Diane McKnight1, Rose Cory1, Matthew Miller1, Colin Stedmon2

1The Institute of Arctic and Alpine Research, University of Colorado, Boulder, CO 2National Environmental Research Institute, Department of Marine Ecology Roskilde, Denmark

Although humic substances are generally resistant to microbial degradation under anaerobic conditions, some microorganisms in soils and sediments can use quinone moieties in humic substances as electron acceptors or as electron shuttles in the microbial reduction of ferric iron. In turn, ferrous iron can reduce nitrate, facilitating the formation of organic nitrogen moieties. Field studies of humic electron shuttling processes can be carried out by characterizing the oxidation state of quinone moieties in humic substances at natural concentrations using fluorescence spectroscopy. Analysis of excitation-emission matrices (EEM's) using parallel factor analysis has identified more than 13 components that occur in the EEM's of most natural waters. The absorption spectra and emission spectra of 6 of the components indicate that they are chemically distinct groups of quinone-containing humic molecules of varying redox state. We have used fluorescence spectroscopy to show that humic substances are important in electron transport reactions in coastal marine sediments and in the water columns of ice-covered lakes. Gradients in humic redox state may also occur as stream water is exchanged with water in associated hyporheic zones. We conducted a conservative tracer injection experiment in an alpine stream-wetland system located in the Front Range of the Colorado Rocky Mountains. In this system, concentrations of nitrate and dissolved organic carbon both increase with the onset of snowmelt as nitrate deposited in the snowpack is mobilized and DOC is flushed from upper soil horizons. During the tracer experiment, we sampled wells adjacent to the stream and found that lower nitrate concentrations occurred in wells with slower hyporheic exchange and more reduced dissolved humic substances. These results suggest that humic redox shuttling may be an important process linking carbon, nitrogen and iron cycling in watersheds.