Soil organic matter stabilization in acidic forest soils is preferential and soil type-specific

Minerals with large specific surface areas promote the stabilization of soil organic matter (SOM). We analysed three acidic soils (dystric, skeletic Leptic Cambisol; dystric, laxic Leptic Cambisol; skeletic Leptic Entic Podzol) under Norway spruce (Picea abies) forest with different mineral compositions to determine the effects of soil type on carbon (C) stabilization in soil. The relationship between the amount and chemical composition of soil organic matter (SOM), clay content, oxalate-extractable Fe and Al (Fe_o; Al _o), and dithionite-extractable Fe (Fe_d) before and after treatment with 10% hydrofluoric acid (HF) in topsoil and subsoil horizons was analysed. Radiocarbon age, ¹³C CPMAS NMR spectra, lignin phenol content and neutral sugar content in the soils before and after HF-treatment were determined and compared for bulk soil samples and particle size separates. Changes in the chemical composition of SOM after HF-treatment were small for the A-horizons. In contrast, for B-horizons, HF-soluble (mineral-associated) and HF-resistant (non-mineral-associated) SOM showed systematic differences in functional C groups. The non-mineral associated SOM in the B-horizons was significantly depleted in microbially-derived sugars, and the contribution of O/N-alkyl C to total organic C was less after HF-treatment. The radiocarbon age of the mineral-associated SOM was younger than that of the HF-resistant SOM in subsoil horizons with small amounts of oxalate-extractable Al and Fe. However, in horizons with large amounts of oxalate-extractable Al and Fe the HF-soluble SOM was considerably older than the HF-resistant SOM. In acid subsoils a specific fraction of the organic C pool (O/N-alkyl C; microbially-derived sugars) is preferentially stabilized by association with Fe and Al minerals. Stabilization of SOM with the mineral matrix in soils with large amounts of oxalate-extractable Al_o and Fe_o results in a particularly stable and relatively old C pool, which is potentially stable for thousands of years.