Selective Adsorption of Dissolved Organic Matter by Calcium Bridging to Mineral Surfaces

Abstract:

It is well established that exchangeable calcium impacts the overall stabilization of organic matter in calcareous soils, but major knowledge gaps remain about how this affects soil dissolved organic matter (DOM) composition. As organic-rich solutions transit from surface litter through mineral soils, mineral surfaces can act as a selective filter that preferentially retain certain components by adsorption, altering how this carbon is ultimately distributed throughout a soil system. Previous studies have demonstrated that calcium cations can mediate the removal of organic matter via formation of inner and outer sphere complexes with mineral surfaces. Here, we systematically investigated the effect of exchangeable calcium content on the adsorption of Suwannee River Natural Organic Matter to calcite and clay minerals across environmentally relevant pH ranges. Our study paired bulk measurements with molecular-level solution phase characterization by Fourier Transform Ion Cyclotron Resonance Mass Spectrometry and spectroscopy to understand how DOM is chemically fractionated by the selective removal of components that preferentially form organic-organic and organic-mineral bridges with Ca that link organic molecules and aggregates to minerals via electrostatic interactions. We will discuss these findings and the implications for how complex DOM likely partitions between solid and solution phases in calcareous soils.