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1. Metabolic and biogeochemical processes in hyporheic zones may depend on inputs of coarse particulate organic matter. Our research focused on how differing quantity and quality of organic matter affects metabolism and nutrient retention in the hyporheic zone of a first-order Appalachian stream.

2. Sixteen plots were established on a tributary of Hugh White Creek, NC, U.S.A. Sediment was extracted and treated with leaves, wood, plastic strips or remained unamended. Following treatment, sediment was returned to the stream and, approximately 3 months later, samples were removed from each plot.

3. Aerobic and anaerobic metabolism were measured as the change in O-2 and CO2 in recirculating microcosms. At the same time, we monitored other possible terminal electron accepting processes and changes in nutrient concentrations. Aerobic metabolism was low in all treatments and respiratory quotients calculated for all treatments indicated that metabolism was dominated by anaerobic processes.

4. Rates of anaerobic respiration and total (combined aerobic and anaerobic) respiration were significantly greater (P < 0.05) in plots treated with leaf organic matter compared to controls.

5. Addition of leaves, which had a low C:N ratio, stimulated respiration in hyporheic sediments. Anaerobic processes dominated metabolism in both control and amended sediments. Enhanced metabolic rates increased retention of many solutes, indicating that energy flow and nutrient dynamics in the subsurface of streams may depend upon the quantity and quality of imported carbon.