Fig. 4: Gradient in organic composition across the organo–organic interface.

Distinct combinations of carbon/nitrogen (C/N), carbon/oxygen (C/O), and aromatic/alkyl C ratio across the organo–organic interface shows a gradient of organic matter (OM) composition at the ~50 nm scale. a Annular dark-field (ADF) scanning transmission electron microscopy (STEM) detail of the interface between organic phases in a soil thin section. Boxes 1 through 5 indicate regions used to compute average electron energy-loss (EEL) spectra across the interface (lateral distance). b Ratios of normalized (maximum = 1 unit) elemental integrated area for C/N, C/O, and aromatic/alkyl C across the interface. The aromatic/alkyl C ratio is the ratio of integrated intensity in EELS spectral regions (defined here as aromatic from 284.25 to 285.75 eV and alkyl from 286.0 to 287.5 eV) normalized to total C integrated area (280.0–315.0 eV). Each location (1–5) across the interface shows a different combination of C/N, C/O, and aromatic/alkyl C. At point 4, alkyl C is enriched relative to the high-alkyl C matrix. The calculations for each point are from ~540 individual spectra (1 spectrum per pixel) in each 11 × 44 nm box. c Rate of change for C/N and aromatic/alkyl ratios across the organo–organic interface relative to box 1. The C/N ratio decreases more rapidly than the aromatic/alkyl ratio for the transitions from boxes 1 to 2 and 1 to 3, which suggests trends in C composition and N quantity are decoupled across the interface. d Average EEL spectra (spline curve, an average of ~540 individual spectra) for boxes 1–5 for the carbon, nitrogen, and oxygen K-edges. Carbon K-edge EELS show similar intensity but a change in fine structure. For carbon, vertical lines at 285.0 and 287.0 eV are putatively associated with aromatic and alkyl C, respectively. Nitrogen and oxygen K-edge EELS show an increase and decrease in intensity across the interface, respectively.