Microbe-Mediated Effects of Enhanced Rock Weathering on Soil Organic Carbon Storage

Noah Sokol | 22-LW-022

Project Overview

To keep planetary warming below 2ºC, it will be critical to actively remove carbon dioxide from the atmosphere via carbon removal technologies. Enhanced rock weathering (ERW) is one carbon removal technology that has recently attracted substantial interest, due to its relatively low cost, ease of scalability, and potential for co-benefits - especially on croplands. ERW involves adding crushed rock dust to soils, which ultimately sequesters carbon in inorganic forms; however, it is entirely unknown how rock dust additions may affect soil organic matter (SOM) - Earth's largest terrestrial organic carbon (C) pool. While increases to SOM under ERW could dramatically enhance the carbon removal benefits of this technology, decreases to SOM could entirely negate its carbon drawdown benefits, and diminish soil fertility. To address this knowledge gap, we studied how ERW affected different SOM pools (i.e., mineral-associated organic matter, ‘MAOM', and particulate organic matter, ‘POM'), and its effect on the soil microbial community - since soil microorganisms are primary agents both of SOM cycling and rock weathering. We leveraged a series of ERW field trials in the Central Valley of California to test field-scale effects of ERW on SOM pools, as well as laboratory incubations, to tease out fine-scale interactions between rock dust, organic matter, and oil microorganisms.  

Across three cropland field trials, we found that MAOM carbon and nitrogen stocks, as well as active soil microbial biomass, were lower in surface soil (0-10 cm) in plots amended with meta-basalt rock dust relative to control plots, though this effect was not present at lower depths (10-30 cm). We also found that when rock dust was co-amended with other organic amendments (i.e., compost and biochar), it reduced the amount of MAOM accrued relative to plots where the organic amendments were added without rock dust. In lab incubations with rock dust, we found visual evidence - via scanning electron microscopy and NanoSIMS - that weathered rock dust may provide extra surface for new mineral-bound SOM to accumulate. Overall, our data suggest that over 2-3 year periods, rock dust amendments may decrease accrual rates of MAOM, but do not reduce MAOM below baseline conditions. We also found evidence that weathering rock dust may provide increased mineral surfaces for new MAOM to form, and ongoing work will determine which bacterial and fungal taxa may be involved in biological weathering and new MAOM accrual. Our data emphasize that before ERW is deployed at large scales, it will be critical for field and lab trials to monitor its effects on SOM dynamics over multiple years, and to investigate the microbial mechanisms underpinning these responses, in order to accurately measure the net carbon removed from ERW and its other effects on the soil system. Our data also encourage future studies to identify how soil microbes can be harnessed to both promote rock weathering and accrual of SOM, in order to maximize carbon removal.

Mission Impact

This research on enhanced rock weathering - a leading carbon removal technology - directly supports Lawrence Livermore National Laboratory's (LLNL) Mission Focus Area on Climate Impacts and Resilience, as well as the soil carbon pillar of the Director's Engineering the Carbon Economy Initiative for negative emissions. The work leveraged unique resources supported by the nuclear, chemical, and isotopic science and technology core competency, through the of imaging tools like scanning electron microscopy and NanoSIMS - to understand mineral-organic matter interactions, as well as measurements with ICP-MS and the X-ray diffraction at the Soil Organic Matter Characterization Lab at LLNL's Nuclear and Chemical Sciences Division's (NACS). It also supports LLNL's Bioscience and Bioengineering Core Competency via our focus on identifying bacteria and fungal taxa involved in rock weathering and soil organic carbon storage via our high-throughout stable isotope probing pipeline. This project also established collaboration with leading academic and industry partners involved in ERW, including the Working Lands Innovation Center at UC Davis, Yale University, and Carleton College, among others. These collaborations - and the ideas in this LDRD - helped to secure funding for a new DOE Energy Earthshot Research Center at LLNL (beginning FY 2024, ‘Terraforming Soil EERC'). This DOE EERC will continue to expand on the scientific ideas from this project, and support continued research with its project collaborators. It also led to members of the LDRD project team to join new initiatives focused on developing measurement, reporting, and verification (MRV) guidelines for the burgeoning enhanced weathering industry.

Our LDRD research addresses DOE's energy and environmental security missions. Its focus on ERW and soil organic carbon cycling is directly relevant to the DOE Office of Science mission to determine the mechanisms that control large-scale C cycling processes in terrestrial ecosystems, as well as the the mission of DOE's Office of Fossil Energy and Carbon Management to understand the efficacy of carbon removal technologies like ERW. Our research also supports the NNSA goal to advance the science, technology, and engineering competencies that are the foundation of the NNSA mission, and our results will inform efforts to develop science and technology tools and capabilities to meet future national security challenges.

Publications, Presentations, and Patents

Sokol, N.W., Whalen, E., Jilling, A., Kallenbach, C., Pett-Ridge, J., Georgiou, K., 2022. "Global distribution, formation, and fate of mineral-associated organic matter under a changing climate: a trait-based perspective." Functional Ecology,doi: 10.1111/1365-2435.14040

N.W. Sokol, "Scientific unknowns for scaling enhanced rock weathering: how do inorganic and organic carbon cycles interact in cropland soils to affect carbon removal?" (Invited Oral Presentation and Panel Discussion featuring six research and industry leaders on carbon removal, American Geophysical Union Annual Conference, Nov. 2023). 

N.W. Sokol, "The formation, function, and fate of mineral-associated soil organic carbon" (Presentation, Invited departmental seminar at Technical University of Munich, Chair of Soil Science, Munich, Germany, 2023).

N.W. Sokol, "Microbe-mediated effects of enhanced rock weathering on soil organic carbon storage" (Presentation, LLNL Biogeochemistry Club Seminar Series. Livermore, CA, 2023).