Controls on mineral-associated organic matter stability: Insights from DNA adsorption and degradation on goethite in the presence of phosphate and cations

Persistence of soil organic carbon (SOC) is largely controlled by interactions between organic matter and mineral surfaces, particularly iron oxides. These interactions are further influenced by the presence of competing species and various cations in soil systems. Despite their importance, the geochemical mechanisms by which competing species regulate organic matter and mineral interactions remain poorly understood, representing a critical knowledge gap in SOC stabilization processes. DNA is a ubiquitous biomolecule in soils and sediments. It is a key component of microbial necromass and extracellular polymeric substances. While DNA represents a small fraction of SOC, the mechanisms of DNA and mineral interaction can reveal broader principles applicable to other organic matter to elucidate mechanisms that contribute towards SOC formation and stabilization.

In this study, we used DNA of varying lengths representing organic matter fractions of different sizes, and phosphate as a model competing anion, to investigate the effects of phosphate on DNA adsorption to goethite. Batch adsorption experiments were complemented by enzymatic hydrolysis studies to assess the influence of phosphate and divalent cations (Ca²⁺ and Mg²⁺) on the degradation of DNA adsorbed on goethite. Our results show that DNA adsorption to goethite is strongly influenced by DNA length, phosphate concentration, and adsorption time. Phosphate significantly reduced DNA adsorption through competitive surface site occupation. Although the presence of Ca²⁺ and Mg²⁺ enhanced DNA adsorption under phosphate concentrations that were otherwise unfavorable for adsorption, this increased adsorption did not translate into protection against enzymatic hydrolysis.

These findings demonstrate that enhanced adsorption alone does not necessarily confer long-term protection of organic matter and highlight the complex roles of competing ions and cations in regulating mineral-associated SOC persistence. Our study provides mechanistic insights into how nutrient and cation availability may influence the stabilization and turnover of reactive, phosphorus-containing organic matter in soils.