Determination of dissolved organic carbon in soils with UV spectroscopy, ultrasonic dispersion pre-treatment and separation with size exclusion chromatography .


This study aimed to reveal differences in UV (ultraviolet) -absorbing dissolved organic carbon (DOC) between three prominent Austrian soil types: a Cambisol and a Chernozem developed from Tertiary marl, both under agricultural management, and a Podzol from a mixed coniferous beech forest stand. Topsoil samples (0–300 mm) were pre-treated, air-dried, sieved and four grams of each probe was added to 80 cm3 of de-ionized water and subjected to ultrasonic treatment with specific energies of 6.7 J cm-3 and 161 J cm-3, respectively, which dispersed the macroaggregates and released formerly occluded soluble carbon. The soils were investigated for morphological differences with a scanning electron microscope after sonication. The suspensions were filtered < 0.45 µm and UV-spectroscopy at 254 nm was performed after the dispersion pre-treatment. In addition the suspension was separated by high performance size exclusion chromatography linked to an UV-vis detector measuring at 254 nm and 210 nm and dissolved organic carbon (DOC) was determined. More DOC was released with higher specific energies for all soil types in the sequence Podzol > Cambisol > Chernozem but the differences in SOM/DOC ratio became less significant with increasing ultrasonic energy. The detected molecules were in the range of 1300-1600 Da for Cambisol, 1500-5400 Da for Chernozem and 1700-10400 Da for Podzol. The different energy levels reached different carbon pools. Based on a model according to von Lützow et al. (2008), the applied energy levels of 6.7 J cm-3 reached the active carbon pool consisting of plant residues and exudates, and microbial/faunal biomass and residues. Sonication with 161 J cm-3 dispersed more aggregate fractions and released carbon from the intermediate carbon pool where biogenic aggregation preserves the organic matter pool.
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