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a Loch Lomond, Watkinsville, GA 30677-2345
b USDA–ARS, J. Phil Campbell Sr., Natural Resource Conservation Center, 1420 Experiment Station Road, Watkinsville, GA 30677-2373
c USDA–ARS, 5601 Sunnyside Avenue, Beltsville, MD 20705-5140
* Corresponding author (afranz{at}arches.uga.edu)
Environmental controls on C cycling in terrestrial ecosystems are difficult to define, because (i) C fluxes from plant vs. microbial activity are difficult to separate, and (ii) controlling variables are often intercorrelated. We investigated temporal and spatial determinants of soil respiration and whole-ecosystem respiration using nighttime exposure of static chambers to alkali absorption during 2 yr on a tallgrass prairie in northeastern Kansas. Soil respiration (mg CO2-C m-2 h-1) was positively related to soil organic C (SOC, kg m-2 0.1 m-1) through linear regression 
. Temporal variations in respiration were related to soil temperature, water-filled pore space (WFPS), and a plant growth rate function, with a combined R2 of 0.76 for soil respiration and of 0.84 for whole-ecosystem respiration. Temporal variograms suggested that both soil and whole-ecosystem respiration became increasingly dissimilar the longer the time between measurements up to 30 d, while dissimilarity in soil temperature and WFPS leveled between 10 and 20 d of separation. A plant growth rate function was an important variable that controlled whole-ecosystem respiration, as well as soil respiration. The ratio of soil respiration to whole-ecosystem respiration was 
0.4 during maximum plant growth (July) and approached a value of 1 during minimal plant growth (November to March). We conclude that whole-ecosystem respiration is under similar environmental controls as soil respiration, the main variables being soil organic C, soil temperature, WFPS, and plant growth rate, which all control the supply of readily mineralizable substrates.
Abbreviations: DOY, day of year • SOC, soil organic carbon • WFPS, water-filled pore space
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