Using Fluorescence Spectroscopy to Link Changes in Soil Chemistry with Stream Dissolved Organic Matter.

Over the last two decades concentrations of dissolved organic matter (DOM) have increased significantly in surface waters throughout much of the northern hemisphere. While much attention has been paid to patterns and drivers of increasing DOM (e.g., declining sulfate deposition), little is known about associated changes in DOM source and quality with either acidification or recovery from acidification.  Changes in DOM quality and source have the potential to affect ecosystem services that involve drinking water, carbon cycling and aquatic habitat. We used a combination of intensive stream sampling, lysimetry and soil leaching experiments at the Bear Brook Watershed in Maine (BBWM) and Fernow Experimental Forest (FEF) in West Virginia, two long-term (>23 years), paired, whole-watershed nitrogen enrichment and acidification experiments using ammonium sulfate amendments to examine the effects of acidification and recovery on DOM source and quality in these forests.  All solutions were analyzed for dissolved organic carbon concentration, UV absorbance, excitation-emission matrices (EEMs) using fluorescence spectroscopy, and fluorescence index (FI).  Fluorescence and absorbance spectroscopy provide information regarding the composition, source, and quality of DOM. A lower FI indicates more aromatic, less labile DOM. Samples from whole-watershed acidification experiments provide evidence in support of an ‘acidification mechanism’ driving changes in DOM quality. DOM from reference watersheds exhibit a consistently lower FI, with the exception of summer base flows that do not interact with upper soil horizons most influenced by acid deposition.