Schulte-Uebbing and colleagues have developed a spatially explicit model to establish regional boundaries for agricultural nitrogen surplus based on thresholds for eutrophication of terrestrial and aquatic ecosystems and nitrate in groundwater. The authors find that half of all agricultural land is located in areas where non-agricultural nitrogen losses alone exceed at least one of the three thresholds for deposition levels, surface water quality and groundwater quality. Nitrogen exceedances are most severe in northwestern Europe (especially in Germany and Benelux), India, Pakistan and eastern China. However, nitrogen thresholds have not been exceeded for agricultural land in sub-Saharan Africa, Central and South America, and southeast Asia, meaning these regions could increase nitrogen use in food production — allowing the planetary nitrogen boundary to increase from 43 megatonnes to 57 megatonnes of nitrogen per year.
Previous assessments of planetary nitrogen boundaries focused only on the agricultural sector, while Schulte-Uebbing and colleagues’ approach accounted for non-agricultural sources, finding that nitrogen discharge from sewage run-off from natural land must also be accounted for in impact assessments. Scenario analyses indicated that reducing non-agricultural nitrogen losses proportionally with agricultural losses, improving nitrogen-use efficiencies and adopting balanced diets could allow for global crop demand to be met while remaining within nitrogen boundaries. Schulte-Uebbing and colleagues call for coordinated action and targeted strategies that recognize multiple environmental impacts — and the regional diversity of agricultural systems.
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