Natural organic compounds compete to sorb to minerals via a wide range of mechanisms (Jardine et al. 1989, Kaiser et al. 2000, Guo and Chorover 2003), and desorption is only partially reversible (Gu et al. 1996). Molecules can entwine, co- dissolve and/or adsorb nearly irreversibly as a function of molecular size, functional group reactivity and diversity, and the concentrations of polyvalent metal ions function of (e.g., Chin et al. 1998, van de Weerd et al. 1999; Aufdenkampe et al. 2001, Aufdenkampe et al. 2003, Sollins et al. 2006). Such processes facilitate the formation of stable foundations for binding other molecules via cation bridging, hydrogen bonding and other mechanisms (Wershaw 1986, Sutton & Sposito 2005, Kleber et al. 2007). Molecules within these assemblages are shielded from degradation (Knicker and Hatcher 1997, Hsu and Hatcher 2005).Thus,intheactivebiospheremineralsaretypicallycoatedwith0.5-1.0mgOC/m2 largely regardless of mineralogy (e.g., Mayer 1994, Fig. 2), because nearly all OM mixtures contain compounds that can bind to every type of mineral surface. Other factors likely exert a secondary control on OC/SA, such as pH, ionic strength and polyvalent cations (Jardine et al. 1989, Day et al. 1994, Gu et al. 1995, and many others), mineralogy and in particular amorphous Al and Fe oxides (Ransom et al. 1998, Kaiser & Guggenberger 2000).

Iron oxides and oxyhydroxides have among the highest specific surface area of all mineral groups, and have long been know to be important to stabilizing soil organic matter (Torn et al. 1997, Massiello et al. 2004). However, iron minerals are subject to transformations across redox gradients, and can be affected by microbially-mediated redox processes. The importance of these redox dynamics on the stability of associated organic carbon became clear to our CRB-CZO team within our first field season. We discovered that suspended sediments during baseflow appear to have very high iron and carbon content and very high specific mineral surface area (Karwan et al, in prep), and that iron oxides co-precipitated with carbon had twice the organic carbon to surface area ratio vs. carbon sorbed onto previously precipitated iron oxides (Chen et al. in prep.). Floodplain sediments at depths where we measure redox oscillations have lower OC/SA than deposits with both higher and lower dissolved oxygen (Lazareva and Chen, unpublished). Our original conceptual model presumed that mineral surface area was conserved over decadal timescales. Since a significant fraction of OC is Fe-associated, we must better constrain the mechanisms and conditions that lead to this association. We ask:

Q1.1. What determines OC complexation potential?
H1.1. Mineral surface area, mineralogy, polyvalent cations and OM composition and concentration determine the capacity of the inorganic matrix to complex and stabilize OC.

Q1.2. What is the role of microorganisms in the formation, aging, and maintenance of Fe oxide-OC complexes?
H1.2. Microbial activity enhances the coprecipitation of Fe oxide-OC complexes which exhibit the highest OC:Fe ratios, and are formed in environments with high microbial Fe redox cycling.