At a Glance
The Bourg group is working to resolve the fundamental mechanisms of soil carbon storage. This research uses a combination of atomistic-level simulations and laboratory and field experiments to gain insight into two natural carbon sinks: the stabilization of soil organic carbon by clay minerals and the sequestration of soil inorganic carbon by mineral weathering. The results will enable more accurate Earth System Model predictions of soil organic and inorganic carbon dynamics and provide practical strategies for enhancing the soil carbon sink.
Soils have two key impacts on the global carbon cycle. First, they store about half of the carbon present near the Earth’s surface (the other half being distributed roughly evenly between atmosphere, the biosphere, and the surface ocean), primarily in the form of organic matter. Second, they mediate mineral weathering reactions that control the drawdown of atmospheric CO2 on geologic time scales. Both processes have strong potential as carbon sinks. They have also been lauded as “triple-win” scenarios, whereby shifting the use of agricultural soils for carbon sequestration would offer increased drought resilience and fertility for food production.
Quantifying the effects different soil management practices have on these sinks remains a stumbling block in understanding the use of soils for carbon sequestration. The Bourg group’s research integrates findings from molecularand pore-scale computational work with those from laboratory and field studies to help efforts aimed at enhancing these soil carbon sinks. In 2020, this research was focused on two major tasks.
In the first task, the Bourg group is using all-atom molecular dynamics (MD) simulations of simple clay-water-organic systems to understand the mechanisms by which clay-rich soils store organic carbon. These soils are known to have a high capacity for accumulating organic carbon, yet the storage mechanism remains poorly understood (Kleber et al., 2021). Recent simulations carried out on U.S. Department of Energy supercomputers (Figure 6.1) demonstrate that anionic organic compounds – compounds with negatively charged ions – have a significant affinity for clay mineral surfaces. This affinity is driven predominantly by hydrophobic (“water repelling”) interactions, yet strongly enhanced by calcium ions (Ca2+) that act as intermediates between the mineral surface and polar organic moieties (Willemsen and Bourg, 2021).
The Bourg group has put forward two concepts that help to clarify the issue of how clay minerals store carbon. Firstly, the interaction between organic matter and minerals in soils, though often represented in soil carbon models as a chemical partitioning phenomenon (i.e., binding of organic matter to mineral surfaces), may in fact be more akin to a wetting phenomenon (i.e., affinity of organic matter for the mineral framework due to capillarity). Secondly, the apparent protectiveness of fine-grained minerals towards soil carbon, though often represented as reflecting a degree of chemical protection, may instead reflect the tendency of organic matter to mechanically reinforce mineral aggregate structures that physically occlude organic matter in pores narrower than soil microbes (Kleber et al., 2021; Yang et al., 2021).
In the second task, the Bourg group is working to determine the rates of mineral weathering in situ in the complex biogeochemical and hydrologic conditions that exist in soils. This is a key parameter controlling the extent to which soil mineral amendments can be used to sequester atmospheric CO2 through enhanced mineral weathering. This task will rely on field work carried out at the Watershed Institute, a conservation NGO located near Princeton, combined with detailed mineral characterizations carried out at Princeton and at U.S. National Laboratories (Figure 6.2). It will leverage a novel capability to determine mineral dissolution rates in soils recently developed by the Bourg group (Wild et al., 2019).
Kleber M., I.C. Bourg, E. Coward, C. Hansel, S. Myneni, and N. Nunan, 2021. Mineral-organic interactions are multifunctional and multidimensional. Nature Reviews Earth and Environment, accepted.
Wild B., D. Daval, J.S. Micha, I.C. Bourg, C.E. White C.E., and A. Fernandez-Martinez, 2019. Physical properties of interfacial layers developed on weathered silicates: A case study based on labradorite feldspar. Journal of Physical Chemistry C 123, 24520-24532. (https://doi.org/10.1021/acs.jpcc.9b05491).
Willemsen J., and I.C. Bourg, 2021. Molecular dynamics simulation of the adsorption of per- and polyfluoroalkyl substances (PFASs) on smectite clay. Journal of Colloid and Interface Science 585, 337-346. (https://doi.org/10.1016/j.jcis.2020.11.071).
Yang J.Q., X. Zhang, I.C. Bourg, and H.A. Stone, 2021. 4D imaging reveals mechanisms of clay-carbon protection and release. Nature Communications 12, 622. (https://doi.org/10.1038/s41467-020-20798-6).