Direct evidence for thickening nanoscale organic films at soil biogeochemical interfaces and its relevance to organic matter preservation

The emerging consensus on organic matter (OM) cycling in soil and sediment proposes that a continuum of biological and geochemical processes in the micro-environment controls the fate of OM. However, spatio-temporal observation of the biogeochemical nature and behaviour of OM at the soil-water interfaces (SWIs) is impeded by the heterogonous and opaque nature of their microenvironment. Herein, 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 provided the first direct evidence that a nanoscale organic film with a distinct composition and thickness gradually formed at the SWI within 21 days of cultivation. Although the OM coating on the SWI quickly reached equilibrium within 4 days, the formation of thicker mineral-organic association (MOA, 20-130 nm) and microbial biomass (>130 nm) continued, partially at the cost of the thin MOA (<20 nm). Consistent with the thickening SWI, the bioavailability of nutrients (dissolved organic C and ammonium) decreased gradually over 21 days, which restrained the microbial activities. Collectively, thickening SWIs act as a biogeochemical gate to regulate the bioavailability of specific organic compounds and determine their preservation or microbial mineralization. Further, thickening SWIs in thez-axis direction provide direct structural insight to increase carbon sequestration in soil and sediment. © The Royal Society of Chemistry 2020.