Revisiting safe and just planetary boundaries for nitrogen and phosphorus

Over the past 15 years, the planetary boundaries (PB) framework has advanced as a scientific approach to define limits for Earth’s system processes, including boundaries for nitrogen (N) and phosphorus (P). These boundaries have been conceptualized and quantified using various approaches, leading to varying estimates and interpretations. In this paper, we review existing control variables and methods used to define and quantify ‘safe’ N and P boundaries for environmental impacts and ‘just’ N and P boundaries for food production. Based on our analysis, we propose that ‘N losses’ (N delivery to surface water, N leaching to groundwater, and NH 3 emissions to air) and ‘P delivery to surface water’ are the most suitable control variables for defining ‘safe’ boundaries for biogeochemical flows of these elements. We present quantitative estimates for both ‘safe’ planetary N and P boundaries, for N based on previous studies and for P based on new calculations. We then assess the contribution of the food system (including agricultural production, aquaculture, and human wastewater streams) to N and P losses. We propose to define ‘just’ boundaries for N and P based on the required input of these nutrients to meet the dietary needs of the population. We indicate possible values for such ‘just’ N and P boundaries and provide options to attain those boundaries while staying with ‘safe’ N and P boundaries.