Opening the Black Box: Compound-Specific Radiocarbon Analysis to Determine the Drivers of Soil Carbon Persistence

Project Overview

Soils store more carbon than plants and the atmosphere combined in a 3000 Pg global reservoir that is critical to soil fertility and the Earth's carbon balance. Our ability to predict soil organic carbon (SOC) response to human impacts on climate and land management is challenged by its complexity, including the processes that drive SOC formation, stabilization, and loss. SOC is highly heterogenous, consisting of diverse compounds that cycle on sub-annual to millennial timescales. Plant inputs and microbial processing drive SOC cycling, producing different classes of SOC (including carbohydrates, proteins, lipids, and nucleic acids), but their distinct residence times are unknown. A mechanistic understanding of SOC persistence with depth and over long-time scales is needed to project future soil C storage and inform C sequestration (negative emissions) strategies. This project focused on fundamental drivers of SOC persistence and developed a novel capability for compound-specific radiocarbon (14C) analysis. 14C is the gold standard for determining both the age and turnover rate of soil C, providing an invaluable metric for evaluating long-term C persistence.

In this project, we developed workflows for isolation and 14C analysis of targeted compound classes from bulk soil and soil physical fractions (lipids, carbohydrates, amino acids, and nucleic acids). We demonstrated workflows for lipids and amino acids in a California grassland soil profile and compared the resulting 14C signatures to those of commonly applied operational soil fractions with less compound specificity (water extraction, density fractionation, size fractionation, and microbial biomass extraction). We found that water-extractable C and CO2 produced by soil microorganisms were both young throughout the top one meter of soil. Microbial biomass was also young in the top 50 cm, but below 50 cm microbial biomass contained older C than was observed in respired CO2. Lipids and amino acids followed a similar trend with depth to microbial biomass, with older C found in these fractions below 50 cm depth. Additional characterization of lipids showed only a small contribution of plant waxes to total soil lipids and that these plant lipids were only present at the soil surface. We also found the insoluble fraction that remained after removal of lipids and amino acids to be the oldest soil C measured in our project and that this fraction likely inherited organic C from parent materials. We conclude that in our soil, young C dominates active C cycles in the top 50 cm, but that below 50 cm depth the presence of old C in microbial biomass, lipid, and amino acid extracts suggest longer persistence of microbial necromass or recycling of old SOC by soil microorganisms. These results provide preliminary data for continued research on the persistence of carbon in soils and management of soils for carbon mitigation.

Mission Impact

This research supported Lawrence Livermore National Laboratory's (LLNL)  Mission Focus Area in Climate Adaptation and Mitigation, LLNL's Mission Area in Climate & Energy Security as well as the former Energy and Resource Security Mission Research Challenge, and the Soil Carbon pillar of the Director's Initiative, "Engineering the Carbon Economy". Our project developed new capabilities that support these mission areas by providing new analytical tools for tracking carbon into persistent soil carbon pools. This work further supported LLNL's core competencies in Earth and Atmospheric Science and the Nuclear, Chemical, and Isotopic Science and Technology through the development of new capabilities for compound-specific measurements at CAMS (an R&D priority). This LDRD project leveraged LLNL's Scientific Focus Areas funded by DOE-OBER that will apply the methodological advancements that we accomplished. This project positioned LLNL well for securing funding from DOE's Energy Earthshot proposal calls, continued and expanded support from multiple DOE-OBER programs (include a new SFA), the ARPA-E Roots program, USDA-NIFA, CA's Cap-and-Trade Climate Change Research Program, and foundations/private industries keen to develop improved accounting for negative carbon emission credits. Our project also supported the NNSA mission to develop science and technology tools and capabilities to meet future national security challenges associated with climate change.

Publications, Presentations, and Patents

Finstad, K., Nuccio, E., Grant, K. E., Broek, T., Pett-Ridge, J., and McFarlane, K.: Radiocarbon analysis of soil microbial biomass via direct chloroform extraction, Radiocarbon, in press. Preprint: https://doi.org/10.31223/X5H952

Finstad, K., Grant, K. E., Morrison, K., Nuccio, E., Pett-Ridge, J., and McFarlane, K.: Linking microbial assimilation of carbon and microbial biomarkers to soil organic matter persistence" (Poster Presentation, Goldschmidt Conference, Lyon, France, July 2023).

Grant, K.E., Repasch, M.N., Finstad, K.M., Broek, T., Pett-Ridge, J., and McFarlane, K.J, "Initial radiocarbon (14C) results of compound class persistence across a climate gradient in California grassland soils" (Poster Presentation, European Geophysical Union Conference, Vienna, Austria, April 2023).

K.Finstad, K. Grant, E. Nuccio, J. Pett-Ridge, and K. McFarlane, "Novel methods for determining the 14C age of microbially assimilated soil carbon" (Poster Presentation. European Geophysical Union Conference, Vienna, Austria, April 2023).

Finstad, K., Nuccio, E., Grant, K. E., Broek, T., Pett-Ridge, J., and McFarlane, K., "Novel methods for determining the 14C age of microbially assimilated soil carbon" (Poster Presentation, International Radiocarbon Conference, September 2021).

K. McFarlane, S. Mambelli, R.C. Porras, D.B. Wiedemeier, M.W. I. Schmidt, T.E. Dawson, and M.S. Torn, "Soil carbon stocks not linked to aboveground litter input and chemistry of old-growth forest and adjacent prairie" (Poster Presentation, International Radiocarbon Conference, September 2021).

K.E. Grant, M.N.Repasch, K.M. Finstad, T. Broek, and K.J. McFarlane, "Divergence of compound class persistence in a California grassland soil" (Poster Presentation, European Geophysical Union Conference, Vienna, Austria, April 2023).