I. Abstract
Most of the terrestrial carbon is stored in the soil with forest soils containing 40-70% of all soil carbon.
Therefore knowledge of SOM composition and decomposition in forest soils is crucial to identify
natural sinks of carbon and enhance carbon sequestration, as a way to mitigate climate change. The
aims of this project were (I) to study soil organic matter (SOM) under coniferous and deciduous forests,
(II) to see how SOM is influenced by substrate type, (III) to search for relationships between
environment, species and SOM chemistry in forests of the temperate climate zone and (IV) to present
data on the effectiveness of both deciduous and coniferous forests to sequester carbon. In the Gaume
(Belgium) and Mullerthal (Luxembourg) study areas, deciduous forests have been partly converted to
coniferous forests, often monocultures of spruce (Picea abies). Soil, vegetation and litter samples were
taken from adjacent coniferous and deciduous forest plots. Those twin plots were located on different
geological substrates. Thermally assisted hydrolysis and methylation (THM) with unlabelled tetra
methyl ammonium hydroxide (TMAH) combined with GC/MS was used to analyse lignin, cutin and
suberin. The lignin S/G ratio was used as a source proxy and the Ac/AlG and ΓG ratio were used as
indicators of lignin decomposition. Cutin and suberin derivatives were used to analyse which part of
the SOM originated from above ground biomass (cutin) and which part from below ground biomass
(suberin). In addition SOC and FFC stocks were determined. The S/G ratio shows that a different litter
input of either needles or deciduous leaves, creates a different SOM composition. Moreover in the
Gaume there is a signal of former deciduous forest in the deeper soil layers under the current spruce
plantations, suggesting a different vegetation history in both study areas. Geological substrate had a
large influence on soil pH and thereby also on soil fertility and SOM composition and decomposition.
In both the Gaume and Mullerthal, lignin is more decomposed in the coniferous forest soils than in the
deciduous forest soils due to faster decomposition of SOM under more acid conditions. In both study
areas, lignin is preferentially decomposed with depth, which is in line with the current dogma that
lignin turns over faster than the bulk SOM. The higher suberin yield found for the sandy substrate
group, with lower pH, compared with the loamy substrate group in the Mullerthal, supports the
hypothesis that at a low pH, root input becomes more important, because of a lack of bioturbation.
Substrate is a more important factor for carbon sequestration than forest type. SOC stocks are higher
on a marl or limestone substrate than on a sandy substrate. This study supports the assertion by
Vancampenhout et al. (2010) that vegetation, through litter input, is a very important determining
factor of the extractable SOM composition in maritime temperate forests when climatic conditions are
comparable. However in addition this study also highlights the important role of substrate.