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 µm and 0.45–1.2 µm 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 µm and the 0.45–1.2 µm 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 (~?100 nm) 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.