As a result, nitrogen levels in the Earth's atmosphere have tripled since 1850. The researchers wanted to know if the extra nitrogen was affecting soil's ability to hold carbon and prevent it from becoming a greenhouse gas. "Because nitrogen is used as a plant fertilizer, we expected that adding more nitrogen would promote plant growth as well as microbial activity, thereby increasing carbon put into soils," said Peter Homyak, study co-author and assistant professor in UCR's Department of Environmental Sciences.
This was not what they saw in dryland soil, which covers much of Southern California. Instead, the researchers discovered that excess nitrogen causes dryland soil to acidify and leach calcium under certain conditions. Calcium binds with carbon, and the two elements exit the soil together. This discovery was published in the journal Global Change Biology. To obtain their findings, the researchers collected soil samples from ecological reserves near San Diego and Irvine that had been nitrogen-fertilized in long-term experiments.
This allowed them to precisely calculate the amount of nitrogen added and account for any effects they observed. In many cases, nitrogen can influence biological processes, which affect how soil stores carbon. These processes include fueling plant growth and slowing the microbes that help decompose dead things in the soil. What the researchers did not anticipate was a significant impact on carbon storage via abiotic, or non-biological, means.
The pH scale determines whether something is acidic or alkaline (basic). Soils, in general, resist drastic pH changes by releasing elements such as calcium in exchange for acidity. As nitrogen acidified the soils at some of the study sites, the soil tried to resist the acidity by releasing calcium. As it did so, some of the carbon stabilized by calcium association was lost. "It's a surprising result because the main effect appears to be abiotic," said Johann Puspok, first author of the study and a graduate student in environmental sciences at UCR.
"That means that bare patches of soil with no plant cover and low microbial activity, which I always thought were areas with little going on, appear to be affected by nitrogen pollution as well. Dryland soil, which has a limited ability to retain moisture and contains low levels of organic matter, accounts for roughly 45 percent of the Earth's land area. It is responsible for storing a significant portion of the world's carbon." Future research may shed more light on how much dryland soil is affected by nitrogen pollution in the study plots.
"We need more information on how widespread such acidification effects are and how they function under non-experimental nitrogen deposition conditions," Puspok said. However, because there is no quick fix and no clear way to reverse the process once it has begun, researchers recommend reducing emissions as much as possible to help soil retain its carbon stores.
"Air pollution caused by fossil fuel combustion has a wide range of effects, including asthma. It can also impact the amount of carbon these dryland systems can store for us. We must reduce air pollution for a variety of reasons," Homyak explained.
