Biodegradability of Dissolved Organic Matter in Forest Throughfall, Soil Solution, and Stream Water

High concentrations of dissolved organic matter (DOM) were leached into rainwater passing through the canopy and forest floor of an oak (Quercus spp.)-hickory (Carya spp.) forest in the southern Appala-chian Mountains. More than 95% of this dissolved organic C (DOC) and N (DON) was removed as water percolated through the soil profile and left the ecosystem in stream water. Our objective was to examine the importance of decomposition in the removal of DOC and DON. Samples of DOM front throughfall, forest floor water, soil water from A and B soil horizons, and stream water were all adjusted to a common initial DOC concentration, inoculated with soil and stream microbes , and incubated in solution for 134 d. In general, only 14 to 33% of the DOC in forest floor, soil solution, and stream samples decomposed during the incubation. The relative order of average decomposition of DOC from the various strata was, from fastest to slowest: throughfall, Oi horizon (forest floor), Oa horizon (forest floor), B horizon, stream, AB horizon, isolated fulvic acid, and upper A horizon. In short, biodegradability of DOM in the ecosystem profile declined vertically from throughfall to the A horizon and then increased with depth. The DON generally did not decay faster than the DOC — results consistent with the idea that hydrolysis of organic N is linked to mineralization of DOC rather than occurring selectively in response to the biochemical need for N. Throughfall DOM could be decomposed during its passage through the upper soil, but decomposition seems too slow to be responsible for the bulk of removal of DON and DOC that occurs in the mineral soil. Adsorption, rather than biodegradation, is more likely responsible for maintaining low DOC substrate concentrations in the mineral soil and preventing its loss into stream water. T HE BIODEGRADABILITY of xenobiotic compounds in soil and water has been the focus of much research. The biodegradability of naturally occurring DOM has received less attention, though there has been considerable research on the fate of fresh autumn leaf litter leachate and on the ability of stream and lake DOM to support the growth of bacteria in those At the same time, there is a widespread belief that dissolved humic substances , which make up much of natural DOM, are quite resistant to decomposition. For obvious reasons, DOM in soil has received much less attention than that in lakes and streams. The …