The concentration of carbon dioxide in Earth’s atmosphere has been steadily rising, largely due to human activities and climate change. However, scientists have also observed an increasing difference between seasonal peaks and dips in atmospheric carbon dioxide levels. Previously, researchers believed that these carbon dioxide fluctuations were primarily caused by warming temperatures and rising CO₂ levels. However, a new study led by Colorado State University has found that agriculture—particularly the use of nitrogen fertilizer—is the dominant factor driving this widening seasonal variation.
The study highlights the impact of agricultural practices on the carbon cycle and suggests that human land management decisions play a larger role in carbon dioxide fluctuations than previously recognized. While climate and atmospheric CO₂ concentrations still contribute to seasonal variations, the research found that nitrogen fertilizer is the single biggest driver of this trend. According to lead author Danica Lombardozzi, agriculture’s role in carbon cycle changes has been undervalued, and it is often missing from Earth system models used for climate projections.
The annual carbon cycle provides insight into biosphere growth. During the spring, plants absorb carbon dioxide as they grow, temporarily reducing atmospheric levels. In the fall, after crops are harvested and vegetation goes dormant, CO₂ levels rise again. Because most global crops are fertilized with nitrogen, they absorb more CO₂ than non-agricultural vegetation, amplifying the seasonal highs and lows. The study found that nitrogen fertilizer is responsible for 45% of the increasing fluctuation in the annual carbon cycle, with atmospheric CO₂ levels and warming temperatures contributing 40% and 18%, respectively.
Although agriculture influences carbon dioxide fluctuations, it does not necessarily contribute to long-term carbon storage. Since crops are harvested yearly, the carbon they absorb is eventually released back into the atmosphere. However, researchers emphasize that adjusting agricultural management techniques could enhance carbon storage in soils, helping mitigate climate change. Co-author Gretchen Keppel-Aleks noted that at a time when people are experiencing extreme climate impacts, from wildfires to floods, understanding agriculture’s role in CO₂ cycles can help develop strategies for positive environmental change.
Many farmers are now adopting regenerative agriculture practices to improve soil health and reduce carbon emissions. This includes techniques such as cover cropping and reduced tillage, which can enhance carbon storage while maintaining food production. However, current Earth system models do not adequately account for agriculture’s role in CO₂ dynamics.
Lombardozzi and her team used the Community Earth System Model, developed by the National Center for Atmospheric Research, to quantify the causes of increasing carbon dioxide fluctuations and found that agricultural nitrogen was the most significant factor. Unlike previous models, which did not include agricultural processes, this model integrates the effects of fertilizer use on carbon cycling.
For climate models to provide accurate projections, they must incorporate agricultural influences, particularly nitrogen fertilizer use. Lombardozzi stressed that while modeling human decisions is challenging, it is essential for understanding and addressing climate change. By improving these models, researchers can better predict future CO₂ trends and explore ways to mitigate the effects of human activities on the planet.
https://phys.org/news/2025-03-agriculture-main-seasonal-carbon-ups.html