Understanding the influences of seasonality and land-use history

on biosphere-atmosphere ¹³CO₂ exchange

Chun-Ta Lai, Andrew Schauer, James Ehleringer

University of Utah

Tel. 801-581-8917

lai@biology.utah.edu

The carbon isotope ratios of carbon dioxide fluxes from terrestrial ecosystems are key measurements needed to constrain interpretations of sinks in carbon cycle analyses. The current research began in January 2001 and is a multi-faceted effort addressing photosynthetic and respiratory isotope exchanges across the biosphere-atmosphere boundary at four AmeriFlux sites (Harvard Forest, Howland, Konza, and Wind River). We have developed, tested, and established automated CO₂ sampling devices at these sites*, capable of continuously sampling CO₂ concentration and filling air flasks for subsequent isotope ratio analyses. We focus on understanding the magnitude of changes in the carbon isotope ratio of ecosystem respiration through the growing season and a mechanistic basis for observed isotope changes. Observations this year indicate that the carbon isotope ratio within a site varied ~ 4 per mil, with changes in magnitude to 4.9 per mil observed across C₃-only sites. These observations appear to relate directly to water stress and stomatal closure dynamics within each of the ecosystems.

Focusing at AmeriFlux sites provides a direct link to NEE measurements. Over the next 2 years, we will continue to measure seasonal observations of the isotope ratio of CO2 fluxes. In addition, we will be combining eddy covariance and isotope observations to partition NEE into assimilation andrespiratory components. This will provide an independent estimate of flux components over a growing season, in response to seasonal stresses, and across these four contrasting ecosystems.

Both short-term and long-term monitoring of the seasonal and interannual variations in ecosystem ¹³C values feed directly into regional and global CO₂ inversion models as a constraint on land-ocean partitioning. Our data sets complement long-term observations of tropospheric CO2, which are the best available long-term data sets constraining interannual estimates of global productivity. Our data sets are being linked with global CO₂ inversion modeling efforts (e.g., TRANSCOM) to better understand the seasonal dynamics of tropospheric CO₂, particularly with respect to how much land/ocean partitioning varies on an interannual basis.

* The automated air samplers have been installed at all but the Konza site; the last sampler will be installed at the Konza site early spring in 2002.