Fluxes and imbalances in the modern global nitrogen cycle

Abstract

Nitrogen (N) limits primary productivity in much of the world. Since the 1940s, synthetic fertilizers produced via the conversion of unreactive atmospheric dinitrogen (N2) into reactive N have supported agricultural production but also perturbed natural nutrient cycles. In this Review, we compile estimates of global N cycling since 1955 and explore changes in the global N balance. Since the 1960s, the annual amount of reactive N entering the biosphere increased fivefold. Meanwhile, N emissions from the biosphere to the atmosphere increased just 12% between the oldest and latest estimates, indicating terrestrial N accumulation on the order of ~50 Tg N yr−1 between 1959 and 2025. The increased N availability due to these inputs has polluted soils, waters and air, negatively affecting ecosystems and economies. Quantifying N pools and fluxes across the atmospheric, terrestrial and aquatic reservoirs is critical to monitor and mitigate this imbalance in the N cycle. Reducing fertilizer demand and use through dietary changes, technological advances to enhance fertilizer efficiencies, and improvements to waste management and nutrient recycling could support a net flux of 51 Tg N yr−1 to the atmosphere, which could help address the existing imbalance in the global N cycle. Synthetic fertilizer use has perturbed the global nitrogen cycle by increasing reactive nitrogen inputs to terrestrial systems. This Review synthesizes estimates of global nitrogen pools and fluxes since 1955, assessing flux imbalances and identifying research and policy priorities to rebalance the global nitrogen cycle.