Exchange of gaseous elemental mercury (Hg⁰) between air, soil, and vegetation in an uncontaminated background ecosystem was studied as part of a multi-year project focusing on effects of interannual climate variability on ecosystem processes. Mercury fluxes from tallgrass prairie soil-plant monoliths were measured using large (7.3x5.5x4.5 m, LxWxD) climate-controlled gas exchange mesocosms (Ecologically Controlled Enclosed Lysimeter Laboratories or EcoCELLs). EcoCELL level Hg⁰ fluxes indicated net system emissions of 102 ± 27 µg m^-2 per year. Mercury exchange was influenced by diel and seasonal changes in solar radiation and temperature, with larger midday and summer Hg⁰ emissions and net deposition in winter months. However, mesocosm Hg⁰ fluxes measured in November and December months when the EcoCELLs housed tallgrass prairie monoliths were similar to fluxes measured when mesocosms were empty. Additionally, in situ dynamic flux chamber measurements were an order of magnitude lower than the mesocosm level fluxes. This led to additional tests and comparisons after the tallgrass prairie soil-plant monoliths were placed in the mesocosms. When monoliths were completely covered and sealed off from the surrounding air using plastic tents in sunny and warm conditions, there was no change in Hg⁰ concentrations compared to control EcoCELLs. These findings indicated that the contribution of EcoCELL infrastructure to the overall mesocosm Hg⁰ fluxes was much greater than emissions from tallgrass prairie monoliths themselves. Indeed, relatively constant regression slopes between EcoCELL Hg⁰ fluxes and solar radiation suggested that only physical and chemical and not biological processes controlled Hg⁰ exchange. Furthermore, removal of vegetation in August did not affect mesocosm Hg⁰ fluxes, nor did watering which has been shown to enhance emissions from soil. Thus, although these mesocosms performed relatively well in a study using Hg contaminated soils (Gustin et al. 2004), there may be limitations for applying EcoCELL technology in a background system.