Global Levels Of Nitrogen Remained Stable For Last 500 Years

Global levels of nitrogen have remained stable for the last 500 years despite humans increasing nitrogen production around the world, a new study from Kansas State University has found.

Although humans have nearly doubled the amount of nitrogen present in the ecosystem, globally nitrogen levels have remained stable over the last half millennium, the study found.

"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," said Kendra McLauchlan, an assistant professor of geography and one of the study authors.

As the largest regulator of plant growth, nitrogen plays a crucial role in the ecosystem and is a key factor in how much food, fuel and fiber the land can produce. Nitrogen also determines how much carbon dioxide plants remove from the atmosphere; excessive amounts of nitrogen can contribute to global warming.

"People have been really interested in nitrogen in current times because it's a major pollutant," said McLauchlan, who also heads Kansas State's Paleoenvironmental Laboratory.

The research team collected and analyzed data from sediment records of 86 lakes in both tropical and temperate climate zones scattered across six continents, which was used to compare past and present nitrogen cycling.  

Researchers also studied how nitrogen availability changed thousands of years ago, finding that after ice sheets melted some 11,000 years ago the Earth continued to experience a global decline in nitrogen for the another 4,000 years.

"That was one of the really surprising findings," said Joseph Craine, research assistant professor in biology, who also worked on the study. "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."

Researchers suspect plants are using more nitrogen than they once were, which is keeping nitrogen levels consistent with what they once were.

"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."

 But that balance may 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. "Now more than ever, it's important to begin monitoring our grasslands and forests for early warning signs."

The study  "Changes in global nitrogen cycling during the Holocene epoch," appears in the current issue of Nature.