December 14, 2021
Soil Organic Deep in the Sierra Nevada Critical Zone
Digging deeper reveals a massive and unrealized pool of terrestrial organic carbon.
The spatial distribution of deep soil organic carbon and its vulnerability to climate change is uncertain. Researchers measured the distribution, stability, and chemical composition of soil organic carbon to 10 m depth across a bioclimate gradient in California’s southern Sierra Nevada. They found that deep soils and weathered bedrock can store over 75% of total soil organic carbon. Climate controls soil carbon storage by influencing vegetation and the thickness of soil and weathered bedrock. Deep soil carbon was a mixture of very old and actively cycling carbon, suggesting a portion of this pool may respond to climate change.
This study illustrates the importance of deep soil organic carbon to the global carbon cycle. These findings indicate that a fundamental understanding of organic carbon storage and dynamics, including the information needed to anticipate and project responses and feedbacks to climate change, requires the inclusion of deep soil organic matter in experiments. Further quantification of the vulnerability and resilience of deep soil organic carbon to shifts in environmental drivers (such as planetary warming) is needed to appropriately represent this large and important carbon reservoir in Earth System Models.
Soil organic carbon is the largest terrestrial reservoir that actively exchanges carbon with the atmosphere. Soils can be tens of meters deep, but few studies on soil organic carbon have included soils below 30 cm. Researchers investigated the distribution and chemical composition of soil organic carbon to the depth of hard bedrock (down to 10 m) along a bioclimate gradient in the southern Sierra Nevada in California. These sites are part of the Ameri-Flux and Critical Zone Observatory networks, allowing the team to evaluate the relationships between ecosystem-level fluxes of carbon and water to their investigations on soil carbon storage, characterization, and age. They found that deep soil and weathered bedrock play a significant role in carbon budgets across a range of environmental conditions.
Researchers found that at their study sites, up to 80% of soil organic carbon is stored below 30 cm depth and up to 30% of total soil organic carbon is stored in deep weathered bedrock (between 1.5 and 10 m depth). Carbon storage in deep soils and weathered bedrock were largest at mid-elevations where soil thickness and ecosystem gross primary productivity were greatest. They also found that mean annual air temperature explained more variability in soil carbon stock than other climatic variables (mean annual precipitation and deep-water percolation), indicating that topsoil and subsoil carbon may be vulnerable to planetary warming.
Using radiocarbon measured at the Center for Accelerator Mass Spectrometry (CAMS) at Lawrence Livermore National Laboratory (LLNL), researchers discovered that organic carbon in deep soil and weathered bedrock ranged in age from 5,000 to 20,000 years old, not only showing that deep soils store carbon for long periods of time but also indicating that relatively young carbon is actively incorporated into some deep layers. In addition, infrared spectroscopy suggested that this deep soil organic carbon is a mixture of organic matter in various stages of decay and transformation by soil biota. These results challenge a long-standing assumption that deep soil carbon pools play a minor role in global carbon cycles and climate by illustrating that carbon in deep soil and weathered bedrock is a larger carbon pool that is potentially more responsive to changes in climate than previously realized.
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Environmental System Science
This work was funded by the Biological and Environmental Research Program within the U.S. Department of Energy’s (DOE) Office of Science (Award SCW1447). Funding was also provided by the U.S. National Science Foundation (NSF), through the Southern Sierra Critical Zone Observatory (Grant/Award Number: EAR-1331939); the University of California (UC) Lab Fees Research Fellowship Program (Award Number: LGF-18-488060); and Lawrence Livermore National Laboratory’s (LLNL) Graduate Student Training Fellowship.
Moreland, K., et al. "Deep in the Sierra Nevada Critical Zone: Saprock Represents a Large Terrestrial Organic Carbon Stock." Environmental Research Letters 16 124059 (2021). https://doi.org/10.1088/1748-9326/ac3bfe.