Soil organic matter and mineral surface interactions are governed by the anchoring pathway

Soil organic carbon (SOC) is the largest terrestrial carbon reservoir and acknowledged to play an important role in climate mitigation. It has been advocated to store additional SOC in stable forms to provide efficient climate change mitigation. For this, mineral associated organic matter (MAOM) is considered to be relatively stable and has longer residence times than faster cycling particulate organic matter. However, we require a better mechanistic understanding of microscale interactions and the controlling factors of MAOM formation. Here we used arable soils across Germany representing three texture classes (sandy, loamy and clayey), two levels of SOC contents (low and high; 7-73 mg OC g^-1 soil) and consequently a range of SOC loading in the MAOM fraction (2-122 mg OC g^-1 fraction). The soils were incubated for two years after an addition of highly ¹³C labelled (8 atom%) barley-litter and fractionated by particle size to extract the MAOM (<20 µm). We applied mid-infrared spectroscopy and synchrotron-based scanning transmission X-ray microscopy coupled with near-edge X-ray absorption focusing on C K-edge spectra (STXM C NEXAFS) to investigate MAOM composition. Following the STXM C NEXAFS analysis, we conducted nano-scale secondary ion mass spectrometry imaging (NanoSIMS) to separate the labelled new from native organic matter. Organic matter composition derived from mid-infrared spectroscopy of the bulk MAOM fraction aligned well with the total STXM C NEXAFS spectra obtained for the different soils. Overall, the composition of the MAOM was controlled by SOC loading rather than texture with more processed and oxidized organic matter in soils with low organic carbon contents. On the microscale, organic matter was patchy distributed and the majority of the mineral surfaces were free of organic matter. Newly formed ¹³C-enriched patches that formed directly on mineral surfaces were more oxidatively-transformed (carboxylic groups) compared to ¹³C co-located with existing organic matter (dominant aromatic moieties). These findings provide direct evidence that soil carbon storage is governed by distinct and preferential accrual pathways shaping local ‘anchoring’ types, and thus, surface attachment mediates the composition of newly incorporated MAOM.