High organic matter accrual in black sand soils linked to microscale aliphatic hotspots

High organic carbon (OC) in agricultural sandy soils (>85% sand) is generally unexpected under prevailing paradigms of organic matter (OM) formation and stabilisation, which emphasise mineral protection. Nevertheless, so-called “black sand” soils are widespread in north-western Europe and store exceptionally large amounts of OC. Much of this OC resides in the finest soil fractions and is chemically enriched in alkyl C, likely reflecting historic heathland vegetation, but it remains unclear whether this enrichment is associated with minerals or occurs within largely organic microstructures. To address this, we combined particle-size fractionation, solid-state ¹³C nuclear magnetic resonance (NMR) spectroscopy, optical photothermal infrared (O-PTIR) microscopy, and nanoscale secondary ion mass spectrometry (NanoSIMS) to investigate OM organisation and composition at the microscale. Fine fractions stored a disproportionate amount of OC, showing pronounced alkyl enrichment and a decline in O/N-alkyl C. NanoSIMS revealed that OM occurred as finely divided domains rather than intact particles. OM-dominated areas expanded with increasing OC, and while some OM was associated with Al-rich domains, much showed no clear mineral association, indicating that OM accumulation is not solely driven by organo-mineral interactions. O-PTIR revealed alkyl-rich microdomains reflecting the chemical signature of historic vegetation. Aliphatic signals increased with OC content and OM coverage, indicating that SOC in black sand soils is stored as micrometre-scale, alkyl-rich micro-particulate OM. This challenges mineral-centric concepts of SOC stabilisation and highlights the importance of OM–OM interactions and land-use legacy in controlling carbon storage in sandy soils.