Humanity's transformation of the nitrogen cycle has major consequences for ecosystems, climate and human health, making it one of the key environmental issues of our time. Understanding how trends could evolve over the course of the 21^st century is crucial for scientists and decision-makers from local to global scales. Scenario analysis is the primary tool for doing so, and has been applied across all major environmental issues, including nitrogen pollution. However, to date most scenario efforts addressing nitrogen flows have either taken a narrow approach, focusing on a singular impact or sector, or have not been integrated within a broader scenario framework – a missed opportunity given the multiple environmental and socio-economic impacts that nitrogen pollution exacerbates. Capitalizing on our expanding knowledge of nitrogen flows, this study introduces a framework for new nitrogen-focused narratives based on the widely used Shared Socioeconomic Pathways that include all the major nitrogen-polluting sectors (agriculture, industry, transport and wastewater). These new narratives are the first to integrate the influence of climate and other environmental pollution control policies, while also incorporating explicit nitrogen-control measures. The next step is for them to be used as model inputs to evaluate the impact of different nitrogen production, consumption and loss trajectories, and thus advance understanding of how to address environmental impacts while simultaneously meeting key development goals. This effort is an important step in assessing how humanity can return to the planetary boundary of this essential element over the coming century.

人类对氮循环的转变对生态系统、气候和人类健康产生重大影响，使其成为当今时代的关键环境问题之一。^{了解 21世纪}的趋势如何演变对于从地方到全球范围内的科学家和决策者来说至关重要。情景分析是实现这一目标的主要工具，并已应用于所有主要环境问题，包括氮污染。然而，迄今为止，大多数解决氮流情景的努力要么采取了狭隘的方法，专注于单一影响或部门，要么没有被整合到更广泛的情景框架中——鉴于氮流对环境和社会经济的多重影响，这是一个错失的机会。污染加剧。利用我们不断扩大的氮流知识，本研究引入了一个以氮为中心的新叙述框架，该框架基于广泛使用的共享社会经济路径，包括所有主要氮污染部门（农业、工业、运输和废水）。这些新的叙述首次整合了气候和其他环境污染控制政策的影响，同时还纳入了明确的氮控制措施。下一步是将它们用作模型输入，以评估不同氮生产、消耗和损失轨迹的影响，从而加深对如何在满足关键发展目标的同时解决环境影响的理解。这项努力是评估人类如何在下个世纪重返这一基本要素的地球边界的重要一步。