Soil Type-Specific and Preferential Stabilization of Soil Organic Carbon in Subsoils of Acidic Forests.

We analyzed topsoil and subsoil samples of three acidic soils (Laxic Cambisol, Skeletic Cambisol, Entic Podzol) under Norway spruce to determine the effects of soil type on SOC stabilization in soil. The relationship between the amount and chemical composition of SOC, clay content, oxalate-extractable Fe and Al, and dithionite-extractable Fe before and after treatment with 10% hydrofluoric acid in topsoil and subsoil horizons was analyzed. 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 SOC after HF-treatment were small for the A-horizons. In contrast, for B-horizons, HF-soluble and HF-resistant SOC showed systematic differences in functional C groups. The non-mineral associated SOC 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 SOC was younger than that of the HF-resistant SOC 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 SOC was considerably older than the HF-resistant SOC. In acid subsoils a specific fraction of the SOC pool (O/N-alkyl C; microbially-derived sugars) is preferentially stabilized by association with Fe and Al minerals. Stabilization of SOC 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.