Estimated carbon fluxes and balance along the measurement transect, showing 30 percent of annual carbon dioxide emissions from deeper than 1 meter below ground, contrary to predictions of less than 1 percent by the Earth System Models.

 

Soil harbors three times as much carbon as Earth’s atmosphere, but until recently research into how warming affects the amount of carbon released from soil into the atmosphere was limited to investigations of the top meter of soil. Researchers at Berkeley Lab are the first to examine soils and sediments as deep as seven meters below ground for carbon concentrations and for the amount of carbon released to the atmosphere as CO2. Their study found that 30 percent of the CO2 released from soil to the atmosphere originates within deeper layers of sediments than previously believed. In winter, 60 percent of this carbon efflux begins from below 1 meter underground.

The study led by staff scientist Jiamin Wan at Berkeley Lab was conducted over two years as part of the Watershed Function Scientific Focus Area (SFA) program at a field site in Rifle, Colorado. Wan set out to quantify carbon fluxes throughout the different compartments of the Earth’s subsurface through which most of the land-related carbon that is relevant to climate change flows, including the soil, the underlying deep vadose zone, and groundwater. Results of the team’s studies were published in the Journal of Geophysical Research: Biogeosciences.

“Our study stands out for its investigation into carbon fluxes in the deep compartments of the Earth’s subsurface,” Wan said. “It’s true that others have drilled deep below the surface to explore carbon efflux to the atmosphere from soils, but others have been satisfied once they’ve obtained the soil samples. Our unique methodology combined continuous monitoring of water saturations, subsurface temperatures, collection and analysis of pore waters and soil gas, and periodic measurement of CO2 fluxes at various depths down to seven meters below ground over two years.”

The study site resides along the Colorado River in the semiarid Colorado Plateau, where drought-tolerant perennial grasses and shrubs are the dominant vegetation. The water table is typically at a depth of 3.5 meters, and the average groundwater velocity is 0.3 m per day. Wan and her team established five monitoring stations along a 250-meter transect aligned with the direction of the flow of groundwater. At each station, a 0.254 meter diameter borehole was drilled down to 3.5 meters. Another borehole was drilled down to the bottom of the upper aquifer for groundwater sampling. Subsurface temperatures, soil gas samples, pore water samples and CO2 fluxes were obtained for field and laboratory analysis from these stations.

The studies at Berkeley Lab and at the Rifle field site took into account how conditions that are characteristic of soils and sediments deep underground in semiarid regions – such as dissolved organic carbon fluxes from the overlying soil and carbon cycling from deep roots and exudates – are important in sustaining microbial respiration. To their surprise, the scientists discovered an unexpectedly high dissolved organic carbon flux from the rhizosphere into the underlying unsaturated zone that supports CO2 at greater depths. Currently, Earth System Model land models assume less than 1 percent of annual CO2 efflux originates from deep sediments.

“Because these deeper sediment processes are characteristic of semiarid climate regions, we believe these results are critical to include in Earth System Models to improve CO2 flux predictions,” Wan said.

The majority of previous field-based soil warming experiments had ignored the extent to which deeper layers of soil respond to warming–even though experts estimate soils below 20 centimeters in depth contain more than 50 percent of the planet’s stock of soil organic carbon. This study by Berkeley Lab scientists is the first to examine soils and sediments as deep as 7 meters below ground in the Earth’s subsurface, through the unsaturated zone and into groundwater.