Environmental Controls on Soil and Whole-ecosystem Respiration from a Tallgrass Prairie

piration and heterotrophic microbial respiration. Net ecosystem exchange of CO2 as an integration of photoEnvironmental controls on C cycling in terrestrial ecosystems are synthesis, plant dark respiration, and soil respiration in difficult to define, because (i) C fluxes from plant vs. microbial activity are difficult to separate, and (ii) controlling variables are often intergrasslands can be obtained with various micrometeorocorrelated. We investigated temporal and spatial determinants of soil logical techniques, which integrate across large land areas respiration and whole-ecosystem respiration using nighttime exposure (Verma, 1990; Norman et al., 1992). Knowing the contriof static chambers to alkali absorption during 2 yr on a tallgrass prairie bution of soil respiration to these fluxes would improve in northeastern Kansas. Soil respiration (mg CO2-C m 2 h 1 ) was posour understanding of the C cycle and help determine rates itively related to soil organic C (SOC, kg m 2 0.1 m 1 ) through linear of ecosystem C sequestration. Separation of soil resregression [CO2-C 44 (40 SOC), r 2 0.71]. Temporal variations in piration from whole-ecosystem respiration is best suited respiration were related to soil temperature, water-filled pore space during the nighttime, when photosynthetic fixation of (WFPS), and a plant growth rate function, with a combined R 2 of 0.76 CO2 is not a factor. There is also a need to better underfor soil respiration and of 0.84 for whole-ecosystem respiration. Temstand whole-ecosystem respiration during the nighttime, poral variograms suggested that both soil and whole-ecosystem respiration became increasingly dissimilar the longer the time between measince micrometeorological techniques for net ecosystem surements up to 30 d, while dissimilarity in soil temperature and WFPS exchange of CO2 are generally less suited during the nightleveled between 10 and 20 d of separation. A plant growth rate functime than during the daytime, because of less reliable ention was an important variable that controlled whole-ecosystem respiergy balance, concentration gradients, and wind speeds ration, as well as soil respiration. The ratio of soil respiration to wholeneeded for calculations (Harper, 1989). ecosystem respiration was ≈0.4 during maximum plant growth (July) Previous studies have indicated a high degree of spaand approached a value of 1 during minimal plant growth (November tial and temporal variability in soil respiration that makes to March). We conclude that whole-ecosystem respiration is under simextrapolations of findings to different ecosystems diffiilar environmental controls as soil respiration, the main variables being cult (Buyanovsky et al., 1986; Kiefer, 1990; Rochette et soil organic C, soil temperature, WFPS, and plant growth rate, which al., 1991). Even when attempting to extrapolate results all control the supply of readily mineralizable substrates. within an ecosystem, major errors may occur because of the limited frequency of observations collected mainly in the summer during active plant growth, at certain G cover 24% of the terrestrial surface times of the day, or with measurement techniques that (Sims and Risser, 2000) and vary with respect to disturb the natural system by removal of vegetation. species composition, net primary productivity, abiotic The frequency of observations needed to estimate soil environment, and management, all of which affect deand whole-ecosystem respiration for a specific period composition and sequestration of organic matter. Reof interest will depend on the day-to-day variability in gion-specific information is needed to characterize C environmental conditions (i.e., temperature and moisfluxes in these vast land areas in order to better quantify ture), which affect respiration. We hypothesized that the role of grasslands in greenhouse gas emissions and variograms could be used as a means of describing the potential C sequestration (Scurlock and Hall, 1998). temporal variability of respiration in order to determine Information exists on net ecosystem exchange of CO2 a reasonable sampling frequency. in tallgrass prairies (Verma et al., 1989; Kim and Verma, Our primary objective was to elucidate whether envi1990; Kim et al., 1992; Polley et al., 1992), as well as some ronmental (i.e., soil organic C, soil temperature, and information on soil respiration in this ecosystem (KucWFPS) and physiological (i.e., plant growth rate) factors era and Kirkham, 1971; Ham et al., 1995; Bremer et al., controlled soil respiration and whole-ecosystem respira1998; Knapp et al., 1998b; Mielnick and Dugas, 2000), tion to the same extent. Secondly, we wanted to deterbut relatively little information is available on soil resmine an optimum sampling frequency for soil and wholepiration measured in concert with whole-ecosystem resecosystem respiration within a year. piration. Carbon fluxes in terrestrial ecosystems are dominated by (i) biochemical fixation of CO2 via photosynthesis and MATERIALS AND METHODS (ii) biochemical release of CO2 via autotrophic plant resSite and Vegetation This study was conducted in 1987 and 1988 at the Konza PraiK. Franzluebbers, Loch Lomond, Watkinsville, GA 30677-2345; A.J. rie, a tallgrass prairie in northeastern Kansas, 14 km south of Franzluebbers, USDA–ARS, J. Phil Campbell Sr., Natural Resource Manhattan (39 3 N, 96 32 W, 445 m above mean sea level). Conservation Center, 1420 Experiment Station Road, Watkinsville, The site was grazed for several years prior to 1986, at which GA 30677-2373; M.D. Jawson, USDA–ARS, 5601 Sunnyside Avenue, Beltsville, MD 20705-5140. Received 4 Jan. 2001. *Corresponding author ([email protected]). Abbreviations: DOY, day of year; SOC, soil organic carbon; WFPS, water-filled pore space. Published in Soil Sci. Soc. Am. J. 66:254–262 (2002).