Submitted to: Agriculture Forest Meteorology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 6/10/2005
Publication Date: 10/3/2005
Citation: Griffis, T.J., Baker, J.M., Zhang, J.M. 2005. Seasonal dynamics of isotopic CO2 exchange in a C3/C4 managed ecosystem. Agriculture Forest Meteorology. 132:1-19. Interpretive Summary: In response to global climate change there is a major effort to better understand the global carbon cycle. A key component of this is atmospheric modeling. Some of the key parameters that are needed for the models involve the differential uptake and release of the stable isotopes of carbon. However, these parameters and their temporal dynamics have historically been difficult to determine with adequate accuracy. We have developed a new approach that uses tunable diode laser spectroscopy to continuously measure surface/atmosphere exchange of both 12- and 13-CO2. This allows for determination of the extent to which a given ecosystem discriminates against the heavier 13-C isotopic form. We also show that in a corn/soybean rotation where the two crops have very different discrimination coefficients, the system can be used to partition night time respiration between the two carbon sources. We found that the discrimination by a corn crop is 3.6%, meaning that the carbon fixed by a growing corn field has a lower fraction of 13-CO2 than the atmosphere by 3.6%. The isotopic data also showed that in a corn/soybean rotation, the midsummer night-time respiration is dominated by the actively growing corn, with more than 90% of the respired carbon coming from it, and less than 10% from soil organic matter from the previous crop. These data will be useful in large scale inversion models that use atmospheric isotope concentrations and ecosystem discrimination coefficients to predict continental-scale carbon uptake and release.
Technical Abstract: Ecosystem-atmosphere fluxes of 12CO2 and 13CO2 are needed to better understand the impacts of climate and land use change on ecosystem respiration (RE), net ecosystem CO2 exchange (NEE), and canopy scale photosynthetic discrimination (delta). We combined micrometeorological and stable isotope techniques to quantify isotopic fluxes of 12CO2 and 13CO2 over a corn-soybean rotation ecosystem in the Upper Midwest United States. Results are reported for a 192 day period during the corn phase of the 2003 growing season. The isotopomer flux ratio, delta 13CO2/ delta 12CO2, was measured continuously using a tunable diode laser (TDL) and gradient technique to quantify the isotope ratios of RE (delta 13CR) and NEE (delta 13CN). Prior to leaf emergence delta 13CR was approximately -26 per mille. It increased rapidly following leaf emergence and warming of near-surface soil horizons and reached a maximum value of -11 per mille at full canopy. Delta 13CR decreased to pre-emergence values following senescence. A mixing model analysis indicated that C4 respiration accounted for about 92% of RE at full canopy. Strong seasonal variation in delta 13CN was also observed. During the main growing period delta 13CN averaged -11.6 per mille. Delta 13CR and delta 13CN values were used in a modified flux partitioning approach to estimate canopy-scale delta and the isotope ratio of photosynthetically assimilated CO2 (delta 13CP) independent of calculating canopy conductance or assuming leaf-scale discrimination factors. The results showed significant day-to-day variation in delta with a mean value of 3.6 per mille. These data and parameter estimates are critical for 1) validating and improving the parameterization of land surface schemes and inverse models that aim to estimate regional carbon sinks and sources, and 2) interpreting changes in the atmospheric signal of delta 13CO2.