March 21, 2013
Atmospheric Nitrogen Levels Have Remained Stable Over Past 500 Years Despite Widespread Emissions
Brett Smith for redOrbit.com - Your Universe Online
Despite widespread use of fertilizers and nitrogen emissions by industrial processes, the amount of atmospheric nitrogen has remained consistent over the past 500 years, according to a new study in Nature.
"People have been really interested in nitrogen in current times because it's a major pollutant," said study co-author Kendra McLauchlan, an assistant professor of geography at Kansas State University. "Humans are producing a lot more nitrogen than in the past for use as crop fertilizer, and there is concern because excess levels can cause damage. The mystery, though, is whether the biosphere is able to soak up this extra nitrogen and what that means for the future."
About 15,000 years ago, many glaciers and ice sheets began to melt as the Earth entered a global warming period. As temperatures rose, the Earth experienced an 8,000-year decline in nitrogen availability as both carbon and nitrogen became locked up in newly exposed soils. According to researchers, the interactions between the nitrogen cycle and the carbon cycle during this time period could be important in understanding the climate of the near future.
"What happened in the past might be a dry run for Earth's future," said co-author Joseph Craine, a research assistant professor in biology. "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."
In the study, the team analyzed sediment data from 86 different lakes across six continents, located in both tropical and temperate zones. This enabled the researchers to compare carbon and nitrogen cycling in various regions.
Through their analysis, the team found that even though the Earth became dramatically warmer 11,000 years ago, it also experienced a decline in nitrogen that lasted 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."
One of the most surprising findings of the study was the impact that human activity is having on nitrogen levels, or rather the non-impact. Although humans have approximately doubled the amount of nitrogen in local ecosystems — the amount of global nitrogen has remained relatively stable over the past 500 years.
McLauchlan said that plants could be utilizing more nitrogen than before, keeping global levels steady despite human activities.
"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 that there is no net effect on the biosphere."
However, she warned that this balance could only be temporary.
"Based on what we learned from the past, if the response of plants to elevated carbon dioxide slows, nitrogen availability is likely to increase and ecosystems will begin to change profoundly," McLauchlan said in a statement. "Now more than ever, it's important to begin monitoring our grasslands and forests for early warning signs."