Fire vs. Metal: A Laboratory Study Demonstrating Microbial Responses to Soil Disturbances

Abstract

Incubation studies are traditionally used in soil microbiology laboratory classes to demonstrate microbial respiration and N mineralization–immobilization processes. Sometimes these exercises are done to calculate a N balance in N fertilizer-amended soils. However, examining microbial responses to environmental perturbations would appeal to soil microbiology students with broader interests in ecology and environmental soil science. In this study, perturbations of fire and heavy metal inputs were used to demonstrate disturbance impacts on microbial respiration and N mineralization, soil β-glucosidase activity, and abundance of soil bacteria and fungi in a series of exercises lasting 28 days. Students constructed duplicate microcosms containing pristine forest soil or forest soil burned by fires varying in severity or polluted with CuCl2. During a 2-week period, students measured changes in soil pH, CO2 evolution by the NaOH-trap method, and extractable soil NH4–N and NO3–N content. After 2 weeks, students tested for β-glucosidase activity and preserved samples for enumeration of total bacteria and fungi. Direct counts of bacteria and fungi were conducted on the third week by epifluorescence and phase contrast microscopy, but alternative methods to assess microbial biomass can be employed. Respiration was determined on the fourth week by titration of NaOH vials. After completion of the exercises, students had an appreciation of microbial responses to different types of disturbances, as measured by multiple biological parameters. Because this study was repeated over 2 years, students in the second year were able to assess soil microbial recovery immediately and 1 year after a forest fire. This study could easily be adapted for other types of disturbance issues, including salinity, land application of waste materials, and contamination of soil by xenobiotics.