"What happened in the past might be a dry run for Earth's future," Craine said. "By looking at what happened millennia ago, we can see what controlled and prevented changes in nitrogen availability. This helps us understand and predict how things will change in the future."
The team collected and analyzed data from the sediment records of 86 lakes scattered across six continents. The lakes were distributed between tropical and temperate zones. With the data, the team was able to compare past and present cycling in various regions.
Researchers found that once most of the glaciers and ice sheets had melted around 11,000 years ago, the Earth continued to experience a global decline in nitrogen that lasted another 4,000 years.
"That was one of the really surprising findings," Craine said. "As the world was getting warmer and experiencing higher carbon dioxide levels than it had in the past, just like we are currently experiencing, the ecosystems were starting to lock carbon in the soils and in plants, also like we are seeing today. That created a long decline in nitrogen availability, and it scrubbed nitrogen out of the atmosphere."
McLauchlan said the most surprising finding, however, was that although humans have nearly doubled the amount of nitrogen to the ecosystems, globally nitrogen levels have remained stable at most sites for the past 500 years.
One reason may be that plants are using more nitrogen than they previously have, keeping nitrogen levels consistent with those thousands of years ago even though humans continue to add carbon dioxide and nitrogen to the atmosphere, McLauchlan said.
"Our best idea is that the nitrogen and carbon cycles were linked tightly back then and they are linked tightly today," McLauchlan said. "Humans are now manipulating both nitrogen and carbon at the same time, which means
|Contact: Kendra McLauchlan|
Kansas State University