Principal Investigator

Ian Bourg

At a Glance

The objective of this initiative is to resolve the physics of soil carbon storage. The carbon storage capacity of soils is known to increase significantly with clay content, and in particular with the content in swelling clay minerals (smectites), but the cause of this relationship remains unknown. Using atomistic-level simulation methodologies, the Bourg group was able to model fully flexible clay particles surrounded by water and interacting with dissolved organic compounds. These results will enable more accurate Earth System Model predictions of the soil carbon sink and inform practical strategies for enhancing this important carbon sink.

 

Research Highlight

Soil are a vast pool of carbon (2,400 gigatonnes of carbon, integrated from the surface to 2 m depth), roughly three times larger than the atmosphere and 240 times current annual fossil fuel emissions. A 0.4% annual increase in global soil carbon content would, on its own, entirely offset global fossil fuel emissions1. According to Earth System Models, soil carbon content will increase significantly over the 21st century. The magnitude of this increase is poorly known (it may range from 0.01 to 0.15% annually) because of a lack of understanding of the fundamental mechanisms that control the rate of microbial degradation of soil organic matter. A 0.02% annual increase in the global soil carbon content would add 25 GtC to the soils, which is a stabilization “wedge” as defined by Pacala and Socolow (2004). Conversely, soil carbon losses have historically led to significant anthropogenic carbon dioxide emissions. The US’s Great Plains lost almost 4% of their soil organic carbon over the last 30 years, and decreases of similar or greater magnitude have been estimated for other regions.

The carbon storage capacity of soils is known to correlate with soil clay content, and in particular with the content in swelling clay minerals (smectites), but the cause of this relationship remains unknown. Clay minerals contribute predominantly to the specific surface area and cation exchange capacity of soils, suggesting that organic molecules may become chemically shielded from microbial degradation by attachment to clay surfaces. Clay minerals also strongly influence the hydraulic permeability of porous media, suggesting that they may slow the degradation of soil organic matter by modulating soil microbiology and/or hydrologic permeability.

A key breakthrough in this initiative in 2016 is the ability to model, using atomistic-level simulation methodologies developed by the Bourg group over the last two years, fully flexible clay particles surrounded by water and interacting with dissolved organic compounds. As a first test of this methodology, the group simulated the adsorption of dissolved gases (noble gases, methane, CO2, or H2) on smectite clay particles (Gadikota et al., 2017). These simulations (Figure 2.1.), which solve Newton’s equations of motion for systems of about 100,000 atoms using semi-empirical models of all relevant interatomic interactions, require about two weeks of time on hundreds of parallel processors. The main challenge is to develop models of these interatomic interactions that accurately predict the properties of real clay-water systems, a research area in which the Bourg group is actively involved, in collaboration with synchrotron scientists at two US National Laboratories. The simulations are carried out on the Cori supercomputer of the US Department of Energy, the world’s fifth fastest computer.

The results show that clay minerals, despite their well-known hygroscopic nature, have a significant hydrophobic character at the atomistic scale. This local hydrophobicity exists because the random distribution of negatively charged sites (mostly isomorphic substitutions of Al by Mg) in the clay structure gives rise to uncharged “patches” on the clay surface, i.e., localized regions where the clay surface is hydrophobic because it carries no exchangeable cations (Na+ ions in Figure 2.1.). The affinity of different dissolved gases for the clay surface further shows unexpected variations related to the size and shape of the adsorbing molecules and the structuring of interfacial water by the clay surface. Results obtained with dissolved gases and preliminary results obtained with uncharged organic compounds suggest that organic molecules containing aromatic rings and/or heteroatoms (O, N, S) should have a significantly greater tendency to attach to (and become physically shielded by) clay surfaces.

Figure 2.1. Snapshot of a simulation cell containing a stack of smectite clay nanoparticles (1 nm thick clay particles with 0.6 nm thick interlayer nanopores) in contact with a mesopore (0.6 M NaCl solution). Clay structural atoms are shown as red, yellow, pink, green, and white spheres (O, Si, Al, Mg, and H, respectively); water molecules are shown as red and white sticks; Na and Cl ions are shown as dark and light blue spheres; five molecules of a monoatomic gas solute (Ar) are shown as large orange spheres. Each simulation required 35 nanoseconds (35 million time steps) to obtain average equilibrium properties of the system of interest. The clay particles have a negative structural charge that is balanced by exchangeable sodium ions in the interlayer nanopores.

The Bourg group is building upon these simulation methodologies to investigate the mechanisms of soil carbon storage through two research efforts. The first effort (carried out under the auspices of CMI) focuses on developing a fundamental knowledge of the thermodynamics of adsorption of a range of organic molecules representative of soil organic matter on smectite clay minerals. The second effort (supported by the US Department of Energy) focuses on developing new constitutive relationships for the impact of clay minerals on the permeability of soils. The two research efforts are independent (and carried out by different team members), but their results inform each other.

In addition to providing an advanced understanding of carbon cycling in soils, this initiative will enable more accurate representations of the interaction of organic or hydrophobic compounds with clay surfaces in other areas, for example, in basin modeling, CO2-enhanced oil recovery, and the remediation of soils contaminated by organic contaminants. Results on the adsorption of dissolved gases on smectite surfaces shed light on long-standing questions associated with the use of noble gases as tracers of fluid migration in the subsurface.

 

1According to Earth System Models, soil carbon content will increase significantly over the 21st century. The magnitude of this increase is poorly known (it may range from 0.01 to 0.15% annually) because of a lack of understanding of the fundamental mechanisms that control the rate of microbial degradation of soil organic matter. A 0.02% annual increase in the global soil carbon content would add 25 GtC to the soils, which is a stabilization “wedge” as defined by Pacala and Socolow (2004).

 

References

Chan, Y., 2008. Increasing soil organic carbon of agricultural land. Primefacts, 735. New South Wales Department of Primary Industries.

Gadikota G., B. Dazas, and I.C. Bourg, 2017. Molecular dynamics simulations of the solubility of gases (CO2, CH4, H2, noble gases) in water-filled clay interlayer nanopores. J. Am. Chem. Soc., in preparation.

He Y., S.E. Trumbore, M.S. Torn M.S., J.W. Harden, L.J.S. Vaughn, S.D. Allison, and J.T. Randerson, 2016. Radiocarbon constraints imply reduced carbon uptake by soils during the 21st century. Science, 353(6306): 1419-1424. doi:10.1126/science.aad4273.

Pacala, S., and R. Socolow, 2004. Stabilization Wedges: Solving the Climate Problem for the Next 50 Years with Current Technologies. Science, 305(5686): 968-972. doi:10.1126/science.1100103.

Sulman, B.N., R.P. Phillips, A.C. Oishi, E. Shevliakova, and S.W. Pacala, 2014. Microbe-driven turnover offsets mineral-mediated storage of soil carbon under elevated CO2. Nat. Clim. Change, 4: 1099-1102. doi:10.1038/nclimate2436.