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_N) into photosynthesis (F_A) and respiration (F_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 (δ_R), net ecosystem CO₂ exchange (δ_N) and photosynthetic discrimination at the canopy scale (Δ_canopy) were examined. During the period of full canopy closure, F_N was partitioned into photosynthesis and respiration using both the isotopic approach and the conventional night-time-derived regression methodology. Results showed that δ_R had significant seasonal variation (-32 to -11‰) corresponding closely with canopy phenology. Daytime δ_N typically varied from -12 to -4‰, while Δ_canopy remained relatively constant in the vicinity of 3‰. Compared with the regression approach, the isotopic flux partitioning showed more short-term variations and was considerably more symmetric about F_N. In this experiment, the isotopic partitioning resulted in larger uncertainties, most of which were caused by the uncertainties in δ_N and the daytime estimate of δ_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.