Soil contaminated by air pollution emits carbon

New research from UC Riverside suggests that nitrogen released from gas-powered machines causes dry soil to release carbon and releases it back into the atmosphere, where it may contribute to climate change.

Industrial manufacturing, agricultural practices, and most importantly, vehicles burn fossil fuels that emit nitrogen into the air. As a result, the amount of nitrogen in the Earth’s atmosphere has tripled since 1850. The research team wanted to understand if this extra nitrogen would affect the soil’s ability to store carbon and keep it from becoming a greenhouse gas.

“Since nitrogen is used as a fertilizer for plants, we expected that supplemental nitrogen would promote plant growth as well as microbial activity, thereby increasing soil carbon input,” said Peter Homyak, study co-author and assistant professor in the Department of Environmental Sciences the UCR .

In dryland soil, the kind that covers much of southern California, this is not what they saw.

Instead, the team found that under certain conditions, extra nitrogen causes dryland soils to acidify and leach calcium. Calcium binds to carbon, and the two elements then leave the soil together. This finding is described in detail in the journal Biology of Global Change.

To obtain their results, the research team took soil samples from ecological reserves near San Diego and Irvine that had been nitrogen-fertilized in long-term experiments. This allowed them to know exactly how much nitrogen was added and explain any observed effects.

In many cases, nitrogen can affect biological processes, which in turn affect how the soil stores carbon. Such processes include encouraging plant growth as well as slowing down the microbes that help break down dead things in the soil.

What the researchers didn’t expect was a large effect on carbon storage by abiotic or non-biological means.

The pH scale measures how acidic or alkaline – i.e. basic – something is. In general, soils resist dramatic pH changes by releasing elements like calcium in exchange for acidity. As nitrogen acidified soils at some of the sites in this study, the soils attempted to resist this acidity by releasing calcium. In the process, part of the carbon stabilized by association with the calcium was lost.

“It’s a surprising result as the main effect appears to be abiotic,” said Johann Püspök, a PhD student in UCR environmental sciences and first author of the study. “This means that even bare soil areas with no vegetation and low levels of microbial activity, which I always thought were low-activity areas, appear to be affected by nitrogen loading.”

Dry soil, characterized by limited ability to retain moisture and low organic matter content, covers about 45% of the Earth’s land area. It is responsible for storing a large amount of the world’s carbon.

Future studies may shed more light on how badly the dryland soil is affected by nitrogen pollution, as was the study plots. “We need more information about how widespread such acidification effects are and how they work under non-experimental nitrogen deposition conditions,” said Püspök.

However, since there is no quick fix for this phenomenon and there is no clear way to reverse the process once it has started, researchers recommend reducing emissions as much as possible so that the soil retains its carbon stores.

“Air pollution caused by burning fossil fuels affects many things, including human health by causing asthma,” Homyak said. “It may also impact the amount of carbon these dryland systems can store for us. We need to get air pollution under control for many reasons.”