Mineral colloids mediate organic carbon accumulation in a temperate forest Spodosol: depth-wise changes in pore water chemistry

Mobile submicron mineral phases within soil pore waters are presumed to play a major role in the transport and availability of organic carbon (OC) in subsurface horizons. This work reports on the composition of the<0.45 mu m and0.45-1.2 mu m size fractions of extracted soil pore waters from horizons of a well-drained-Spodosol-soil to reveal conditions favorable for carbon mobility and accumulation. These operationally defined quantities are abbreviated SF-S (size fraction small) and SF-L (size fraction large) to identify the<0.45 mu m and the 0.45-1.2 mu m size fraction filtrates, respectively. It is found that in the SF-S fraction, OC mass concentrations are more than 30-50 times higher than metal (M=Fe+Al) mass concentrations in all Spodosol horizons, with metal-to-carbon (M/C) atomic ratios of 0.15-0.03. Chemical equilibrium modeling calculations estimate>95% of the total Fe and Al in SF-S are complexed with OC in all Spodosol horizons. In contrast with the SF-S, the SF-L had much greater OC concentrations and even lower M/C (<0.01), except in the Bh horizon (M/C=0.05). In Bh, major accumulation of organic matter occurred above the lesser-accumulating Bhs, the latter having higher pH, but much lower OC in all forms (soil, colloidal). Infrared spectroscopy indicates the SF-L fraction of soil pore waters contains both organic and inorganic constituents, including amorphous silica, the second-most abundant component after OC. Mineral-organic associations such as mineral crystallites embedded in OC are observed in the SF-L fraction by transmission electron microscopy (TEM). Transmission electron microscopy also reveal carbon-rich amorphous structures containing traces of Fe, Al and Si, and small (similar to 100nm) spherical amorphous SiO2 particles. These observations provide support for the main mechanism of OC accumulation in Spodosols being the downward movement of colloids (organic, OC-sorbed mineral and organo-mineral), followed by colloid immobilization due to a combination of increases in pH and M/C ratio. The occurrence of these three types of colloidal structures in pore waters seems to depend on the pH and the relative supply of OC and Fe+Al to pore waters. Similar colloidal structures might also contribute to the transport and availability of OC in subsurface horizons of soils that range in the accumulation of organic and organo-metallic compounds, that is, in the expression of spodic properties.