Biodegradation of soil-derived dissolved organic matter as related to its properties

Quantifying the contribution of dissolved organic matter (DOM) to C sequestration in soils requires knowledge about extent and rate of its biodegradation. Since degradation experiments are time consuming, estimating the biodegradability of DOM by more easily measurable properties seems valuable. Our goal was therefore to investigate the biodegradation of DOM of different origin and to relate its extent and rate to properties such as UV absorbance, synchronous and emission scan fluorescence, XAD-8 sorption chromatography and 1H-NMR spectroscopy. We extracted DOM from 13 different samples (maize straw, forest floors, peats, agricultural soils) and carried out a 90-day liquid incubation experiment. DOM biodegradation was quantified by CO2 evolution. Rapidly and slowly mineralizable portions of dissolved organic carbon (DOC) as a measure of labile and stable DOC and the respective mineralization rate constants and half-lives were calculated by a double exponential model. The extent and rate of DOM biodegradation from less humified organic material (straw, litter and fermentation layers of forest floors) were high resulting in 61–93% of DOC being mineralized. The labile fraction comprised 59–88% of total DOC. DOM extracted from agricultural soils was of an intermediate biodegradability with a CO2 evolution comprising 17–32% of total DOC. Labile DOC represented 14–25% of total DOC. DOM extracted from peats and Oa forest floor layers was relatively stable (mineralization of 4–9% of total DOC, labile DOC: 3–6%). The half-life of the labile DOC pool was short (2–5 days), whereas that of the stable DOC pool ranged from 0.2 years (DOM from less humified material) to 8.6 years (DOM from the Oa layer under spruce). Extent and rate of DOM biodegradation were closely but nonlinearly related to DOM properties. Solutions exceeding a threshold value of UV absorbance, aromaticity, XAD-8 adsorbable C or of humification indices derived from fluorescence spectra or having carbohydrate contents below a certain level were stable against biodegradation. Relatively simple methods like UV spectroscopy, XAD-8 fractionation and fluorescence emission spectroscopy were suitable to estimate the biodegradation of DOM.