Warming Response of Regional Deep Soil Organic Carbon

Project Overview

The overarching objective of this LDRD project was to determine the vulnerability of deep soil organic carbon (SOC) to warming in the Sierra Nevada region. Deep soils (>30 cm) store more than 70% of global SOC, and increased SOC decomposition and CO2 emissions caused by warming are potentially large climate-change feedbacks. According to the Intergovernmental Panel on Climate Change, temperatures are expected to increase by 4°C by the year 2100, warming the land and underlying soil and making understanding of how warming will influence deep SOC storage and persistence critical to projecting the land carbon sink. However, uncertainty remains in our process-level understanding and ability to quantify how projected warming will impact the stability of carbon in deep soils. We investigated warming effects on deep (up to 16 m) SOC stability across climate, vegetation, and soil-mineralogy gradients in California. We sampled soils from the surface to bedrock (down to 16 meters) at four sites representing vastly different ecosystems across the northern and southern Sierra Nevada mountains. This study quantified the response of SOC concentration, distribution, and vulnerability to warming using a soil-incubation experiment to warm the whole soil profile and radiocarbon and stable isotopes to assess which pools are vulnerable to loss under warming. Our results refine our understanding of the terrestrial carbon cycle by revealing the vulnerability of deep SOC to future changes in climate and how minerology may influence that vulnerability.

Mission Impact

Soil is the foundation of all life on earth. Soil security, resiliency, and health is central to solving big issues of climate-change abatement, food security, fresh-water regulation, and biodiversity. Soil is increasingly recognized as a strategic asset as its ability to produce food and store carbon (reduce climate change) underpins peace and civil stability. Understanding how soil carbon will respond to a changing future is critical to LLNL's mission "to enable U.S. security and global stability and resilience by empowering multidisciplinary teams to pursue bold and innovative science and technology." Our LDRD project results justify the inclusion of deep soils to research and development in climate resilience, a new LLNL mission-focus area. It was previously assumed that despite storing more carbon than the atmosphere and plants combined, deep soil is not an active pool of carbon because it is buffered from changes on the surface. In contrast to this paradigm, our results show that warming of the surface could cause a significant amount of deep-soil carbon to be respired and released into the atmosphere. Our results highlight a critical need for protection of deep soils and show that deep soils are a critical component of soil response to climate change. Furthermore, the absence of deep-soil carbon cycling in previous assessments of terrestrial carbon response to future change adds to the large uncertainties in predictions of the future land carbon sink.

This project supported LLNL's workforce development by hiring Kimber Moreland as a post-doctoral researcher in Karis McFarlane's group at CAMS. This project has given Kimber the opportunity to build internal and external collaborations and contributed to her development as an expert in soil science and biogeochemistry, in addition to building LLNL's expertise in this area. This project formed new collaboration between LLNL and UC Riverside and the Southern Sierra Critical Zone Observatory. It also supported an LLNL ROTC summer intern who learned new laboratory skills and will be included on the future manuscript. Kimber has created multiple online videos to educate and engage with other scientists and the public about the importance of deep soils and LLNL research in this area. Furthermore, this research supports LLNL's science mission area in energy security and climate resilience, the energy-and-resources-security mission research challenge, the soil pillar of the director's initiative "Engineering the Carbon Economy," earth and atmospheric science core competency (climate change impacts R&D priority) and nuclear, chemical, and isotopic science and technology core competency.

Publications and Presentations

Moreland, K. and K. McFarlane. "Digging Deeper: Sampling to Bedrock Changes our Understanding of Soil Carbon Warming Response in the Sierra Nevada." American Geophysical Union (AGU) Fall Meeting, Chicago, IL. 2022.

Moreland, K. and K. McFarlane. Deep Soil Carbon Is Sensitive to Warming and Influenced by Minerology in Two Oak Savannah Grasslands in the Sierra Nevada." American Geophysical Union (AGU) Fall Meeting, New Orleans, LA. 2021.

Broek, T. A. B. et al. "Optimization of the LLNL/CAMS Gas-Accepting Ion Source and 1 MV Compact AMS for Natural Abundance Radiocarbon Analysis." 15th International Conference on Accelerator Mass Spectrometry (AMS-15), Virtual. November 2021.

Broek, T. A. B. et al. 2021. "Conversion of the LLNL CAMS 1 MV Biological AMS System to a Semi-Automated Natural Abundance 14C Spectrometer: System Optimization and Performance Evaluation." Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 499, p. 124-132 (2021); doi: 10.1016/j.nimb.2021.01.022.