The adsorption of dissolved organic matter (DOM) to metal (oxy)hydroxide mineral surfaces is a critical step for C sequestration in soils. Although equilibrium studies have described some of the factors controlling this process, the molecular-scale description of the adsorption process has been more limited. Chemical force spectroscopy revealed differing adhesion strengths of DOM extracted from three soils and a reference peat soil material to an iron (oxy)hydroxide mineral surface. The DOM was characterized using ultrahigh-resolution negative ion mode electrospray ionization Fourier Transform ion cyclotron resonance mass spectrometry. The results indicate that carboxyl-rich aromatic and N-containing aliphatic molecules of DOM are correlated with high adhesion forces. Increasing molecular mass was shown to decrease the adhesion force between the mineral surface and the DOM. Kendrick mass defect analysis suggests that mechanisms involving two carboxyl groups result in the most stable bond to the mineral surface. We conceptualize these results using a layer-by-layer "onion" model of organic matter stabilization on soil mineral surfaces.

13The competition between orthophosphate and water-extractable organic matter 14 (WEOM) for adsorption to iron (oxy)hydroxide mineral surfaces is an important factor in 15 determining the plant bioavailability of P in soils. Chemical force spectroscopy was used to 16 determine the binding force between orthophosphate and iron (oxy)hydroxide that was coated 17 onto atomic force microscopy (AFM) tips and adsorbed with WEOM. The force measurements 18 were conducted at pH 4.65 and 0.02 M ionic strength which are representative of typical acid 19 soil solutions. The chemical composition of the WEOM was determined by ultrahigh resolution 20 electrospray ionization Fourier transform ion cyclotron mass spectrometry. The results indicate 21 a correlation between aromatic WEOM molecules that are greater than 600 Daltons and the 22 reduced binding force of orthophosphate to WEOM-adsorbed iron (oxy)hydroxide AFM tips 23 suggesting that the molecular mass of aromatic WEOM molecules plays a critical role in 24 regulating the WEOM-P interactions with surface functional groups of minerals. Based on the 25 results of this study, we show the importance of obtaining a detailed, molecular-scale 26understanding of soil processes that can help develop better management strategies to reduce 27 waste of limited P resources and adverse environmental impacts. Specifically, soil amendments 28 with greater content of high molecular mass aromatic components may positively affect 29 dissolved P use efficiency in soils by maintaining P is soil solution.