Direct Evidence for Thickening Nanoscale Organic Film at Soil Biogeochemical Interfaces and its Relevance to Organic Matter Preservation
Emerging consensus on organic matter (OM) cycling in soil and sediment proposes that a continuum of biological and geochemical processes in micro-environment controls the fate of OM. However, spatio-temporal observations of the biogeochemical nature and behaviour of OM at the soil-water interfaces (SWI) are impeded by the heterogonous and opaque nature of its microenvironment. Here, we used a novel SoilChip method (soil microarrays incubated with a predefined solution) to continuously mimic and trace the OM biogeochemistry at SWIs for 21 days. Combining X-ray photoelectron spectroscopy and ion sputtering on SoilChips, we provide the first direct evidence that nanoscale organic film of distinct composition and thickness gradually formed at the SWI in 21 days of cultivation. Although the OM coatings on the SWI quickly reached equilibrium within 4 days, the formation of thicker mineral-organic association (MOA, 20-130 nm) and biofilm (>130 nm) continues, partially at the cost of thin MOA (< 20 nm). Consistent with the thickening SWI, the bioavailability of nutrients (dissolved organic C and ammonium) decreased gradually over 21 days, conversely restrained the microbial activities. Collectively, thickening SWI act as a biogeochemical gate regulating bioavailability of specific organic compounds and determining its preservation or microbial mineralization. Further, thickening organic film at SWIs in the z-axis direction provides a direct structural insight to increase carbon sequestration in soil and sediment.