Abstract

Catchment exports of dissolved organic matter (DOM) in streamflow can be influenced by multiple sources, which, may vary with hydrologic conditions or seasons. Thus, understanding the concentrations and quality of DOM for potential watershed sources is critical to assessing the dynamics of DOM. We investigated the quality of DOM across various watershed sources in a 12 ha forested watershed located in the Piedmont region of the mid-Atlantic USA. Sampling was performed over a two-year time period (2008-2009) and included: rainfall, throughfall, litter-leachate, soil water, riparian and wetland waters, seeps, stream runoff, and shallow and deep groundwaters. DOM constituents were characterized using ultraviolet (UV) absorption and PARAFAC modeling of fluorescence excitation emission matrices (EEMs). Specific indices that were used include: UV absorption coefficient at 254nm (a254), specific UV absorbance at 254nm (SUVA254), spectral slope ratio (SR), humification index (HIX), fluorescence index (FI), biological index (BIX), and percent humic-like and protein-like components. Our results indicated that of all the watershed DOM sources litter-leachate had the highest aromatic (high values of a254, SUVA, % C5) and humic (high HIX) content. Aromatic and humic content of DOM then decreased with soil depth with lowest values for deep groundwaters and seeps. In addition, the SR index indicated a decrease in molecular weight of DOM with soil depth. Taken together, these indices suggest that the aromatic and high molecular weight fractions of DOM were preferentially removed by sorption as runoff water percolated through the soil profile. While throughfall was less aromatic than litter leachate, it was more aromatic than the other watershed compartments. The aromatic and humic content of soil and stream water was intermediate between litter-leachate and deep groundwaters. In contrast to the trend in aromatic DOM, the % of protein-like DOM component increased with soil depth and was highest for deep groundwaters and seeps. The trend in protein-like DOM was also supported by an increase in FI and “microbial” DOM component (% C3) with soil depth. These high values of % protein, FI, and % C3 suggests that in comparison to surficial sources of DOM, deep groundwaters and seeps had a greater proportion of bioavailable DOM. This study highlighted that DOM amount and quality can vary considerably across watershed compartments and assessing this variability is critical for characterizing DOM. Lastly, this study also showed that fluorescence techniques can be useful and can provide important insights into DOM quality.

Contact Information
Shatrughan Singh, Newark, Delaware, USA, 19713

Citation

Singh, S., S.P. Inamdar, N. Finger, M.J. Mitchell, D.F. Levia, D. Scott, H. Bais. (2010): Quality of Dissolved Organic Matter (DOM) in Watershed Compartments for a Forested Mid-Atlantic Watershed. Abstract B13D-0508. AGU Fall Meeting, San Francisco, CA, December 13-17..