Using continuous stable isotope measurements to partition net ecosystem CO 2 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 CO 2 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 12 CO 2 and 13 CO 2 were measured continuously using tunable diode laser (TDL) absorption spectroscopy. The dynamics of the isotope ratio of ecosystem respiration ( δ R ), net ecosystem CO 2 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.