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"Because nitrogen is used as a fertilizerfor plants, we expected the supplemental nitrogen to promote plant growth as well as microbial activity, thereby increasing soil carbon,” said Peter Hamyak, co-author of the study and assistant professor of environmental science at the University of California, Riverside.
In the arid soil that covers most of Southern California, they have not seen anything like it.
Instead, the team found that whenUnder certain conditions, additional nitrogen causes soil acidification in dry areas and calcium leaching. Calcium binds to carbon and the two leave the soil together. This finding is detailed in the journal Global Change Biology.
The research team took soil samples fromecological reserves near San Diego and Irvine, which were fertilized with nitrogen in long-term experiments. This allowed the scientists to know exactly how much nitrogen was being added and to account for the observed effects.
In many cases, nitrogen can change biologicalprocesses that in turn influence how soil stores carbon. Such processes include fueling plant growth as well as slowing the growth of microbes that help break down dead organisms in the soil. What the researchers didn't expect was the large impact on carbon storage through abiotic or nonbiological processes.
As a rule, soils "resist" sharppH changes, releasing elements such as calcium in exchange for acidity. As nitrogen acidified the soils in some of the sites studied in this study, the earth tried to counter this acidity by releasing calcium. At the same time, part of the carbon stabilized by association with calcium was also lost.
“This is an amazing result, because the mainthe effect seems abiotic,” said Johann Puspock, a UC Riverside graduate student in environmental sciences and the first author of the study. “This means that bare patches of soil with no vegetation and low microbial activity, which I have always thought of as areas where little happens, appear to be susceptible to nitrogen pollution as well.”
Dryland soil characterized bylimited ability to retain moisture and low organic matter content, covers approximately 45% of the Earth's land area. It is responsible for storing much of the world's carbon.
“We need more information about how widespread these acidification effects are and how they work under non-experimental nitrogen deposition conditions,” Puspock said.
However, since there is no quick fix for thisphenomenon and there is no clear way to reverse the process once it has begun, the researchers recommend cutting emissions as much as possible to help the soil conserve its carbon stores.
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