Nitrogen pollution causing soil to lose carbon

As I recently covered, sulfuric air pollution once acted as free fertilizer for farms. As efforts to clean the air succeeded, farmers had to increase their use of sulfur fertilizers to compensate. With that knowledge, it seems entirely reasonable to assume that nitrogen, another element often put in fertilizer, would behave in a similar manner. Apparently not.

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

To obtain their results, the research team sampled soil from ecological reserves near San Diego and Irvine that have been fertilized with nitrogen in long-term experiments. This allowed them to know precisely how much nitrogen was being added, and account for any effects they observed.

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

What the researchers did not expect was a big effect on carbon storage through abiotic, or non-biological means.

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

“It is a surprising result because the main effect seems to be abiotic,” said Johann Püspök, UCR environmental sciences graduate student and first author of the study. “That means bare patches of soil with no plant cover and low microbial activity, which I always thought of as areas where not much is going on, appear to be affected by nitrogen pollution too.”

Dryland soil, characterized by limited ability to retain moisture and low levels of organic matter, covers roughly 45% of 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 much dryland soil is being affected by nitrogen pollution in the way the study plots were. “We need more information as to how widespread such acidification effects are, and how they work under non-experimental conditions of nitrogen deposition,” Püspök said.

However, since there is no quick fix for this phenomenon, 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 generated by fossil fuel combustion has an impact on many things, including human health by causing asthma,” Homyak said. “It can also impact the amount of carbon these dryland systems can store for us. For many reasons, we have to get a handle on air pollution.”

Oh, joy.

Given that we already knew that high temperatures cause soil to release carbon, that was already a source of concern for me. Now it turns out that the air pollution I’m always raving about is doing that as well.

On the plus side, this means that as we end fossil fuel use, this particular effect should reverse, which could make a reduction in greenhouse gas levels happen more quickly. We’ll still get that temperature spike from decreasing particle pollution, but anything that causes levels to drop faster is a win for us, I think.

So while this is bad news in our current situation, I’ll take it as good news by looking to the future. .


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