Extended Abstract (220K)4.4
Seasonal variation in the isotope ratio of ecosystem respiration and canopy-scale discrimination of a corn-soybean ecosystem
Timothy J. Griffis, University of Minnesota, St. Paul, MN; and J. M. Baker and J. Zhang
Micrometeorological and stable isotope techniques were combined to study the seasonal variation in the net ecosystem CO2 exchange (NEE) of a corn (C4 grass) – soybean (C3 dicot) rotation ecosystem in the Upper Midwest United States. Results are reported for the corn phase of the 2003 growing season, which was warmer and drier than normal. Isotopic fluxes of 12CO2 and 13CO2 were measured using a tunable diode laser absorption spectroscopy (TDLAS) system and gradient technique. Continuous isotopic fluxes and mixing ratios of 12CO2 and 13CO2 were used to quantify the isotope ratio of ecosystem respiration (δ13CR) and canopy-scale discrimination (Δe) to better interpret and understand variations in NEE and ecosystem respiration (RE). Values of (δ13CR) prior to leaf emergence were approximately -27 per mil and increased rapidly (became more positive) following leaf emergence and reached a maximum of -16 per mil at full canopy. A mixing model was used to partition RE into C4 and C3 contributions and indicated that respiration from corn accounted for greater than 60% of RE at full canopy. Following senescence, the contribution of C4 substrates to respiration rapidly decreased and respiration from C3 substrates constituted the major fraction of RE. The seasonal variation in the isotopic ratio of the surface layer showed the strong effect of canopy photosynthesis on the enrichment of 13CO2. Variation in canopy-scale discrimination indicated that midsummer drought caused a significant reduction in photosynthesis and the net gain of CO2. The combination of micrometeorological and continuous stable isotope measurements provides additional information at diurnal and seasonal timescales that can be used to better understand the processes controlling NEE.
Session 4, Carbon dioxide exchange 2
Tuesday, 24 August 2004, 2:30 PM-5:15 PM
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