A new study featuring research conducted at an experimental forest in West Virginia is shedding light on how the carbon-storing ability of soils, and the billions of microbes within them, may fare as both carbon dioxide and nitrogen increase in the future.
The research, published recently in the journal Global Change Biology, examined how increased nitrogen affects the ability of forests and soil to store carbon.
"So in general, adding nitrogen to soils forests causes less decomposition, more soil carbon storage, but the mechanism for as to why that happens, again, largely unknown," said Joe Carrara, a Ph.D candidate in the Department of Biology at West Virginia University and co-author of the paper. "Most research shows that it’s due to a decline in certain fungal guilds that are really good at breaking down lignin, or leaves or really recalcitrant, sort of hard to decompose soil organic matter."
Humans have more than doubled the amount of nitrogen being deposited across ecosystems worldwide through the burning of fossil fuels and agriculture.
More nitrogen is on the way as developing countries invest in new coal-fired power plants. Carrara said understanding how nitrogen affects soils and ultimately the amount of carbon dioxide in the atmosphere can help scientists improve future climate change predictions.
In the summer of 2015, Carrara and the team spent a few days in the Fernow Experimental Forest near Parsons, West Virginia. They sampled soils in two parts of the U.S. Forest Service site -- one area that has been left alone and another, which has been treated with nitrogen pellets since 1989.
"What we were interested in seeing is how the relationships between trees and these soil microbes, fungi, bacteria, mycorrhizal fungi, change under conditions with elevated nitrogen and can these relationships or changes in these relationships sort of give us some insight into why soil carbon decomposition goes down under elevated nitrogen," he said.
The scientists found more nitrogen meant trees used less of their energy taking up carbon and creating things like roots. Bigger root systems mean more carbon stored underground. They found the trees also spent less carbon creating relationships with symbiotic, or beneficial, fungi that live in soils.
One surprising thing they found is that bacteria in the soil they sampled seemed to undergo changes when there was more nitrogen to contend with.
'Mother Nature Threw Up on Wheeling, West Virginia'
West Virginia, as it turns out, is a great place to study this because of how close it is to the coal-burning power plants that have historically dotted the Ohio Valley. The state has some of the historically highest levels of past nitrogen deposition of course in the country.
"I think that the lowest ever recorded acid rain was actually in Wheeling, West Virginia," Carrara said. "I don’t know exactly what the pH was, but it the same as stomach acid. One of our co-authors likes to say, I can’t remember what year it was, but he says basically mother nature threw up on Wheeling, West Virginia."
Since the passage of the Clean Air Act, pollution in the U.S. has decreased and the amount of nitrogen falling onto forests and soils has dropped. But as developing nations bring coal-fired power plants and other manufacturing facilities online -- which are sources of nitrogen -- Carrara says it’s important to study how more nitrogen impacts soil carbon storage.
"This sort of gives us an idea of what, in the future, if nitrogen deposition is to continue to rise in some places in developing countries, places in the developing word, even some areas in the United States, how the forest will respond to that elevated nitrogen," he said. "So, it’s sort of like speeding up the process so we can see now what the future might look like."
Still, more research is needed to know how nitrogen fertilization worldwide might affect the total amount of carbon stored by soil worldwide, and thus serve as an important sink of carbon dioxide.
Carrara said the study helped illuminate the ways in which forests take up carbon dioxide and allocate it to different sorts of compartments in the forest like leaves and soils.
"Knowing that this link between the plants themselves and the microbes in the soils is important, provides us with some sort of mechanistic understanding of how nitrogen will impact where the carbon’s allocated in the forest," he said.
For his next research study, Carrara hopes to take tackle more questions related to the forest soil micro biome. Last summer, he collected similar information in an experimental forest in Maine. Together with data from West Virginia, he says he hopes to soon know more about how different tree species that associate with the same fungi react to high levels of nitrogen.
The study was made possible in part by a grant from the National Science Foundation. Additional authors of the study include Christopher Walter at the University of Minnesota, Colin Averill of Boston University and Jennifer Hawkins, William Peterjohn and Edward Brzostek at WVU.