Using continuous stable isotope measurements to partition net ecosystem CO₂ exchange

Ecosystem-scale estimation of photosynthesis and respiration using micrometeorological techniques remains an important, yet difficult, challenge. In this study, we combined micrometeorological and stable isotope methods to partition net ecosystem CO₂ exchange (F[subscript N]) into photosynthesis (F[subscript A]) and respiration (F[subscript R]) in a corn-soybean rotation ecosystem during the summer 2003 corn phase. Mixing ratios of ¹²CO₂ and ¹³CO₂ were measured continuously using tunable diode laser (TDL) absorption spectroscopy. The dynamics of the isotope ratio of ecosystem respiration ([delta][subscript R]), net ecosystem CO₂ exchange ([delta][subscript N]) and photosynthetic discrimination at the canopy scale ([Delta][subscript canopy]) were examined. During the period of full canopy closure, F[subscript N] was partitioned into photosynthesis and respiration using both the isotopic approach and the conventional night-time-derived regression methodology. Results showed that [delta][subscript R] had significant seasonal variation ([-]32 to [-]11[per mille]) corresponding closely with canopy phenology. Daytime [delta][subscript N] typically varied from [-]12 to [-]4[per mille], while [Delta][subscript canopy] remained relatively constant in the vicinity of 3[per mille]. Compared with the regression approach, the isotopic flux partitioning showed more short-term variations and was considerably more symmetric about F[subscript N]. In this experiment, the isotopic partitioning resulted in larger uncertainties, most of which were caused by the uncertainties in [delta][subscript N] and the daytime estimate of [delta][subscript R]. By sufficiently reducing these uncertainties, the tunable diode laser (TDL)-micrometeorological technique should yield a better understanding of the processes controlling photosynthesis, respiration and ecosystem-scale discrimination.