Submitted to: Agricultural and Forest Meteorology Conference Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: 7/16/2004
Publication Date: 8/30/2004
Citation: Zhang, J., Griffis, T.J., Baker, J.M. 2004. Using continuous isotope measurements to partition net ecosystem CO2 exchange into photosynthesis and respiration [abstract]. Agricultural and Forest Meteorology Conference Proceedings. p. 46-47.
Technical Abstract: Field-scale estimates of gross ecosystem photosynthesis (Pg) and respiration (Re) using micrometeorological techniques remains an important yet difficult challenge. Here, we combined micrometeorological and stable isotope techniques to partition net ecosystem CO2 exchange (NEE) into Pg and Re in a corn-soybean rotation ecosystem in the Upper Midwest, United States during the summer 2003 corn phase. Mixing ratios of the isotopomers, 12CO2 and 13CO2, were measured continuously from May 21 to October 14 at three heights (two above the canopy and one within the canopy) using tunable diode laser absorption spectroscopy (TGA100, Campbell Scientific Inc.). Half-hour fluxes of NEE were measured using the eddy covariance technique and isotopic fluxes were measured using the gradient method. The diurnal and seasonal variations of the critical partitioning parameters (isotope ratio of Re (delta 13CR) and isotope ratio of NEE (delta b)) were examined. The variation of delta13CR and delta b showed a strong seasonal pattern related to changes in phenology. Half-hour values of NEE were partitioned into Pg and Re based on the principle of mass conservation by using a set of equations that constrain the canopy conductance and plant discrimination. The partitioned Pg and Re, based on the isotopic approach, were in close agreement with the more traditional nighttime temperature regression methods at full canopy, but differed significantly during other growth stages. A sensitivity analysis was performed to evaluate the uncertainty in the isotope partitioning method. Partitioning was most sensitive to the estimates of delta 13CR and delta b, which were derived from the gradient measurements of the mixing ratios of 12CO2 and 13CO2.