Millennium timescale carbon stability in an Andisol: How persistent are organo-metal complexes?

•Two Holocene buried Andisol A horizons of 4.1 kyrs apart were studied in depth.•PP-extractable Al + Fe strongly correlated with C among all layers and size fractions.•Metals correlated more with aromatic C in younger and with alkyl C in older horizon.•Molecular recalcitrance did not fully account for the long-term C stability.•Size fraction analyses suggested reduced stability of microaggregate over time.