Abstract
Abstract. Freezing can affect concentrations and spectroscopic properties of dissolved organic matter (DOM) in water samples. Nevertheless, water samples are regularly frozen for sample preservation. In this study we tested the effect of different freezing methods (standard freezing at −18 °C and fast-freezing with liquid nitrogen) on DOM concentrations measured as organic carbon (DOC) concentrations and on spectroscopic properties of DOM from different terrestrial ecosystems (forest and grassland). Fresh and differently frozen throughfall, stemflow, litter leachate and soil solution samples were analyzed for DOC concentrations, UV-vis absorption and fluorescence excitation–emission matrices combined with parallel factor analysis (PARAFAC). Fast-freezing with liquid nitrogen prevented a significant decrease of DOC concentrations observed after freezing at −18 °C. Nonetheless, the share of PARAFAC components 1 (EXmax < 250 nm (340 nm), EXmax: 480 nm) and 2 (EXmax: 335 nm, EXmax: 408 nm) to total fluorescence and the humification index (HIX) decreased after both freezing treatments, while the shares of component 3 (EXmax: < 250 nm (305 nm), EXmax: 438 nm) as well as SUVA254 increased. The contribution of PARAFAC component 4 (EXmax: 280 nm, EXmax: 328 nm) to total fluorescence was not affected by freezing. We recommend fast-freezing with liquid nitrogen for preservation of bulk DOC concentrations of samples from terrestrial sources, whereas immediate measuring is preferable to preserve spectroscopic properties of DOM.
Highlights
In addition to dissolved organic carbon (DOC) concentrations, properties of dissolved organic matter (DOM) are crucial for its role in biogeochemical cycles of carbon and nutrients as well as for its effect on pollutant dynamics (Bolan et al, 2011)
Spectroscopic methods like UV-vis absorption and fluorescence spectroscopy used as single excitation– emission scans, synchronous scans and excitation–emission matrices (EEMs) in combination with different indices and/or parallel factor analysis (PARAFAC) are increasingly applied to characterize chromophoric dissolved organic matter in various environments (e.g., Murphy et al, 2008; Yamashita et al, 2010; Stedmon and Markager, 2005; Graeber et al, 2012; Otero et al, 2007, Traversa et al, 2014; Kalbitz et al, 1999)
Due to different sources of DOM in land and water environments (Bolan et al, 2011) and different chemical characteristics, it is unlikely that insights regarding the alterations of samples during storage can be transferred from one sample type to another. To help close this gap, we investigate in this study the influence of freezing and thawing on DOM concentrations measured as organic carbon (DOC) concentration, spectral absorption and fluorescence properties for a wide range of water samples from different terrestrial ecosystems
Summary
In addition to dissolved organic carbon (DOC) concentrations, properties of dissolved organic matter (DOM) are crucial for its role in biogeochemical cycles of carbon and nutrients as well as for its effect on pollutant dynamics (Bolan et al, 2011). DOM properties depend on many physicochemical and biological boundary conditions, so that artifacts caused by sample storage or sample pre-treatment may be produced . For these reasons it is recommended to directly filter samples after collection and store them in the cold and dark prior to measurement as soon as possible (Santos et al, 2010; Spencer and Coble, 2014). Immediate measurement is often not possible for practical reasons such as a large number of samples or remote or separated sampling sites, so that freezing of filtered DOM samples is often the selected storage method
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