Submitted to: Plant Cell and Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/10/2005
Publication Date: 4/1/2006
Citation: Zhang, J.M., Griffis, T.J., Baker, J.M. 2006. Using continuous stable isotope measurements to partition net ecosystem CO2 exchange. Plant Cell and Environment. 29:483-496. Interpretive Summary: In response to the problems associated with rising atmospheric carbon dioxide (CO2) levels, much research has been conducted to measure the impact of different land management practices on the net CO2 exchange. Unfortunately, when only the net exchange is measured, it is often impossible to determine cause and effect, so it is important to separately determine the competing processes of photosynthesis and respiration that together determine net CO2 exchange. We have developed a method for doing this by concurrently measuring the net exchange of the two major isotopic forms of CO2, known as 12-CO2 and 13-CO2. The basis for the method is that while all plants preferentially absorb the lighter 12-CO2 during photosynthesis, there are significant differences between the degree of discrimination among species, with soybeans exhibiting much more discrimination than corn. During respiration there is no discrimination. We show that by measuring the isotopic composition of the crop and of the soil organic matter, along with measurements of the isotopic exchange rates, it is possible to separate the processes of photosynthesis and respiration. This approach will allow scientists to examine the impacts of both climate and management practices on crop growth, carbon gain, and carbon loss.
Technical Abstract: Field-scale estimation of ecosystem 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 (NEE) into photosynthesis and respiration 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 absorption spectroscopy (TDLAS). The dynamics of the isotope ratio of ecosystem respiration (delta 13 Cr), photosynthesis (delta 13 Cp), and NEE (delta 13 Cn) were examined. During the period of full canopy closure, NEE was portioned into photosynthesis and respiration using both the isotopic approach and the conventional nighttime-derived regression method. Results showed that delta 13 Cr had significant seasonal variation (-28 per mille ~ -11 per mille), corresponding closely with canopy phenology. The daytime delta 13 Cn typically varied between -11 per mille and -3 per mille, while delta 13 Cp remained relatively invariant (-11 per mille). The isotopic partitioning agreed well in magnitude with the regression method, but showed more short-term fluctuations. Isotopic partitioning can be used to better understand the impacts of phenology and other environmental factors on short-term variations in photosynthesis and respiration. It could also be used to constrain the nighttime regression method that is currently used at more than 200 global FLUXNET sites.