An assessment of a mesocosm approach to the study of microbial respirationin a sandy unsaturated zone

Microbial respiration rates were determined through a 3.2 m thick, sandy un saturated zone in a 2.4 m diameter x 4.6 m high mesocosm. The mesocosm was maintained under near constant temperature (18 degrees to 23 degreesC) and reached steady moisture content conditions after several hundred days. Soil -gas CO2 concentrations in the mesocosm ranged from 0.09 % to 3.31 % and in creased with depth. Respiration rates within the mesocosm were quantified o ver a 342-day period using measured CO2 concentrations and a transient, one -dimensional finite-element model. Microbial respiration rates were 2 x 10( -1) mug C.g(-1).d(-1) throughout most of the system, but decreased to 10(-4 ) to 10(-3) mug C.g(-1).d(-1) within the capillary fringe. Microbial respir ation rates were also determined in minicosms (500 g sample mass) over a ra nge in temperatures (4 degrees to 30 degreesC) and volumetric moisture cont ents (0.044 to 0.37). The functional dependence of CO2 production on temper ature and soil-moisture content was similar for the two scales of laborator y observation. Respiration rates in the minicosms, for temperatures and moi sture contents in the mesocosm, were up to an order of magnitude greater th an those determined for the mesocosm, The higher respiration rates in the m inicosms, compared to the mesocosm, were attributed to greater disturbance of the samples and to shorter acclimation time in the minicosms. Extrapolat ing the laboratory respiration rates to field conditions yielded rates that were two to three orders of magnitude greater than rates previously determ ined in situ for C-horizon material. Results show that in situ microbial re action rates determined using disturbed samples in minicosms and mesocosms yielded respiration rates that greatly exceeded field conditions. Mesocosms can, however, provide a useful environment for conducting process-related research in unsaturated environments.