Forest conversion effects on SOM composition: Disentangling effects of parent material and litter input chemistry

Mr Olaf Brock1, Dr. Annemieke Kooijman1, Dr. ir. Klaas Nierop2, Prof. Dr. Bart Muys3, Dr. Karen Vancampenhout4, Dr. Boris Jansen1

1University Of Amsterdam (IBED-ELD), Amsterdam, Netherlands, 2Utrecht University (Geolab, Faculty of Geosciences), Utrecht, Netherlands, 3KU Leuven (Department of Earth and Environmental Sciences), Leuven, Belgium, 4KU Leuven (Cluster for Bioscience Engineering), Geel, Belgium

Increasing forest carbon stocks through forest conversion is being considered in Western-Europe as potential climate change mitigation measure. Edaphic factors, such as soil pH, as well as tree species affect forest carbon cycles, but are difficult to disentangle. We studied how conversion of deciduous stands to mono-culture spruce plantations affected the soil organic matter (SOM) composition along a lithological gradient in the Mullerthal (Luxembourg) and Gaume (south-east Belgium) regions. Parent materials in these regions range from decalcified sands to calcareous marls. We used a twin plot setup of adjacent deciduous and coniferous stands on the same parent material to evaluate the effect of edaphic factors versus litter input chemistry on SOM composition and soil organic carbon (SOC) stocks. Lignin and cutin/suberin molecular proxies were identified with thermally assisted hydrolysis and methylation (THM), to distinguish litter sources (coniferous vs. deciduous and leaf litter vs. roots) in the studied stands. In this study, SOC stocks were influenced more by parent material than by forest type. Lignin yield, composition and degradation state were influenced both by litter input chemistry as by the edaphic context. Moreover, there appear to be important interaction effects between the two, as the relative importance of parent material and litter quality was site specific. These conclusions seem also valid for cutin yield, as tree type effects were absent from the Mullerthal loamy plots with more favourable pH values. We suggest that models used to estimate carbon stocks should always combine data on vegetation history and edaphic context. For forest management this study shows that if forest conversion is applied to increase forest SOM stocks, as climate change mitigation measure, this can only be a success when substrate differences are taken into account, including relatively minor substrate-directed differences in soil properties such as pH and texture.


I am a PhD student at the University of Amsterdam (UvA) in the Netherlands. My PhD is part of a larger project in which we are developing a method to reduce water permeability through sandy layers, e.g. in dykes. This method consists of injecting a mixture of dissolved organic matter (DOM) and aluminum that together form flocs or Al-DOM precipitates. These flocs/precipitates block the soil pores, thereby reduce water permeability and inhibit unwanted water flow. In my research I study the role of organic matter in the precipitation process, also known from podzolization.  I conduct this by investigating the molecular composition of different natural and commercially available DOM sources using liquid chromatography coupled to high resolution mass spectrometry (LC-QTOF-MS). I also work on the effect of forest conversion on SOM composition (former master thesis topic). For this pyrolysis-GC–MS was used to study SOM beneath deciduous and (converted) spruce forests, with a focus on lignin and cutin/suberin.


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