Scientists find new truths about how forests impact air quality

Researchers are looking at how forests impact air quality, based on the classification of temperate trees and what that means for how the forest really works.

A team from Indiana University, West Virginia University, Jet Propulsion Laboratory, the University of Virginia and the University of Warwick are looking into how tree species impact ecosystems – beneath the soil, as well as in the general forest.

Trees are universally perceived as forms of air quality goodness. However, among them, there are countless differences that impact the world around them.

Scientists are now looking at how this minutiae works – especially because knowing what kind of tree is best for forest restoration could improve efforts to build them back.

Because of the large number of tree species on Earth, it is impractical to study each unique effects on carbon and nutrient cycling. Recently, there has been a push to classify trees into groups to help predict the consequences of tree species shifts.

“There’s been a shift in our thinking over the past decade about what controls soil carbon storage,” said Richard Phillips, professor of biology in the IU Department of Biology and co-author on both studies.

“We used to think that slow decaying leaf detritus was the main driver of soil carbon storage, but we now know that fast-decaying compounds released by roots may be what causes soil carbon to persist.”

So, how does fungi impact the storage of carbon and nitrogen?
In two studies, the team reported that forest stands dominated by trees that associate with arbuscular mycorrhizal (AM) fungi differ from stands are dominated by trees that associate with ectomycorrhizal (ECM) fungi in terms of how they store and retain carbon and nitrogen.

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The first study found that some trees handle nitrogen better
In the first study, the authors found that AM-associating trees such as maples, tulip trees, cherry, and ash, which produce fast-cycling detritus, promote soil microbial communities that have more genes capable of processing nitrogen.

This means that there is an increased amount of nitrogen gases that reduce air quality. In contrast, ECM-associating trees such as oaks, hickories, beech, and hemlock produce slow-cycling detritus that promotes microbial communities with few nitrogen-cycling genes, leading to lower gaseous nitrogen losses.

“Regardless of which tree species were present, we found nearly 5-fold more nitrogen cycling potential in the plots dominated by AM trees,” said Ryan Mushinski, the lead author of the study.

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“It’s very exciting that the trend is consistent across the eastern United States, indicating we may be able to predict nitrogen-cycle activity, and more importantly the gaseous loss of nitrogen, in other temperate forests around the world.”

To understand the link between tree species and the functioning of soil microbes near these trees, the researchers collected soils from 54 plots spread evenly across six forests in the eastern United States. Each site had both AM- and ECM-associating trees.

They extracted DNA from the soils in each plot and looked for the abundance of genes critical to nitrogen cycling. They then placed soils in closed chambers in the laboratory to measure how much nitrogen gas is released from the soil and to determine whether this relates to the abundance of nitrogen-cycling genes.

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The second study found out which trees release a lot of carbon
Adrienne Keller, a PhD student in the IU Department of Biology at the time of the study and now a postdoc at University of Minnesota, lead the team that found forests dominated by AM trees enhance soil carbon storage by releasing carbon from their roots.

Adrienne commented: “It’s challenging to measure how much carbon plants shuttle from their roots to the soil. Here we were able to not only quantify the amount of root carbon sequestered in the soil, but also show that its magnitude rivals that of aboveground plant inputs.”

Keller packed mesh cores with root-free soil and inserted the cores in the same 54 forest plots as in the first study.

Because the soil inside the cores had a unique chemical signature, she was able to separate the carbon released from roots from the carbon already present in the soil. Keller found that roots of AM trees release more carbon to soil than the roots of ECM trees and that much of the root carbon sticks to the surface of soil minerals where it is protected from microbial decay.

This means that root carbon may persist for decades or longer, especially in AM-dominated stands.

Read the full study here.

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