PNNL

Abstract

The formation of mineral-associated organic matter (MAOM) is a key phenomenon used to explain the slow turnover rates of carbon in soil organic matter (SOM). Despite this, many details pertaining to the structure and dynamics of MAOM remain unknown. In the present study, we use replica-exchange molecular dynamics (REMD) 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 Ca2+ charge balancing counter-ions. Our results are consistent with the hypothesized multilayer sorption (“onionskin”) model of MAOM. They also reveal a previously unknown level of complexity. In particular, the SOM coatings are partial and laterally heterogeneous, and water retains extensive access to mineral surfaces even when significant SOM sorption occurs.