Monday, August 2, 2021

Forest Soils Release More Carbon Dioxide Than Expected in Rainy Season

Current carbon cycle models may underestimate the amount of carbon dioxide released from the soil during rainy seasons in temperate forests like those found in the northeast United States, according to Penn State researchers. Tree roots and microbes use oxygen to convert organic carbon in the soil to carbon dioxide (CO2) for energy through a process called aerobic respiration.

This release of CO2 from soils to the atmosphere represents the largest flux of carbon from terrestrial ecosystems, making it a key component of the global carbon budget. Aerobic respiration is the dominant process contributing to this flux, but the researchers found that under wet conditions, anaerobic respiration — or respiration without oxygen — also significantly contributes to the flux. Caitlin Hodges, a doctoral candidate in the Department of Ecosystem Science and Management, said:

One benchmark for interpreting soil processes affected by soil carbon dioxide and oxygen is to calculate the apparent respiratory quotient (ARQ), which combines carbon dioxide and oxygen concentrations into one value. Hodges added:

The scientists studied soil respiration in a shale watershed and a sandstone watershed in the National Science Foundation-funded Susquehanna Shale Hills Critical Zone Observatory.

Current carbon cycle models may underestimate the amount of carbon dioxide released from the soil during rainy seasons in temperate forests like those found in the northeast United States, according to Penn State researchers. (Image: CAITLIN HODGES)
Current carbon cycle models may underestimate the amount of carbon dioxide released from the soil during rainy seasons in temperate forests like those found in the northeast United States, according to Penn State researchers. (Image: CAITLIN HODGES)

They measured soil carbon dioxide and oxygen levels about 8 and 16 inches below ground and just above the bedrock layer where the soil ends. Susan Brantley, distinguished professor of geosciences and director of the Earth and Environmental Systems Institute (EESI) at Penn State, said:

The team found that ARQ sometimes signaled significant anaerobic respiration by the microbes. During anaerobic respiration, the microbes shifted from using oxygen to oxidized metals, like iron and manganese, for growth. Jason Kaye, professor of soil biogeochemistry, said:

The Penn State team’s research, reported in the Soil Science Society of America Journal, is the first to use ARQ to find evidence of a seasonal pattern of anaerobic respiration in temperate forests. The researchers also calculated the total amount of carbon dioxide — 36 grams per square meter — that leaves the soil system every year due to anaerobic respiration.

A soil oxygen meter and field book for recording soil oxygen concentrations in the Shale Hills watershed. In the background are soil gas wells for sampling 8-inch and 16-inch soil depths. (Image: CAITLIN HODGES)
A soil oxygen meter and field book for recording soil oxygen concentrations in the Shale Hills watershed. In the background are soil gas wells for sampling 8-inch and 16-inch soil depths. (Image: CAITLIN HODGES)

They said the conservative estimate constitutes 10 percent of all respiration done by soil microbes at their research sites, which is a large number since scientists don’t think of these humid temperate forests as posting much anaerobic respiration. Hodges said:

Brantley said novel approaches like soil gas sampling are needed to understand how soils will respond to changing climate.

Provided by: Francisco Tutella, Pennsylvania State University [Note: Materials may be edited for content and length.]

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Troy Oakes
Troy was born and raised in Australia and has always wanted to know why and how things work, which led him to his love for science. He is a professional photographer and enjoys taking pictures of Australia's beautiful landscapes. He is also a professional storm chaser where he currently lives in Hervey Bay, Australia.
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