Microbial Respiration on Decaying Leaves and Sticks in a Southern Appalachian Stream

Microbial respiration on sticks and leaves, measured as rate of oxygen uptake, was compared among four sites (2nd-4th order) along Ball Creek/Coweeta Creek, Coweeta Hydrologic Laboratory, North Carolina. Senescent rhododendron and birch leaves were placed in the stream at each site on 21 October 1990. Microbial oxygen uptake rates were measured for both leaf species beginning 15 December 1990 and continuing monthly until leaves were no longer intact (birch--2 mo, rhododendron--7 mo). Oxygen uptake rates also were measured monthly for 1 yr on sticks (1-3 cm diameter) collected from the stream at each site. Oxygen uptake rates (mg O2/hr) were calculated on the basis of surface area and ash-free dry mass (AFDM). Mean respiration rates per unit surface area of substrate were highest for sticks, followed by rhododendron, then birch, indicating that substrate stability and persistence may affect microbial respiration in streams. When expressed on an AFDM basis, respiration rate was highest for birch, followed by rhododendron, then sticks. Respiration rates were significantly correlated with temperature for both rhododendron leaves and sticks on both an AFDM and surface area basis. Respiration rates on rhododendron also increased with exposure time. Rhododendron leaves represent a long-lasting leaf source for microbial activity in streams. Differences in respiration rates among sites, for both rhododendron leaves and sticks, can be explained primarily by differences in water temperature. The high respiration rate per unit surface area on sticks suggested that small woody debris can play a significant role as a substrate for microbial metabolism.