Elucidating the composition of organic matter in water-repellent forest soils using analytical pyrolysis combined with gas chromatography–mass spectrometry (Py-GC–MS)

Soil water repellency (SWR) is an inability of soil to spontaneously absorb water. It affects the spatial distribution of soil moisture, may cause formation of preferential flow, increase surface runoff and the risk of water erosion, or affect nutrient availability for crops. Although there is a general agreement among published data that SWR is caused by hydrophobic components of soil organic matter (SOM), more comprehensive information is needed. Additionally, the link between SWR occurrence and SOM genesis has not been studied sufficiently. In this work, we analyzed SOM in water-repellent soils by pyrolysis combined with gas chromatography and mass spectrometry (Py-GC–MS). The samples were taken from two forest ecosystems in Slovakia: relatively dry pine–oak forest, located in the Záhorská nížina lowland (Záhorie), and spruce forest in the High Tatras with a colder and more humid climate. The results showed that the composition of SOM at both sites is dominated by partially decomposed residues of plant biomass. Whereas most compounds detected in the Záhorie samples were identified as pyrolysis products of lignin, the pyrolysates of the High Tatras soils originated mainly from polysaccharide-like substances. Although lauric (12:0), myristic (14:0) and palmitic (16:0) fatty acids were detected in pyrograms of water-repellent samples, the results suggest that the residues of lignin decomposition may contribute to SWR as well. Pyrolysates such as methylguaiacol, vinylguaiacol, dimethylphenol, or the methyl ester of vanillic acid are examples of less polar lignin-like moieties. Py-GC–MS data, as well as other soil properties, suggest that SWR at both sites is associated with the accumulation of certain SOM components in the topsoil. Acidic pH, higher C/N ratio and coarse soil texture prevent organic carbon stabilization in the studied soils. As a result, SOM is in great part composed of accumulated particulate residues with lignin-like origin and lipidic coatings, which contribute to SWR development during drier conditions.