From a thermodynamic point of view, soil matter should easily turn over, but minerals can remain in soils for many years due to the complex and reactive interactions between the minerals, plants and microbes. Due to issues with probing the interface of minerals, it has been inherently difficult to develop knowledge, other than basic qualitative data.
A team of Researchers from the USA have now used force spectroscopy to directly measure the binding interactions between organic ligands with known functional groups and soil minerals in aqueous environments.
The minerals in soil do not filter through and turnover as you would expect, and stay longer in the soil as a result. Reasons for the long residence times have been attributed to the organic molecules in the soil becoming absorbed onto the reactive surface of the mineral(s).
The absorption of such species is known to create a physical isolation and chemical stabilization at the interface which has been very difficult to probe with current techniques. This has resulted in a lack of knowledge surrounding the micron and molecular scale organic–mineral interactions, with most of the data being purely qualitative and empirical in nature.
In an effort to yield new data, the Researchers have adopted a new proof-of-concept approach using dynamic force spectroscopy (DFS). DFS is an in-situ technique which uses a functionalized AFM cantilever tip to connect with a mineral surface, creating a new surface bond, to produce quantitative molecular scale measurements of the molecular binding at a mineral surface.
In addition, DFS can also be used with very complex molecular interaction scenarios. DFS outputs a numerical value for the energy landscape of the bond by causing the cantilever bond to break via a cantilever pullback mechanism. The numerical output of DFS also allows for mechanistic insights to be generated for the interactions at long length scales and for the bond kinetics at the interface.
The Researchers have used this DFS technique to probe the interaction between mineral and organic matter in soil, to provide a quantitative outlook and evaluate the similarities (and differences) between specific functional groups and mineral types under varying environmental conditions.
The DFS analysis was performed on an ES Environmental AFM instrument using Bruker (SNL-10) cantilever tips. Atomic force microscopy (AFM) was also performed on the same instrument. The Researchers performed the experiments at room temperature and 5 loading rates between 10 and 3000 nm/s were chosen.
A constant approach velocity of 100 nm/s was implemented and a minimum of 250 data points per sample were collected. The Researchers analyzed the data using multiple bond theory and explored the near-equilibrium and far-from-equilibrium regimes. The Researchers also used X-ray diffraction (XRD, Rigaku Rapid II microbeam diffractometer) to characterize the mineral crystals.
Through their studies in aqueous environments and using model minerals, the Researchers have deduced that both the chemistry of the organic ligand and the mineral surface both contribute to the binding free energy. By varying the organic functionalities, the Researchers also found that changes in pH and ionic strength in the surrounding environment have a profound effect on the interactions and produce significant differences in the binding energies.
The data provided clear face-specific measurements that were used to address the complexity of the soil using a bottom-up approach. In this, it was found that the ligand chemistry, local environment or specific mineral faces can be probed directly.
Although the focus in this study surrounded simple functional groups, the data produced still showed the effects of binding and environment, laying a platform for more complex functional groups to be tested– molecules such as realistic biomolecules and systems characterized by ongoing biological activity.
The direct measurements surrounding the molecular binding of the organo-mineral interactions could be used to deduce mechanistic insights and form land-carbon models that specifically include mineral-bound carbon pools.
With climate change being ever more apparent in everyday life, especially through naturally occurring events such as droughts and floods, it is crucial for the scientific community to understand how the local, nano and micron scale environmental conditions affect organo–mineral interactions and the stability of soil organic matter (SOM). The novel approach to quantify the interfacial interaction binding energies at the organo-mineral interfacial boundary could be used to provide an initial molecular basis for understanding organic–mineral interactions, and ultimately help to provide a greater understanding of soil-organic-mineral interactions and improve predictions of SOM persistence.
“Developing a molecular picture of soil organic matter–mineral interactions by quantifying organo–mineral binding”- Newcomb C. J., et al- Nature Communications, 2017, DOI: 10.1038/s41467-017-00407-9