). The hotspots were mainly located in the developed and coastal regions that also have high population densities. Urbanization was associated with overall decreases in agricultural Nr emission and increases in industrial and residential Nr emissions. The overall increase in residential Nr emission per capita played a large role in driving the growth of national Nr emission. Continuing urbanization poses a significant challenge to future Nr mitigation for ecosystem sustainability and a range of strategies, covering improvement of N-use efficiency, slowdown of Western China's urbanization, and promotion of low N intensive lifestyle, are proposed that can promote Chinese low-nitrogen development.

Phosphorus footprint in China over the 1961-2050 period: Historical perspective and future prospect

The phosphorus footprint (PF) is a novel concept to analyze human burdens on phosphorus resources. However, research on PF approach is still limited, and current several PF studies include incomplete phosphorus sources and have limited quantitative interpretation about the drivers of PF changes, which can help understand future trends of PF. This study develops a more comprehensive PF model by considering crop, livestock and aquatic food, and non-food goods, which covers the mainly phosphorus containing products consumed by human. The model is applied to quantify China's PF from 1961 to 2014, and the results of the model are also used to analyze the factors driving the PF changes and explored China's PF scenarios for 2050 using an econometric analysis model (STIRPAT). The result shows that China's PF increased over 11-fold, from 0.9 to 10.6 Tg between 1961 and 2014. The PF of livestock food dominated China's PF, accounting for 57% of the total in 1961 and 45% in 2014. The key factors driving the increase in China's PF are the increase in population and urbanization rate, with contributions of 38% and 33%, respectively. We showed that in the baseline scenario, China's PF would increase by 70% during 2014-2050 and cause the depletion of China's phosphate reserves in 2045. However, in the best case scenario, China's PF would decrease by 15% in 2050 compared with that in 2014, and it would have 50% of current phosphate reserve remaining by 2050. Several mitigation measures are then proposed by considering China's realities from both production and consumption perspective, which can provide valuable policy insights to other rapid developing countries to mitigate the P footprint.

From Production to Consumption: A Coupled Human-Environmental Nitrogen Flow Analysis in China

Anthropogenic inputs of reactive nitrogen (Nr) provide sufficient food, energy, and industrial products to meet human demands; however, only a fraction of Nr is consumed as food and nonfood goods, and the rest is lost to the environment and negatively affects ecosystems. High-resolution studies of nitrogen flows are invaluable to increase nitrogen use efficiencies and reduce environmental emissions. In this study, a comprehensive substance flow analysis of nitrogen for China in 2014 is presented. Based on the conceptual framework, which highlights the key roles of human drivers, the analysis of the synthetic ammonia supply and demand balance shows that 75% of ammonia is used for agricultural purposes. Moreover, the life cycle analysis of food nitrogen shows that human food consumption accounts for approximately 7% of the total Nr inputs. A quantitative analysis of pollutant emissions shows that industrial and crop production are the main sources of atmospheric emissions, while livestock farming and crop production are the main sources of water emissions. Finally, we investigate four scenarios (efficiency improvement, high recycling rate, nitrogen oxide emission reduction, and a combined scenario) and provide relevant policy recommendations (large farm size, standardized agricultural production model, flue gas denitration, etc.) for improving nitrogen management practices.

Driving forces and impacts of food system nitrogen flows in China, 1990 to 2012

Food nitrogen (N), which includes animal-food (AN) and plant-food N (PN), has been driven by population growth (PG), dietary changes associated with income growth (DC) and rural-urban migration (M) over the past three decades, and these changes combined with their N cost, have caused some effect on N use in China's food system. Although there is an increasing literature on food N and its environmental impacts in China, the relative magnitude of these driving forces are not well understood. Here we first quantify the differences in per capita AN and PN consumption in urban and rural areas and their impacts on N input to the food system during 1990-2012, and then quantify the relative contributions of DC, PG and M in the overall N change during this period. Our results show that a resident registered as living in city required 0.5kg more ANyr^-1 and 0.5kg less PNyr^-1 than one living in a rural area, in 2012. DC, PG and M accounted for 52%, 31% and 17% of the total AN increase, respectively. These three factors caused 46% of the increased N use for food production over the past two decades. Another 54% was mainly caused by the declining in N use efficiencies of the food system. Food-sourced N loss intensity in urban and rural areas were 502 and 162kgNhm^-2 in 2012, a three-fold difference due to the increasing amount and a linear rural-urban flow of N input, and inadequate N recovery via solid waste and wastewater treatment in cities. Our study highlights China is facing higher risks of environmental N pollution with urbanization, because of the high demand for AN and higher food-sourced N loss intensity in urban than in rural areas.

Reactive nitrogen losses from China's food system for the shared socioeconomic pathways (SSPs)

Food production in China has been changing fast as a result of socio-economic development. This resulted in an increased use of nitrogen (N) in food production, and also to increased reactive nitrogen (Nr) losses to the environment, causing nitrogen pollution. Our study is the first to quantify future Nr losses from China's food system for the Shared Socio-economic Pathways (SSPs). We show that Nr losses differ largely among SSPs. We first qualitatively described the five SSP storylines for China with a focus on food production and consumption. Next, we interpreted these SSP scenarios quantitatively for 2030 and 2050, using the NUFER (NUtrient Flows in Food chains, Environment and Resources use) model to project the Nr losses from China's food system. The results indicate that Nr losses from future food system in China are relatively low for SSP1 and SSP2, and relatively high for SSP3 and SSP4. In SSP5 Nr losses from China's food system are projected to be slightly lower than the level of today.