Evaluation of Watershed Characteristics on Dissolved Organic Matter (DOM) quality and quantity at the Hubbard Brook Experiment Forest, NH

The evaluation of DOM attributes in surface watersheds that have different abiotic and biotic characteristics can be used in evaluating factors that influence the concentration and chemical attributes of DOM. This understanding also can be used for determining the role of dissolved organic carbon (DOC), dissolved organic nitrogen (DON) and dissolved organic sulfur (DOS) in affecting the watershed cycling of carbon, nitrogen and sulfur. The objectives of our study are to evaluate the fate and quality of DOM with particular emphasis on comparisons among watersheds with different cutting histories (W5—whole tree harvest versus W6—reference watershed) and the relative importance of wetlands (W9 versus W6) on the mineralization of organic matter with particular focus on those factors affecting sulfur biogeochemistry We have determined that stoichiometry (C, N and S ratios) of labile and refractory DOM and the stable carbon and sulfur isotopic composition. Preliminary results suggest a shift of DOM stoichiometry by microbial degradation resulting in the more refractory DOM having a higher C:N ratio compared to labile DOM. Also, there is a higher proportion of labile DON as a fraction of TDON compared to the proportion of labile DOC as a fraction of TDOC. These results suggest that the nitrogen in DOM is more labile than carbon. We are also looking at the stable isotopic values to evaluate differences in elemental cycling among watersheds. The range of stable carbon isotopic values in DOC is much narrower than the range of stable sulfur isotopic values in DTS and hence the δ34S values may be particularly useful in making comparisons of DOM characteristics among watersheds (Figure 1). We will be using the comparison of the values of δ34S-DOS versus δ13C-DOC values at Hubbard Brook to make inferences about differences in the cycling of sulfur and carbon in these watersheds. Figure 1. The relationship between stable C isotopes in DOC and stable S isotopes in DTS. W5; clear-cut watershed, W6; reference watershed, and W9; wetland-affected watershed.