This document deals with significant impact of agricultural nitrogen (N) fertilization on increasing the seasonal amplitude of CO2 exchange between land and atmosphere, especially in northern latitudes. The authors analyzed simulations of Earth climate models (ESMs), in particular the Community Earth System Model (CESM) and its land component Community Land Model version 5 (CLM5), and compared them with observations of CO2 concentrations.

Key findings of the study:

• Observations over the past 50 years show increase in seasonal CO2 amplitude in northern latitudes, with significant increases recorded at stations such as Mauna Loa and Point Barrow.
• Older versions of ESM (e.g. CMIP5) they did not adequately simulate this increased amplitudeNewer models (e.g. CMIP6) including CESM2 are getting closer to observations.
• The study showed that agricultural nitrogen fertilization is the largest contributor (45%) to the increased seasonal amplitude of CO2 in the Northern Hemisphere in CESM simulations. In agricultural hotspot areas, it can contribute up to 9 ppm of amplitude increase.
• The increased amplitude is also contributed by higher CO2 concentrations (40%) and warmer temperatures (18%), but to a lesser extent than nitrogen fertilization. The impact of increased CO2 is more pronounced in northern latitudes.
• Increased photosynthesis is the dominant mechanism responsible for the increase in the seasonal amplitude of CO2. Agricultural nitrogen fertilization directly supports plant growth and thus photosynthesis.
• While it was previously considered increase in atmospheric CO2 concentration as the dominant factor contributing to the increased amplitude of carbon fluxes in northern latitudes, this study highlights the importance of agricultural nitrogen fertilization.
• The impact of agricultural nitrogen fertilization on net biome production (NBP), which represents the net flux of CO2 between land and atmosphere, shows similar spatial patterns as changes in CO2 amplitude in CESM2 simulations.
• Increased use of fertilizers in agriculture over the past fifty years, initiated by the Green Revolution, has significantly increased crop productivity.
• Most older ESMs underrepresented agriculture and land management, including nitrogen fertilization, which led to an underestimation of the impact of agriculture on the seasonal amplitude of CO2. Newer models, such as CESM2, include explicit modules for agricultural simulation, including different crops, irrigation and fertilization.
• Irrigation has a smaller impact on the seasonal amplitude of carbon fluxes in the Northern Hemisphere (3.7%) compared to nitrogen fertilization, but can be locally significant. Land use changes have a significant impact on the overall carbon cycle, but not as strong on the seasonal amplitude of carbon fluxes.
• The magnitude of the change in carbon fluxes between land and atmosphere in 2010-2015 due to industrial nitrogen fertilization (0.54 gC m−2 d−1) is similar to the combined impact of CO2 and climate (0.55 g C m−2 d−1).
• Authors recommend including a representation of agricultural nitrogen fertilization into future carbon cycle simulations given its key role in regulating carbon fluxes.
• Study findings they question the assumptionthat climate change is the primary driver of the increased seasonal amplitude of CO2, and they emphasize the important role of agricultural fertilization.

The study highlights that agricultural nitrogen fertilization represents a significant anthropogenic influence on the seasonal amplitude of CO2 and its inclusion in models is crucial for more accurate predictions of future carbon cycle and climate change. Spring

Glossary of Key Terms

• Seasonal CO2 Amplitude: The difference between the maximum and minimum monthly average concentrations of atmospheric carbon dioxide (CO2) over a year. It reflects seasonal changes in the uptake and release of CO2 by ecosystems.
• Earth System Models (ESMs): Complex computer models that simulate interactions between different components of the Earth, such as the atmosphere, oceans, land, ice sheet, and biosphere, including biogeochemical cycles such as the carbon cycle.
• CMIP (Coupled Model Intercomparison Project): A standardized framework for comparing the outputs of different global climate models. CMIP phases (e.g. CMIP5, CMIP6) allow scientists to evaluate models and gain insight into climate projections.
• CESM (Community Earth System Model): A specific open-source Earth system model developed and used by a community of scientists to simulate climate and biogeochemical processes.
• Agricultural Nitrogen (N) Fertilization: Adding nitrogen fertilizers to agricultural soil to increase crop yields. Nitrogen is an essential nutrient for plant growth and photosynthesis.
• Net Biome Production (NBP): The total accumulation of organic carbon in an ecosystem over a period of time, representing the net flux of carbon from the atmosphere to vegetation and soil minus carbon released by respiration and disturbances (e.g. fires, land-use change). A positive NBP indicates net carbon uptake.
• CO2 Fertilization: An increase in the rate of photosynthesis in plants due to higher concentrations of atmospheric carbon dioxide.
• CLM (Community Land Model): The land component of the CESM model, which simulates physical, chemical, and biological processes on the Earth's surface, including vegetation growth, soil interactions, and carbon, water, and energy flows.
• LUMIP (Land Use Model Intercomparison Project): A CMIP6 project aimed at comparing the impact of land use and management changes on climate and biogeochemical cycles using different Earth system models.
• Green Revolution: A period of widespread change in agriculture in the mid- to late 20th century, characterized by the development of high-yielding crop varieties, increased use of fertilizers, and irrigation, leading to significant increases in food production.