Submitted to: Global Biogeochemical Cycles
Publication Type: Peer reviewed journal
Publication Acceptance Date: 4/29/2010
Publication Date: N/A
Citation: Interpretive Summary: The isotopic composition of atmospheric CO2 is an important diagnostic tool, because different plant types vary in the degree to which they utilize the heavier isotopes in photosynthesis, with C4 plants such as corn discriminating less than C3 plants. For this reason, regional-scale measurement of isotopic exchange can be a key indicator of land use change, if it involves replacement of one type of vegetation with another. Also, since there is isotopic discrimination in photosynthesis but not in respiration, separate measurement of isotopic fluxes is useful in testing models of CO2 exchange that are important in predicting climate change. Unfortunately, to this point there has been no reliable method for continuous, regional scale isotopic flux measurements. We have installed recently developed high-frequency tunable diode laser technology on a tall (244 m) radio tower south of St. Paul, MN, and have developed methodology to make the first continuous eddy covariance measurements of surface/atmosphere isotopic exchange over the course of a growing season. The results show that carbon exchange in this region is dominated (70%) by C4 vegetation, over spatial scales ranging from 1 km to 100 km. As these measurements are continued they will provide a valuable diagnostic tool for tracking land use change and the impacts of global climate change.
Technical Abstract: Agricultural crops with a C4 photosynthetic 3 pathway rapidly expanded across North America as early as 800 A.D. Their distribution continues to expand globally as demands for food and biofuel production increase. These systems are highly productive, having a significant impact on carbon and water exchange between the land and atmosphere. Here, we investigate the relative impact of agricultural C4 vegetation on the local and regional 13CO2 biosphere-atmosphere discrimination and atmospheric isotopic forcing in the Upper Midwest, United States. We address three questions: 1. What is the relative importance of C3 and C4 species to the regional CO2 budget? 2. How do these different photosynthetic pathways influence the regional biosphereatmosphere isotope discrimination? 3. To what extent do changes in C4 vegetation impact atmospheric isotopic forcing and the isotopic signature of the atmosphere? These questions are addressed using measurements obtained from the University of Minnesota tall tower (240 m) trace gas observatory (TGO) over the period 2007 to 2008 and are supported with scaled-up values of isotopic forcing based on ecosystem-scale eddy flux observations and high resolution land use data. Our analyses indicate that local and regional C4 production was higher by 10% in 2007 due to increased demand for biofuel. Isotopic flux measurements from the tall tower confirm that this increase had a significant impact on the growing season biosphere-atmosphere 13CO2 photosynthetic discrimination, which ranged from 9.7 to 13.2 h in 2007 and 11.2 to 14.8 h in 2008. Net ecosystem CO2 exchange was partitioned into its C3 and C4 contributions using a simple mixing model. C4 species accountedfor as much as 65% of the flux during peak growth 26 in 2007. C3 and C4 isotopic observations from ecosystems located within the vicinity of the tall towerwere used to estimate the local scale 13 CO2 isoforcing. These scaled up values were in relatively good agreement with direct observations of isoforcing at the tall tower and indicate that C3 photosynthetic discrimination dominatedthe atmospheric budget during spring and fall. The tall tower observations of biosphere-atmosphere 13CO2 discrimination support recently published modeling studies that explicitly accounted for increases in C4 cropland, which has significant implications for estimating the terrestrial carbon sink strength based on inverse modeling techniques.