HR: 16:00h
AN: B14B-01 INVITED     [Abstracts]
TI: Partitioning Ecosystem-Atmosphere CO2 Exchange: Recent Progress and Emerging Issues
AU: * Griffis, T J
EM: tgriffis@umn.edu
AF: Department of Soil, Water, and Climate, University of Minnesota-Twin Cities, Soil Science, Room 331, 1991 Upper Buford Circle, St. Paul, MN 55108
AB: Ecosystem-atmosphere fluxes of C16O2, 13CO2 and C18O16O are needed to better understand the impacts of climate and land use change on net ecosystem CO2 exchange (FN) and its partitioning into respiration (FR) and photosynthesis (FP). In this paper we examine isotopic CO2 fluxes (13CO2, C16O2 and C18O16O) measured using tunable diode laser (TDL) spectroscopy and micrometeorological methods (flux-gradient and eddy-covariance) over a corn-soybean rotation ecosystem in the Upper Midwest, United States during three growing seasons. The objectives were to: 1) determine the diurnal, seasonal and interannual variation in the isotopic composition of FN, FR and FP; 2) estimate the relative contributions of heterotrophic and autotrophic respiration; and 3) partition FN into FP and FR. Initial results illustrate strong seasonal variation (up to 14 per mil) in the isotopic signature of FN and FR that is driven by changes in phenology and associated changes in the isotopic composition of autotrophic and heterotrophic respiration (i.e. switching between C3/C4 substrates). For example, during the 2003 corn (C4) phase, the isotopic composition of FR was approximately -26 per mil prior to leaf emergence. It increased rapidly following leaf emergence and reached an average value of -12.5 per mil at full canopy. The isotopic composition of FR decreased to pre-emergence values following senescence. Strong seasonal variation in the isotopic composition of FN was also observed and during the main growing period averaged -11.6 per mil. Micrometeorological and stable isotope data were used to help partition FR into its autotrophic and heterotrophic components based on the numerical optimization of a mass balance model. On average autotrophic respiration accounted for 44 percent of growing season FR and reached a maximum of 59 percent during peak growth of the corn phase. The isotopic signature of heterotrophic respiration was -26 per mil prior to leaf emergence, and became increasingly 13CO2 enriched as the canopy developed, indicating that recent photosynthate became the dominant substrate for microbial activity. Sensitivity analyses substantiated that heterotrophic respiration had a major influence on the seasonal pattern of the isotopic signature of FN and FR. During the period of full canopy closure we attempted to partition FN into FP and FR using both the isotopic approach and the conventional nighttime-derived regression methodology. Our initial results showed that the isotopic flux partitioning produced more short-term variations in FR and FP that were considerably more symmetric about FN. In this experiment, the isotopic partitioning resulted in larger uncertainties, most of which were caused by the uncertainties in measuring the isotopic composition of FN. By sufficiently reducing this uncertainty the TDL-micrometeorological technique should yield a better understanding of the processes controlling FP, FR and ecosystem-scale discrimination. While these initial results show considerable promise, a number of methodological limitations require further attention. These limitations and future directions will be discussed.
DE: 0315 Biosphere/atmosphere interactions (0426, 1610)
SC: Biogeosciences [B]
MN: Fall Meeting 2005