Changes in DOM Quantity and Quality in a Southern Rockies Forested Catchment

Forest fires change the processes governing dissolved organic matter (DOM) transport. These shifts may result in adverse water quality impacts in headwater catchments and on receiving water bodies downstream. DOM is a key control on water quality and DOM provides nutrients that are consumed by the surrounding ecosystem. In order to investigate how DOM in the hydrologic system is impacted by a forest fire, data from three catchments in the Valles Caldera National Preserve were examined. This study sought to address two research questions. First, after a forest fire occurs, is there a significant impact on DOM quantity? Second, how does DOM quality vary in response to catchment processes and post-fire processes? Source water mixing analysis was combined with cutting edge DOM indices analysis using fluorescence excitation–emission matrix spectroscopy (EEMs). Source water mixing results identify groundwater contributions from short and long residence time waters as the predominant sources of water to streams in the Valles Caldera. The relative source water compositions identified as present in the stream were used to develop calculated DOM quality indices in the stream. These calculated compositions consistently showed expectations that were less ‘humified’ and less ‘microbially processed’ than observed compositions. These pre-fire results indicate that DOM is consumed, likely by microbial processes, between the source water locations and the location of stream observations. The shift in index values indicates that these consumption processes remove relatively labile DOM and leave behind more recalcitrant DOM with a ‘humic-like’ signature. In the post-fire period this pattern continued but with significantly higher DOM and more humic DOM index values. Enhanced aromaticity of DOM suggests that some of this humic signature enhancement may result from solubilization of biochar.