Agricultural Fertilization Drives Increased Seasonal Amplitude of CO2 Exchange

Abstract:

The terrestrial carbon sink is growing with anthropogenic emissions and increasing the amplitude of the annual CO₂ cycle by up to 50% in northern latitudes over the past 50 years. While previous work highlights the important role of increasing CO₂ concentrations and warmer temperatures, many studies cannot fully explain the factors contributing to the increased amplitude. Here, we use CESM2, which simulates CO₂ amplitude at Barrow more accurately than older model versions, to explore the extent to which CO₂ concentrations, climate change, land use change, agricultural N fertilization and irrigation contribute to changes in the annual CO₂ cycle in the Northern Hemisphere. Our analysis highlights that industrial nitrogen (N) fertilization has a disproportionately large influence on the increasing CO₂ amplitude in northern latitudes, increasing the CO₂ amplitude in the northern hemisphere during 1995-2015 by 3 ppm on average and up to 9 ppm in some regions. Higher CO₂ concentrations and warmer temperatures also increase the amplitude of carbon fluxes, although the overall impact is smaller. Land use change and irrigation are locally important, but have negligible large-scale impacts. These results illustrate the importance of agricultural N fertilization in regulating annual carbon cycle fluxes and highlight that ESMs can improve the seasonal CO₂ cycle by representing agricultural N fertilization.