Microbial respiration on decaying leaves and sticks along an elevational gradient of a southern Appalachian stream

Abstract

Microbial respiration on sticks and leaves, measured as oxygen uptake rate, was compared among four (2nd-4th order) sites along an elevational gradient at Coweeta Hydrologic Laboratory, North Carolina. Rhododendron and birch leaves were placed in mesh bags in the stream at each site on 21 Oct 90. Oxygen uptake rates were measured for both leaf species beginning 15 Dec 90 and continuing monthly until leaves were no longer intact (birch-2 months, rhododendron-7 months). Microbial oxygen uptake rates were also measured monthly for one year on qualitative collections of sticks (1-3 cm diameter) found in the stream at each site. Oxygen uptake rates (mg O₂/hr) were calculated on both a surface area and AFDM basis. Overall mean respiration rate per unit surface area was highest for sticks, followed by rhododendron, and birch. When expressed on an AFDM basis, respiration rates were highest for birch, followed by rhododendron, and sticks. Based on continuous measurements, mean monthly stream temperatures were significantly different among sites, but mean incubation temperatures were not significantly different among sites, 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. Results indicate that differences in respiration rates among sites, for both rhododendron leaves and sticks, can be explained primarily by changes in temperature. Additionally, the high respiration rates per unit surface area on sticks indicate that small woody debris may play a significant role as a substrate for microbial metabolism.