Growth in the global N₂ sink attributed to N fertilizer inputs over 1860 to 2000

Cropland expansion and fertilizer applications are among the most important substantial effects of human actions on the global nitrogen (N) cycle. However, questions remain over the fate of anthropogenic N inputs, particularly whether a significant fraction of N-based fertilizers have been lost to inert N₂ or reactive N forms. Here, we combine natural N isotope constraints on the pre-industrial N cycle with global mass-balance modeling to investigate the role of cropland conversion on gaseous N emissions and hydrological N leaching fluxes. We estimate that cropland expansion has been accompanied by > 9-fold increase in N input rates to cropping systems, roughly doubling the baseline N budget of the terrestrial biosphere. As a consequence, approximately 10 times more N is exported from modern croplands to the hydrosphere than in 1860, with a five-fold increase in cropland N gases emission to the atmosphere. Atmospheric NH₃, NO, N₂O and N₂ fluxes increased from 8.6, 16.6, 11.7 and 31.9 Tg N yr⁻¹, respectively, in 1860 to 17.7, 23.6, 15.2 and 39.7 Tg N yr⁻¹, respectively, by 2000. Thus, the growth in N₂ accounted for ~ 20% of cropland-driven N losses (dissolved plus gaseous pathways), with the remaining 80% exported as reactive N forms. Although the increase in N₂ emissions has mitigated some of the unwanted side-effects of N fertilizer applications on human health, the economy, and climate change, this inert sink has been unable to keep pace with the increase in N inputs for enhanced food production. Our results imply that, unless new management steps are taken, an increasing fraction of N fertilizers will mobilize to reactive N forms in the global land, air and water systems, thus further accelerating the negative consequences of human modifications of the N cycle this century. - Highlights: • A stable isotope model was used to constrain the soil gaseous N losses. • Soil N₂ emission from natural and cropland ecosystem was investigated. • Terrestrial soil N budget increased two-fold from 1860 to 2000. • Cropland expansion increased the soil reactive N (N₂O, NO_x, NH₃ and leaching) losses.