Nitrogen (N) is one of life’s fundamental building blocks – a core ingredient in amino acids and proteins – and essential for agrifood systems. With the invention of the Haber–Bosch process, humans have converted unreactive N2 to reactive forms of N that can be used as mineral fertilizer, which has significantly contributed to increased crop production and yields to feed a growing world population. This technological breakthrough has altered the global N cycle, resulting in excess N release to the envi- ronment, negatively impacting air and water quality, human health and biodiversity. The anthropogenic release of N impacts both terrestrial and aquatic ecosystems, as altered N flows have profound effects on natural ecosystem structure, function and services upon which humanity depends. Nitrogen losses occur through emissions of ammonia (NH3) and nitrogen oxides (NOx), which lead to air pollution, and nitrous oxide (N2O), a potent greenhouse gas (GHG), which contributes to climate change. Additionally, N can be lost through nitrate (NO3-) leaching in soil and water bodies. This causes eutrophication and acidification, ultimately harming terrestrial and aquatic ecosystems, contributing to biodiversity loss, and affecting the provision of clean air and water. As a result, the substantial losses of anthropogenic N pose a risk to human health and contribute to the triple planetary crisis of climate change, pollution and biodiversity loss.
Agrifood systems play a significant role in the alteration of N dynamics. Nitrogen is an essential building block for crop and livestock production. While some plants, such as legumes, can access atmospheric N through biological N fixation, most oth- ers depend on N availability in soils. Synthetic N fertilizer has complemented natural processes and significantly increased crop yields. The over-application of synthetic N fertilizer has resulted in substantial losses of N through the leaching of nitrates when the absorption capacity of soils is surpassed. Additionally, with the rising demand for livestock products, the livestock sector has undergone significant changes, transitioning from traditional and small-scale to intensive production systems in which large amounts of concentrated feeds are used. Production of this feed is partly linked to deforestation and heavy synthetic fertilizer use, causing gaseous N emissions and NO3- leaching.
As feed production is typically decoupled from livestock-dense production areas, high concentrations of manure are accumulated in the latter areas. Significant emissions occur in livestock-dense housing systems where manure is accumulated and stored for prolonged periods, causing emissions of NH3, N2O and NOx. The livestock sector is the main contributor of N losses by agriculture and represents about one-third of total N emissions from anthropogenic activities.
Conversely, many low- and middle-income countries still face opposite challenges, as N fertilizers are more difficult to access. Here, soil health degradation occurs when crops are harvested without compensating for the harvested N through the application of organic (such as manure, compost and crop residues) or synthetic fertilizer, which in turn results in crop yields well below their potential. Additionally, manure is often not collected or correctly handled, resulting in emissions of N and loss of nutrients that could otherwise be returned to the agricultural system.
Because of its significant role in environmental N pollution, the agricultural supply chain must implement sustainable N management practices to minimize N losses and increase balanced N cycling within the agrifood system. Sustainable N management is defined as management practices that seek to minimize external N inputs and losses and increase recycling of N within the production system. Increasing N use efficiency (NUE) can contribute to sustainable N management. Nitrogen use efficiency is the ratio of N xi
recovered in the final output to the total N used as input. Increasing NUE aims to recover as much as possible of the N input as possible in the final product, thereby minimizing the amount of N lost in the production process. Improved fertilization strategies contribute to improving NUE and sustainable N management in cropping systems. In livestock production, strategies at the farm level to increase NUE should focus on minimizing N excretion through manure. When feeding high-protein diets, a significant amount of N is excreted via manure (faeces and urine). Improved feeding strategies, including low-protein feed, can decrease manure N excretion and associated N losses. Through improved manure handling and storage and an adequate use of manure N in crop production, overall NUE can be increased substantially. Reducing N losses from manure can be achieved through innovative livestock housing systems, improved storage, and low-emission application of manure to cropland. Beyond farm-level measures, crop–livestock integration and recoupling of livestock to local feed production can enhance sustainable N management on a regional scale. In general, livestock decrease the overall NUE of the food production system compared with plant-based production systems, as they add additional steps in the process where N can be lost to the environment. Integrating livestock systems with crop production systems improves NUE of the system as a whole, thereby contributing to an increase in sustainable resource use.
Improving resource use efficiency, including NUE, can be achieved by the adoption of circular bioeconomy principles. The circular bioeconomy aims to provide sustainable solutions in the production, utilization, conservation and regeneration of biological resources within and across all economic sectors to enable a transformation to a more sustainable economy. Within agricultural production systems, circularity principles are proposed to improve resource use efficiency and NUE and can contribute to sustainable N manage- ment as they aim to maximize the efficiency with which food is produced and utilized. The main principles of a circular agrifood system are to reduce food losses and waste, recycle inevitable food losses and waste back into the agrifood production chain, use arable land primarily for direct human consumption to maximize resource use efficiency and available food, and use livestock to convert biomass and waste streams unsuitable for human consumption. Across these principles, many solutions are present to increase the recycling of N and increase NUE of the agrifood system. In recognition of the importance of these processes, countries can now use NUE as one of the indicators of the productivity and sustainability of their agrifood systems when reporting on Sustainable Development Goal (SDG) Indicator 2.4.1.
Nitrogen management policies in agrifood systems present disparities across differ- ent regions. Policies often prioritize food security and productivity gains, leading to high N inputs and low NUE. For instance, Asia’s Green Revolution saw significant crop yield increases as a result of fertilizer subsidies. These policies have led to massive environ- mental pollution from the overuse of synthetic fertilizers. In response, Asian countries have implemented reforms to reduce N fertilizer use and improve NUE. Africa has chal- lenges related to low crop yields and soil nutrient depletion due to inadequate policies, low-fertility soils, and limited access to affordable synthetic fertilizers. The European Union and North America have achieved higher NUE through nutrient management guidelines and environmental regulations. Conversely, Latin America and the Caribbean face challenges due to heavy reliance on imported fertilizers, which are affected by fluctuating prices and disruptions of supply chains.
Adoption of agricultural technologies and financial mechanisms, such as crop insur- ance and joint responsibilities among agrifood chain stakeholders to decrease N loss and share abatement costs are needed. Countries are encouraged to adopt policies that promote sustainable N management and address other environmental challenges, such as climate change, water use and biodiversity loss.
Sustainable N management is crucial for achieving the SDGs by 2030, particularly those related to ending hunger (SDG 2), health (SDG 3), clean water (SDG 6), sustainable xii
production and consumption (SDG 12), climate action (SDG 13), and preserving life under- water (SDG 14) and on land (SDG 15). In developing countries, improving NUE can improve soil health and fertility, increase crop production and yields, and increase food production. Furthermore, improving NUE can contribute to improved human and environmental health by reducing harmful emissions and protecting water bodies from pollution. For these efforts to be successful, policies need to reconcile the dual role of N as an important nutrient necessary for economic growth, human advancement and food security and as a pollutant that causes serious ecosystem damage.
Key actions and policy options to promote sustainable nitrogen management should focus on the following.
• Improve nitrogen management in crop production through promoting the use of biological N fixation in locally suitable crop rotations and encouraging the use of manure as organic fertilizer. Low- and middle-income countries should enhance access to synthetic fertilizers and promote agroecological practices, while the fertilizer industry should take urgent action to cut GHG emissions during the production of synthetic fertilizers.
• Improve nitrogen management in the livestock sector through developing guide- lines to adopt best practices in manure management and processing techniques, enhance spatial integration of crop and livestock production, and implement circu- lar bioeconomy principles at the landscape level. Livestock farmers should improve feed formulation to optimize protein intake and improve feed use efficiency. Agrifood system policies should focus on improving spatial planning and reducing livestock numbers in areas with high geographical concentration to ensure crop and livestock systems are balanced and integrated.
• Reduce nitrogen loss and waste. Countries should bolster efforts to reduce food loss and waste throughout the agrifood production chain and promote recycling and treatment of food unsuitable for human consumption as livestock feed.
• Promote public and private investment. National governments, the private sector, international financial institutions, and local agricultural banks should mainstream sustainable N management into development projects and programmes in agri- food systems and promote investment in high-efficiency, low-emission mineral fertilizers and production of organic residues to enhance system efficiency and reduce waste of resources and environmental pollution. Agrifood system stake- holders should promote investment in agroecology and sustainable crop–livestock integrated development projects to enhance sustainable N management.
• Capacity building at scale. Countries and international development partners should support national capacity building on sustainable N management among different agrifood system stakeholders, including the public, private sector, civil society organizations, farmers, and producer organizations; strengthen national extension services, research, and knowledge transfer; and promote sustainable N management practices through farmer field schools, and low-N footprint diets.
• Policy options. Countries should promote the integration of sustainable N management in nationally appropriate mitigation actions and nationally determined contributions, including targets to reduce N2O from agrifood systems to keep the Paris Agreement goal of 1.5 °C in sight. Countries should set national commit-
ments to reduce N pollution, including NH3 and NOx emissions to air and NO3- losses to water, in line with Target 7 of the Kunming-Montreal Global Biodiversity Framework and SDGs 6, 12, 13, 14, 15 and 17. Finally, countries should address consumption patterns and promote healthy diets with low environmental impact.