Effects of Acidification on Leaf Decomposition in Streams

Effects of acidification on leaf decomposition in streams were studied in four second-order streams in the Great Smoky Mountains National Park. The streams ranged in pH from 4.5 to 6.4 at baseflow. Mass loss of leaves incubated in mesh bags placed in pools in each stream was measured periodically over 15 wk beginning in late August. Measurements were also made of C, N, P, and Al in leaves, microbial biomass (adenosine triphosphate ATP) and respiration rate and bacterial production (thymidine uptake) associated with leaves, and the number and biomass of macroinvertebrates in leaf bags. Rates of leaf mass loss were significantly lower in streams with pH ≤5.7 compared with a stream with pH 6.4. Although rate of leaf mass loss among the streams varied directly with pH, differences between streams with pH values between 4.5 and 5.7 were not significant. Microbial ATP and respiration rates and bacterial production rates followed the same pattern as leaf mass loss rate, i.e., low for more acidic streams and highest in the stream with the highest pH. Accumulation of aluminum by the leaf-microbe complex was also greatest in the most acidic streams. The number and biomass of macroinvertebrate shredders found in leaf bags was lowest at the highest pH site and therefore cannot account for the higher rate of leaf mass loss found at this site. Our results suggest that the lower rate of leaf decomposition in the more acidic streams is due largely to low rates of microbial activity.