Effect of dissolved organic carbon quality on microbial decomposition and nitri®cation rates in stream sediments ERIC A. STRAUSS and GARY A. LAMBERTI Department of Biological Sciences, University of Notre Dame, Notre Dame, IN, U.S.A SUMMARY 1. Microbial decomposition of dissolved organic carbon (DOC) contributes to overall stream metabolism and can in¯uence many processes in the nitrogen cycle, including nitri®cation. Little is known, however, about the relative decomposition rates of different DOC sources and their subsequent effect on nitri®cation. 2. In this study, labile fraction and overall microbial decomposition of DOC were measured for leaf leachates from 18 temperate forest tree species. Between 61 and 82% (mean, 75%) of the DOC was metabolized in 24 days. Signi®cant differences among leachates were found for labile fraction rates (P < 0.0001) but not for overall rates (P  0.088). 3. Nitri®cation rates in stream sediments were determined after addition of 10 mg C L ±1 of each leachate. Nitri®cation rates ranged from below detection to 0.49 lg N mL sediment ±1 day ±1 and were signi®cantly correlated with two independent measures of leachate DOC quality, overall microbial decomposition rate (r  ±0.594, P  0.0093) and speci®c ultraviolet absorbance (r  0.469, P  0.0497). Both correlations suggest that nitri®cation rates were lower in the presence of higher quality carbon. 4. Nitri®cation rates in sediments also were measured after additions of four leachates and glucose at three carbon concentrations (10, 30, and 50 mg C L ±1 ). For all carbon sources, nitri®cation rates decreased as carbon concentration increased. Glucose and white pine leachate most strongly depressed nitri®cation. Glucose likely increased the metabolism of heterotrophic bacteria, which then out-competed nitrifying bacteria for NH 4 + . White pine leachate probably increased heterotrophic metabolism and directly inhibited nitri®cation by allelopathy. Keywords: dissolved organic carbon (DOC) quality, leaf leachates, microbial decomposition, nitri®cation, nutrient cycling, stream Introduction Metabolism in stream ecosystems often is dependent on inputs of allochthonous organic matter (Fisher & Likens, 1972; Vannote et al., 1980). Direct input and lateral blow-in of forest litter, especially leaves, can account for a substantial portion of the total carbon budget of streams (Webster, Wallace & Ben®eld, 1995). Because of the importance of leaves to stream meta- bolism, many studies have focused on determining species-speci®c processing rates or understanding the factors in¯uencing those rates in stream ecosystems (e.g. Webster & Ben®eld, 1986; Maloney & Lamberti, 1995). Considerably less is known about how micro- bial decomposition rates of dissolved organic carbon (DOC) leached from leaves differ among tree species. Dissolved organic carbon enters streams through several pathways including overland ¯ow, ¯ushing of soil water, groundwater inputs, autochthonous inputs from algal exudates, and direct leaching from litter after it enters the stream (Kaplan & Newbold, 1993; Correspondence and present address: Eric A. Strauss, U.S. Geological Survey, Upper Midwest Environmental Sciences Center, 2630 Fanta Reed Road, La Crosse WI 54603 USA. E-mail: eric_strauss@usgs.gov Freshwater Biology (2002) 47, 65±74 Ó 2002 Blackwell Science Ltd 65