B Five future scenarios for TN required to meet the hunger eradication target. A total of countries were included in the analysis, including 69 low-income countries with a food deficit see Supporting Information. Our baseline scenario, even with its optimistic hypothesis about diet, therefore implies a continuous and large increase of TN in the future as a consequence from meeting the global hunger alleviation target through an increase of food production in LIFD countries.
This large increase of TN is likely to have very detrimental environmental and climatic impacts. We therefore investigated possible pathways to reduce TN while alleviating hunger by constructing four alternative scenarios. The efficiency improvement scenario assumes that the PTN could be reduced everywhere to reach the current average of European countries by year This implies that less reactive N will be used to produce a unit of crop biomass dry matter so that the global nitrogen use efficiency of agriculture will be greatly improved.
Finally, the combined scenario combines the storylines of the previous three scenarios, assumed to be additive. The evolution of TN for each scenario Fig. The combination of food waste reduction and improved nitrogen-use efficiency thus represents the most effective strategy to prevent a large future growth of TN.
The healthy diet scenario only slightly reduces TN compared with the baseline case, in both and , which is not surprising because a healthy diet was already part of the baseline scenario storyline. Joint implementation of waste reduction , efficiency improvement and healthy diet policies has the benefit of meeting hunger alleviation targets while keeping TN slightly below the level.
The estimate of Bouwman et al. Leach et al.
Hence, their definition of nitrogen footprint is equivalent to the total amount of N used, which is similar to our TN definition. Yet, one difference is that Leach et al. According to Leach et al. The Dutch per capita nitrogen footprint calculated by Leach et al. Our result shows that losses dominate the TN of food production.
Such a finding is in line and consistent with some existing literature. The only well known report on food waste from The Food and Agricultural Organization of the United Nations FAO also indicated that around one-third of food produced for human consumption is lost or wasted globally Bouwman et al. The estimates from Bouwman et al. Our scenarios are based on coincident N-reduction measures in food consumption and production, whereas Bouwman et al.
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Our baseline scenario projection for global TN is consistent with the independent estimate of reactive N inputs to the world agriculture from Bouwman et al. Erisman et al. Both our study and Bodirsky et al. In comparison with previous studies, we present the first attempt to integrate global nitrogen footprints from the perspective of food production, consumption and trade Table 1.
Most previous global N studies focused on nitrogen for food production e. Smil 27 , Galloway et al. Bodirsky et al. Galloway et al. As for food consumption, Bodirsky et al. As for food trade, Galloway 6 assessed N in traded fertilizer, grain and meat products, while we assessed N embedded in the trade commodities and N used to produce these commodities in exporting countries, a term that is similar to virtual water flows Another novelty is that this study quantitatively demonstrates the importance of reducing food losses and waste in mitigating TN.
Reducing food waste is recommended as one of the key actions to produce more food with less pollution However, quantitative estimates for N mitigation potential of reduction of food losses and waste remain extremely rare, except for ref. Our main result is that reducing food losses and waste could help significantly reduce nitrogen footprint. There are several possible improvements of our methodology. First, We did not account for vegetables in the TN of crop production.
Vegetables have the highest TN after meat due to the small fraction of harvested plant biomass, high N input rates especially in developing countries and high waste fraction. There is a sharp increase in vegetable demand in China due to population growth, and increasing N inputs is one important way to enhance vegetable production.
Hence, omission of vegetables will not lead to large systematic errors in TN of crop production. However, there is no information about country-specific healthy calorie intake requirements. Several other authors also used such a level as an indicator for food security, e.
Kummu et al. Nevertheless, access to country- even local-specific caloric values will produce more reliable results. We also acknowledge the fact that our estimates are based on the level of calorific values, while protein contents were not considered. This is a shortcoming of most available literature, which estimated natural resources requirements e. Future research should put more efforts to investigate the relations between human diets and protein intakes. Human diets that are lacking of meat generally require more high-protein plants e. For example, on average, it takes Our results highlight the central role of combining different policies in order to jointly achieve hunger alleviation and mitigate the increase of TN of food consumption.
Reducing food waste within supply chains is a problem that has defied solution and that threatens agricultural and environmental sustainability 1 , 20 , 33 , There has been little progress towards the food loss reduction target established by FAO in Many obstacles make this target difficult to tackle, including a lack of monitoring programs in most countries 40 and a lack of governance and institutions The ratio of applied nitrogen fertilizer to per unit of crop yield has decreased in Japan 42 , and the USA 43 , Reducing PTN and improving nitrogen use efficiency need a strict nitrogen policy from governments 42 , 43 , and improved nitrogen management practice by farmers 45 , However, regions differ in nitrogen challenges and policies: sub-Saharan Africa and Latin America have low nitrogen inputs in agricultural fields largely due to poor transport, market infrastructure and poor ratio of yield increase to fertilizer cost; many countries there emphasized the need for fertilizer subsidies to ensure food production Policies are needed not only to encourage more nitrogen use, but also to invest in transport, market, and targeted research, as well as to expand irrigation and improve other agronomic factors.
In contrast, regions e. Europe and China have faced pollution problems largely because they use too much nitrogen. These regions have started to lower the fertilizer subsidies and emphasized the reduction of nutrient losses and pollution, but the effectiveness of nitrogen policies may be challenged by insufficient investment and high costs of wastewater treatment 33 , In these regions, there is a need to shift toward a new sustainable agriculture paradigm based on precision farming and sustainable soil management.
For both the nitrogen-poor and nitrogen-rich regions, future efforts need to not only demonstrate best nitrogen practices in the field, but also emphasize long-term dialogue, education and training among all actors in the whole supply chain of nitrogen pathways Food trade partly allows maintaining food security in regions where domestic production is insufficient. The consequence of economics driven trade choices on PTN are not taken into consideration, even though they also have environmental impacts and economic costs for producing countries.
In some cases, importing a specific crop product comes with a lower PTN than if the same product was produced domestically e. Mitigation actions, such as reducing food waste, and improving nitrogen use efficiency, will not only reduce local but also global environmental pollutions. The positive externalities from these active actions cannot be fully internalized under uncoordinated local policies 31 ; hence, inter-governmental effort 33 and global collaboration 31 will be required to prevent a further growth TN that would cause high damage to the environment and economic costs.
The total nitrogen input TN of crop production is the sum of the inputs of all nitrogen types t associated with crop production for all crop products c. Per capita TN is calculated by dividing TN by the population within the study area. The 2. Six types of N use were accounted for in calculating TN for the production of each crop product: mineral nitrogen fertilizer TN 1, c ; manure TN 2, c ; wet and dry atmospheric deposition TN 3, c ; biological nitrogen fixation TN 4, c ; nitrogen inputs from water and from soil particles deposited by the wind or by flowing water TN 5, c , and nitrogen input from recycled crop residues TN 6, c.
Estimates of the use of each nitrogen type were obtained from Liu et al. PTN is the amount of nitrogen used to produce one unit of crop product. For crop c , it can be calculated as the ratio of TN to crop production P :. We calculated the average PTN for a group of crops e. We calculated the average PTN of a specific crop within a geographically delineated area as the ratio of the total TN to the total production of this crop within the geographic area. The spatial distribution of crop production in was obtained from the Spatial Production Allocation Model SPAM , which has a spatial resolution of 5 arc-minutes 47 , SPAM produced reliable spatially explicit crop production data, even for many developing countries e.
The N requirement per unit of calorie supply is the amount of N of all types that is used to produce one unit of caloric energy. We calculated the aggregate N requirement per unit of calorie supply for a group of crops by dividing the total TN of all the crops by their total caloric energy. The TN of the consumption of a crop c is calculated by multiplying the PTN of this crop by the food intake from this crop. The total caloric energy intake from a crop c is calculated by multiplying the weight of food intake from crop type c by the caloric content of crop c.
The data on the weight of food intake and caloric content of various crops are obtained from the Food and Agriculture Organization FAO We calculated the mean N requirement per unit of calorie supply of plant products n p with the equation below:. We estimated that an average of This value may represent a conservative estimate because we did not include vegetable oils in the calculation due to a lack of data. Vegetable oils account for This translates into 48 Tg N that is used to produce animal feed each year. FAO reports that animal products provided 1. On average, it thus takes In our analysis, we considered 1 N accumulated in the harvest crop i.
Estimates of each flux were obtained from Liu et al. After a crop is harvested, part of the crop is lost through the processes of storage, processing, and transportation before it can reach humans for consumption. We also calculated N in such losses from the N that is accumulated in the crop yield.
From a production perspective, we performed this calculation for all the crops based on the assumption that food loss rate l was constant for crops within the same crop category e. Country- and crop-specific l values were obtained from Gustavsson et al. Thus far, Gustavsson et al. When our calculations were conducted from a production perspective, we did not consider food waste during the consumption process. But when we conducted such calculations from a consumption perspective, the food waste in the consumption process was taken into account.
Instead, we calculated TN based on dietary caloric intakes for plant and animal products. We used the l value of cereals to represent plant products, and that of meat to represent animal products. Country- and crop-specific trade data were obtained from FAO 1. D p and D a represent the dietary energy intakes of plant products and animal products per country, respectively. We estimated the future values based on the scenario analysis. We obtained data on historical and present national dietary energy supply per capita both plant and animal calorie supply from the FAO food balance sheets Population figures in each country from to were also obtained from FAO The future TN is compared with the result of the base year We analyzed the hunger eradication targets for the FAO countries in two years: and This means that the proportion of undernourished people would decrease by half between and Shifts of food consumption patterns and rising population are two key driving forces for food production.
This change was enormous for countries with the fastest growth of income and the fastest urbanization rate Hence, in our analysis, we considered the dietary calories from both animal products and plant products. Adequately nourished people would at least reach and possibly exceed both the grand calorie target and the animal calorie target, and the caloric intakes should be no less than the level of For , countries with a grand calorie supply in higher than the grand calorie target will remain at that level.
For countries that have not achieved the grand calorie target, we assumed that they would reach this target in and that the calorie supply from animal products would be no less than the animal calorie target or the level of The proportion of undernourished people in was based on available data for each country for the period from to The nitrogen requirements to produce each unit of dietary calorie of plant or animal products were calculated in this study, as described above.
In addition to the baseline scenario, we also set up four scenarios to study the potential for the reduction of TN:. In this scenario, we assumed that all the population with food security would have the balanced diet i. For the malnourished population, we assumed that their dietary energy intakes would remain the same as in the baseline scenario i. Such a consumption pattern means that currently affluent people would have to reduce their consumption, and particularly animal product consumption.
We used this commitment in our scenario, and assumed that progress would be made at a roughly equal rate each year. In S2, we assumed that this reduction task could be fully achieved in each country.
The food loss rates in were based on an FAO report 19 that summarized the rates for seven commodities for five phases of the supply chain in eight global regions. In this estimate, the food loss rates of the LIFD countries for plant and animal products were calculated by weighting the total food waste in all five supply chains for cereals, roots and tubers, oilseed and pulses, fruits and vegetables, and for meat, fish and seafood, and milk estimated by FAO based on the total food caloric supply of each food category in sub-Saharan Africa, North Africa, West and Central Asia, South and Southeast Asia, and Latin America from the FAOSTAT data 1.
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PTN varies significantly among countries and continents. Here, we assumed that PTN in all countries would be reduced to the European level by at a linear rate over time. This assumes that the efficiency of N use will improve and that the N requirements per unit of calorie supply will consequently decrease. The results are shown in Table S3. How to cite this article : Liu, J. Reducing human nitrogen use for food production.
Mueller, N. Closing yield gaps through nutrient and water management. Nature , — Bouwman, A. Global and regional surface nitrogen balances in intensive agricultural production systems for the period — Pedosphere 15 , — Liu, J. A high-resolution assessment on global nitrogen flows in cropland. USA , — Smil, V. Nitrogen and food production: proteins for human diets. Ambio 31 , — Schlesinger, W. On the fate of anthropogenic nitrogen. Galloway, J. Transformation of the nitrogen cycle: Recent trends, questions, and potential solutions.
Science , — Sutton, M. Too much of a good thing. A safe operating space for humanity. Planetary boundaries: Exploring the safe operating space for humanity. Cirera, X. Income distribution trends and future food demand. B , — Godfray, H. Food security: The challenge of feeding 9 billion people.
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Vitousek, P. Tilman, D. Forecasting agriculturally driven global environmental change. De Willigen, P. Stehfest, E. Modelling of global crop production and resulting N 2 O emissions. Estimation of global NH 3 volatilization loss from synthetic fertilizers and animal manure applied to arable lands and grasslands. Cycles 16 , 8—1 Ito, A. Simulated impacts of climate and land-cover change on soil erosion and implication for the carbon cycle, to Gustavsson, J. Global food losses and food waste - Extent, causes and prevention.
FAO, Rome, Additionally, the presentation of the indicator can identify minimum nitrogen use levels that denote minimum food production thresholds. Food systems, such as many smallholder farmers in Africa, that use too little nitrogen would therefore be encouraged to increase nitrogen use. Finally, the graphs can specify the acceptable nitrogen balance surplus for each food system. Such a graph is illustrated schematically below Figure 7.
All values are purely indicative and for illustration purposes only. The example only serves to illustrate the interpretation of the proposed indicator. Targets for crop nitrogen use efficiency are context-specific, primarily depending on climate, yield, current nitrogen use, soil quality, irrigation, and other crop management practices. This indicator needs to be interpreted in relation to other indicators, such as the crop yield gap indicator and the water productivity indicator.
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A possible target range for this indicator would require careful consideration. Tracking nitrogen will require major improvements of the necessary data collection systems in two ways: i annual nutrient use and crop removal statistics at sub-national level and by crops fertilizers and other nutrient sources and ii regular field monitoring of nitrogen use efficiency and other nutrient-related indicators e.
Currently this indicator is not used widely. Food production systems are extremely diverse and context specific. Therefore it is important that nitrogen indicators can be tracked at different geographic scales local, national, global as well as by farming systems e. Nitrogen use efficiency can be estimated at different scales. This indicator tracks only nitrogen use and is complemented by a national indicator for phosphorus.