Impacts of sampling dissolved organic matter with passive capillary wicks versus aqueous soil extraction

Quantitative characterization of dissolved organic matter (DOM) in soil and vadose zone solution is needed to interpret mechanisms of nutrient and C cycling as well as bio-weathering processes. Passive capillary wick samplers (PCaps) are useful for soil solution sampling because they can provide measures of water and associated DOM-constituent flux in the unsaturated zone, however potential impacts of the wick material on DOM chemical properties has not been investigated yet. We therefore conducted experiments where aqueous soil extracts were transported along PCap fiberglass wicks in flow-through experiments. Results indicated limited dissolved organic carbon (DOC) sorption and DOM fractionation, and related parameters (total dissolved nitrogen [TDN], DOM fluorescence components) also remained largely unaffected. We note that this experiment does not account for the extent to which soil hydrologic processes may be affected by PCap field installations. However, given that the wicks did not fractionate significantly DOM we compared field-based PCap DOM solution collected in situ with laboratory-based aqueous soil extraction (ASE) of DOM from the same soils to assess differences in DOM quality. Spectroscopic analysis of DOM in ASE solutions showed lower O-H stretch/carboxlyate band intensity ratios, more pronounced aliphatic C-H stretching (Fourier Transform Infrared analysis) higher specific ultraviolet-absorbance (SUVA₂₅₄) values as well as greater abundance of fluorescence components in the region attributed to fulvic acids. We conclude that difference in molecular properties of DOM derived from laboratory ASE vs. PCap field collection of the same soils is attributable to differential disturbance effects of the two methods of soil solution collection.