The foundation under our feet has a reputation for being an inhospitable place. On the other hand, the soil is known to be teeming with life – from microbes to plant roots to insects.
From this perspective, the soil is the main source of carbon dioxide produced by forests, the CO2 be a natural byproduct of the life in it. But according to a study led by the University of Texas at Austin, the prevailing opinion is only scratching the surface.
The study found that CO2 can also be produced deeper underground in rock fractures, and that this source can account for up to 29% of the daily mean CO2 emitted from the country, depending on the season.
This finding doesn’t mean landscapes emit more CO2 into the atmosphere, but it challenges conventional wisdom about where CO2 is produced. It can also help improve climate change models, as understanding how and where CO2 is produced is an essential part of making accurate predictions.
The study linked CO2 from production in the rock to the seasonal uptake of water by deep tree roots many feet below the surface, a finding that suggests that tree roots and the microbial communities around them are the source of the CO2 – and that fractures in the rock are a place for life to thrive.
“This is a paradigm shift in terms of where the action is,” said Daniella Rempe, an assistant professor at the UT Jackson School of Geosciences who co-authored the study. “Soils may not be the only protagonist in forests.”
The study was published in the journal Dec. 6, 2020 JGR Biogeosciences.
Alison Tune, a graduate student at Jackson School, led the investigation. Other co-authors include Jackson School Professor Philip Bennett, Jia Wang, a graduate student at the University of Illinois Urbana-Champaign, and Jennifer Druhan, an assistant professor at the University of Illinois Urbana-Champaign who played key roles in designing and conducting the research. .
Soil is not on solid rock. On the contrary, a transition zone of broken and weathered rock sits between these two extremes. This altered rock is notoriously difficult to sample. The study was based on a specialized sampling device buried in a hillside in Northern California, which stretched from the top of the fractured rock to the bottom, about 44 feet.
This tool quickly revealed that this region was an active site of CO2 production.
“There is a large CO2 well underground, ”said Tune. “When we first entered the [CO2] concentration profiles in the field, we were quite enthusiastic about what we found. “
By analyzing thousands of samples collected from 2017-2019, the researchers found that the CO2 did not stay put. During the dry season, the CO2 mainly traveled into the soil where it was released into the atmosphere. During the wet season, when groundwater rose to fill the fractures, nearly 50% of the CO2 dissolved in the water, which eventually flows to streams and rivers.
The researchers discovered that this dissolved CO2 accelerates the weathering of rocks, with as much as 80% of the dissolved carbon in the groundwater leaving the study area and coming from the fractured rock. This finding is significant, Rempe said, because it’s the first time scientists have been able to pinpoint where sustained rock weathering occurs in the hills.
This study builds on a growing body of knowledge showing that fractured rock is an ecologically important region. For example, in a 2018 study, Rempe and colleagues found evidence of rock moisture in broken rocks that support trees during droughts.
Mark Torres, an assistant professor at Rice University who studies how carbon circulates through environments, said the research is important in that it sheds light on an area of the landscape that is considered a “black box” between the soil and groundwater. .
“The work I do usually scoops up river water and I have to deduce what is going on under a hill,” he said. “What’s really impressive about the work is how they observed things that are incredibly difficult to see.”
The researchers plan to investigate fractured rock in other places, including a local research site at the Jackson School’s White Family Outdoor Learning Center, a 266-acre site in Dripping Springs, Texas.
“Crushed rock is very common in Texas, where the soil is very thin and there is a lot of deep rooting,” said Tune. “It could be an important part of the carbon cycle in these ecosystems and it could be important to understand that as we move forward and the climate changes over time.”
Reference: “Sub-soil carbon dioxide production contributes substantially to forest carbon cycle” by Alison K. Tune, Jennifer L. Druhan, Jia Wang, Philip C. Bennett and Daniella M. Rempe, November 27, 2020, JGR Biogeosciences.
DOI: 10.1029 / 2020JG005795
The Department of Energy, the National Science Foundation, and the Geological Society of America supported the research.