Poorly crystalline minerals protect organic carbon in clay subfractions from acid subsoil horizons

Minerals with less than perfect crystallinity (poorly crystalline minerals) are ubiquitous in pedogenetic active soils as a result of mineral weathering. However, systematic investigations into the importance of poorly crystalline phases for carbon stabilisation in soils are scarce. We used clay subfractions from 12 acid subsoil horizons of differing pedogenetic provenance to test the influence of poorly crystalline minerals on carbon stabilisation. Stable organic carbon (OCstable) was isolated by chemical treatment using NaOCl. Poorly crystalline minerals were characterised chemically by selective dissolution with acid oxalate and dithionite–citrate and physically by surface area determinations (BET–N2). Fine clay contained slightly but insignificantly more stable OC and total N than coarse clay fractions. The majority of the variability of stable OC concentrations (r2=0.58, p<0.01 in coarse and r2=0.84, p<0.001 in fine clay) was explained by linear regression using a parameter, which represents poorly crystalline Fe and Al minerals [OCstable=f(Fe+Al)o]. Stable OC was neither related to dithionite-extractable Fe (Fed) nor to mineral surface area. The absence of a relation between stable OC and total mineral surface area points to a coupling mechanism requiring specific surface sites on mineral surfaces.