Mineral-associated organic matter is heterogeneous and structured by hydrophobic, charged, and polar interactions

Journal Article · 07 November 2024 · Proceedings of the National Academy of Sciences of the United States of America

DOI: https://doi.org/10.1073/pnas.2413216121 · OSTI ID:2476758

Underwood, Thomas R.  ^[1]; Bourg, Ian C.  ^[2]; Rosso, Kevin M.  ^[3]

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1. Physical and Chemical Sciences Directorate, Pacific Northwest National Laboratory, Richland, WA 99352, Department of Civil and Environmental Engineering, Princeton University, Princeton, NJ 08544
2. Department of Civil and Environmental Engineering, Princeton University, Princeton, NJ 08544, High Meadows Environmental Institute, Princeton University, Princeton, NJ 08544
3. Physical and Chemical Sciences Directorate, Pacific Northwest National Laboratory, Richland, WA 99352

The formation of mineral-associated organic matter (MAOM) is a key phenomenon that may explain the slow turnover rates of carbon in soil organic matter (SOM). Despite this, important details pertaining to the structure and dynamics of MAOM remain unknown. In the present study, we use replica-exchange molecular dynamics simulations to gain insight into the structure of MAOM on the surface of prototypical phyllosilicate clay and Fe-oxide minerals, montmorillonite and goethite, fine-grained minerals that strongly impact soil carbon dynamics in temperate and tropical regions, respectively. We examine the impact of aqueous chemistry through the presence of either Na ${}^{+}$ or Ca ${}^{2+}$ charge balancing counterions. Our results are consistent with the hypothesized multilayer sorption (“onion-skin”) model of MAOM and help to explain previous observations regarding the patchy distribution of SOM on mineral surfaces. In particular, the SOM coatings are partial and laterally heterogeneous, and water retains extensive access to mineral surfaces even when significant SOM sorption occurs. Low molecular weight neutral SOM molecules ( $<$ 200 Da) infrequently interact with the mineral surfaces nor their sorbed organic matter coatings and are increasingly labile with decreasing molecular weight. This observation is inconsistent with a central feature of the predominant soil continuum model of SOM and suggests that further iterations of the conceptual model may be required.

• Sponsoring Organization: USDOE
• Grant/Contract Number: SC0018419
• OSTI ID: 2476758
• Journal Information: Proceedings of the National Academy of Sciences of the United States of America, Journal Name: Proceedings of the National Academy of Sciences of the United States of America Journal Issue: 46 Vol. 121; ISSN 0027-8424
• Publisher: Proceedings of the National Academy of SciencesCopyright Statement
• Country of Publication: United States
• Language: English