Genesis of Spodic Material underneath Peat Bogs in a Danish Wetland

Abstract

The chemistry of wet Spodosols (Aquods) differs from well‐drained Spodosols. Two different hypotheses have been suggested to explain the contrasting genesis of wet spodic horizons. This study attempted to determine whether Aquods at a Danish peat‐bog wetland site are a result of (i) in situ illuviation under a fluctuating water table, or (ii) degradation of a former Fe‐rich, well‐drained spodic horizon. Methods included soil surveys, wet chemical analyses, micromorphology, pollen analysis, and radiocarbon dating of soil organic matter (OM) fractions. Aquods, and soil material with spodic features, were exclusively found in sandy material at the margins of or underneath sphagnum peat bogs, whereas Inceptisols were found on well‐drained sandy deposits only. Aluminum content was very high and Fe low in spodic materials with ortstein properties. Solid‐state 13C nuclear magnetic resonance (NMR) spectra of wet sandy 2Bs horizons were dominated by alkyl C (70%) from water‐insoluble OM and were clearly distinct from NMR spectra of overlying 2Bhsm material. Pollen analysis revealed that an open forest with a thick mor layer dominated until shortly after 5000 yr BP, when sphagnum was first recorded. Radiocarbon ages of bulk soil C in the spodic horizons had mean residence times of 4500 to 4400 yr. Accordingly, the spodic B horizon was probably formed by strong in situ illuviation of Al–OM complexes before the sphagnum peat bog formation. This suggests that spodic material formation and thus strong C accumulation underneath this Danish peat bog took place for a limited period, only in susceptible parent material, and at the margins of the expanding peat bog.