Vertical distribution, age, and chemical composition of organic carbon in two forest soils of different pedogenesis

Recent carbon inventories have shown that significant amounts of soil organic matter (SOM), even though at low concentrations, can be stored in the subsoil (B and C horizons). Its quantity, turnover and chemical composition are largely unknown. The objective of the present study was to analyse the organic matter in the mineral horizons of two forest soils with different pedogenetic history and to assess the quantity, turnover and chemical composition of SOM stored in the subsoil compared to the topsoil (A horizon and litter layers). Samples were taken from a Dystric Cambisol under beech and a Haplic Podzol under spruce forest down to a depth of 140 and 80 cm, respectively. They were analysed for elemental composition, carbon storage and chemical structure of SOM by 13C CPMAS NMR spectroscopy, radiocarbon age by accelerator mass spectrometry and plant derived phenols as well as hydroxyalkanoic acids by CuO oxidation. Special attention was drawn to the contribution of phenols and hydroxyalkanoic acids, because they are major contributors of root litter. Up to 75% of the organic carbon present in the mineral soil of the two profiles was found below the A horizon. Radiocarbon measurements showed that the organic carbon in the subsoil had an apparent age of several thousand years. The structural analyses indicated a similar chemical composition of the organic matter present in the litter layers of both soils. In the mineral soils, the chemical composition of the SOM differed according to the pedogenetic processes operating at the two sites. A high contribution of alkyl carbon was recorded in the B horizons of the Dystric Cambisol which may be partly explained by the contribution of hydroxyalkanoic acids which are preserved preferentially in this soil compared to phenols. In the Haplic Podzol, spectra of the B horizons indicate a higher contribution of O-alkyl and carboxylic carbon due to carbon leaching during podzolisation. In the C horizons of both soils, most of the organic carbon was mobilised after demineralisation by treatment with 10% hydrofluouric acid (HF) and may therefore be adsorbed to the soil minerals. Our data indicated that there is, apart from root litter, a strong influence of soil-forming processes on the composition of organic carbon in subsoils.