Soil CO2 flux (JCO2) was measured at midday over a 2-yr period in undisturbed tallgrass prairie (Konza Prairie, Kansas, USA) to quantify seasonal and annual budgets, to evaluate temperature and moisture as determinants of soil CO2 flux, and to assess the effect of a common land management tool, spring fire, and fire history on soil respiration. We hypothesized that: (1) maximum rates and annual estimates of soil JCO2 would be greater in more productive burned sites than in unburned sites, (2) soil JCO2 would be greater in newly burned sites with a history of fire exclusion than in annually burned sites (consistent with differences in aboveground production), and (3) soil temperature and water availability would be primary abiotic determinants of soil JCO2 in tallgrass prairie. A preliminary assessment of the effects of large herbivores on soil JCO2 was included to evaluate the hypothesis that removal of aboveground biomass would reduce soil JCO2. Results indicated that spring fire increased maximum monthly soil JCO2 by 20–55% relative to unburned tallgrass prairie, with greatest monthly differences measured in April (fourfold higher in burned sites). In burned sites that differed in fire history, maximum monthly JCO2 in annually burned prairie was 33% greater than in burned sites with a history of fire exclusion. Soil JCO2 in these latter sites was still significantly higher than in unburned sites. Soil JCO2 in sites grazed by bison was reduced by as much as 30% relative to adjacent ungrazed areas. Reduced root biomass and activity in grazed areas, unburned sites, and sites with a history of fire exclusion suggest that plants play a major role in determining soil JCO2 in this grassland. Soil temperature at 5 cm was related strongly to midday JCO2 in both annually burned sites (r2 = 0.58) and unburned sites (r2 = 0.71). In contrast, differences in soil moisture among sites, enhanced by comparing irrigated grassland to control areas, increased maximum monthly JCO2 by only 8%. Thus, soil temperature was the primary abiotic determinant of soil JCO2 during this study.

Maximum monthly estimates of soil JCO2 in tallgrass prairie ranged from 10.3 μmol CO2·m−2·s−1 in unburned sites to 15.1 μmol·m−2·s−1 in annually burned irrigated sites, whereas annual estimates varied from 4.7 to 7.8 kg CO2/m2. Over the 2-yr period, spring fire increased estimated annual soil JCO2 by 38–51% relative to unburned sites, while irrigation increased annual soil JCO2 by 13%. These estimates for tallgrass prairie are much higher than those reported for most temperate ecosystems but are similar to estimates for tropical forests. Characteristics of undisturbed tallgrass prairie that may lead to high levels of soil JCO2 include: high above- and belowground productivity; a relatively high proportion of C stored belowground; levels of soil microbial biomass and activity that are among the highest in native ecosystems in the United States; and the lack of a single dominant factor such as temperature, moisture, or nutrient availability, that consistently limits biotic processes during the growing season. The sensitivity of soil JCO2 in tallgrass prairie to different land use practices (fire and grazing) suggests that it is critical to include these factors in the development of grassland C budgets, as well as in regional models that estimate biogeochemical responses to land use change.