A Model Laboratory System for Teaching Concepts Important in Soil Microbiology M. Siobhan Fennessy,* Gerald K. ABSTRACT An experiment wasdeveloped in which the interrelated na- ture of redox changes couldbe demonstrated to students.Con- cepts relating to redox status; respiration pathways; electron acceptors; the C, N, and S cycles; andbiodegradation of aro- matic organic molecules were explored through a single labora- tory incubation study. Three redox conditions were established: aerobic,nitratereducing, and sulfate reducing. Other treatment variables included the presence or absence of an inoculum source andthe presenceor absenceof p-hydroxybenzoate (PHB) a C source. ThePHB was used as a modelaromaticC com- pound because it mimics characteristics of both naturally oc- curring andxenobiotic aromatic organic compounds foundin the environment. The PHB and sulfate disappearance as well as accumulation of nitrous oxide were monitored as measures of biodegradation. In addition to teaching subject matter, this exercise provides an opportunity for students to gainexperience withdifferent analyticaltechniques. Although the experiment presented here employs some instrumentation that may not be availablein some undergraduate laboratories, it can be adopt- ed to less sophisticated procedures withonly minor modifica- tions. Students used the data generated in the experiment to write a manuscript in the style of a professional journal. A S IN OTHER AREAS OF AGRONOMY, teaching soil microbiology effectively requires that students integrate soil science with other fields such as biology and chemistry. Simultaneously presenting course content, method, and field or laboratory experience provides a framework to guide student thought, which enhances crit- ical thinking (Aide, 1989). Laboratory courses can pro- vide an opportunity to increase critical thinking if students are required to use data to identify patterns in information, make predictions, propose causative factors, and present consistent arguments to support a position (Ruggiero, 1988). As a course topic, soil microbiology poses several challenges in the development of laboratories, not the least of which is the diverse and interrelated nature of the subject matter. For instance, elemental cycles do not really function independently (as they are often taught), but rather are linked, as are topics such as redox condi- tions, microbial population dynamics, and the fate of xenobiotic compounds. The instructors sought to develop a laboratory that would compliment lecture materials and M.S. Fennessy, Dep. of Environmental Biology, 2021 Coffey Rd., Rm. 210, Ohio State Univ., Columbus, OH 43210-1085 (Current address, Univ. College London, Dep.of Geography, 26 Bedford Way, London, England WCIH 0AP); G.K. Sims, Environmental Chemistry, DowElan- co, Midland, MI;and S.R. Kehrmeyer, Dep. of Agronomy, 2021 Coffey Road, Ohio State Univ.,Columbus, OH 43210-1086. Received 14 Jan. 1991. *Corresponding author. Published in J. Nat.Resour. Life Sci. Educ. 21:79-83 (1992). Sims, and Staci R. Kehrmeyer demonstrate the complexities of soil microbial processes without confusing students. Topics of environmental con- cern were incorporated into the course, not only because it is an area of current research, but also becauseof the high level of student interest in environmentalproblems. The laboratory experiment was devised to reinforce most of the major topics addressed in the lecture por- tion of the course. The class was structured such that as the experiment progressed, lectures covering the above topics were delivered. Material covered included C and energy metabolism, microbial interactions, elemental cy- cles, fate of xenobiotic compounds, and the effect of re- dox status on all of the above. At the end of the term, the experiment was developed into a manuscript modeled after a journal article as if for publication, and peer- reviewed by classmates (Sims, 1989). The class is taught on a quarter system, but the laboratory developed here would work equally well on a semester system. The experiment consisted of an incubation study in which an aromatic compound (in this case p- hydroxybenzoate or PHB)was introduced into microbi- al communities under aerobic, denitrifying, and sulfate- reducing conditions. During the course of the incubation, PHB and sulfate disappearance as well as denitrification rates (measured as N20 accumulation) were monitored. Procedures for additional measurements including nitrite, oxygen, CO2, hydrogen sulfide, and bacterial numbers were also developed, although these were not used in the classroom. The experiment reported herein was piloted prior to classroom use, and was tested in a classroom sit- uation with students from a broad range of academic backgrounds including agronomy, environmental biolo- gy, microbiology, and plant pathology. MATERIALS AND METHODS Experimental Design Three different redox levels were established so that relationships among the C, N, and S cycles could be ob- served in a single experiment. The treatments consisted of supplying electron acceptors that woulddetermine the redox regime and composition of the microbial commu- nity. The experiment included an aerobic treatment (open to the atmosphere), a nitrate-reducing treatment in which NO~- was provided as an electron acceptor, and a sulfate-reducing treatment, in which SO42-was added as an electron acceptor. A reducing agent (dithiothreitol) was added to the anaerobic treatments to scavenge any 02 in the system. The inocula for the experiment con- sisted of sediment gathered from an aquifer. Many other sources of inocula could be used including sediment from wetland areas, anaerobic sludge from a sewage treatment J. Nat.Resour. Life Sci. Educ., Vol. 21, no. 1, 1992 ¯ 79