Understanding seasonality and interannual variability of biosphere-atmosphere ¹³CO₂ exchange in the southeastern region of the United States

Chun-Ta Lai, San Diego State University

Abstract

The stable isotope ratios of carbon dioxide (CO₂) fluxes from terrestrial ecosystems are key measurements needed to constrain interpretations of sources and sinks in carbon cycle analyses. We propose the addition of flask-based stable isotope measurements at two southeast AmeriFlux sites to complete coverage of a national AmeriFlux-isotope network that spans the US major biomes. This research will fill gaps in regions under-represented by current activities coupling atmospheric isotope and NEE measurements.

Air samples will be collected using 100ml flasks on a weekly basis at two AmeriFlux sites: 1) Chestnut Ridge, a deciduous forest in eastern Tennessee and 2) Freeman Ranch, a C₄-dominated grassland/savanna in southern Texas.

For the past 5 years, continuous atmospheric ¹³CO₂ measurements indicate considerable seasonal and interannual variations of carbon isotope ratios of respired CO₂ fluxes (δ¹³C_R) in northeast deciduous forests (Harvard Forest), grasslands in the Great Plains (Rannells Prairie), and in both northeast (Howland Forest) and northwest (Wind River Canopy Crane) coniferous forests of the United States. These seasonal isotopic signals are critical to improving large-scale atmospheric ¹³C inversion technique for identifying carbon sources and sinks on land. The proposed research will test two hypotheses that are fundamentally connected to NACP objectives: 1) southeast deciduous forests and grassland/savanna ecosystems respond to projected climate change differently to their northern counterparts, and 2) a national AmeriFlux-isotope network provides continental-scale information to constrain North American carbon budget.

Air sample will be collected on weekly intervals and analyzed for the CO₂ mixing ratio and its carbon and oxygen isotope ratios to quantify seasonal variations of ¹³C and ¹⁸O signatures associated with ecosystem CO₂ fluxes. These isotope measurements provide a direct link to NEE measurements, allowing NEE to be separated into gross assimilation and respiration components at the ecosystem level. Measurements from the proposed work will contribute to other atmospheric isotope sampling activities across AmeriFlux sites, most significantly by the DOETCP project, to provide a national AmeriFlux-isotope network.

Currently, frequent and consistent atmospheric δ¹³C measurements are missing from the southeastern USA region. The proposed research will focus on expanding an atmospheric isotope sampling effort to include major biomes in the southeast. The two proposed study sites complement existing AmeriFlux-isotope sampling effort and will provide an opportunity for cross-site comparison to contrasting climatic effects on ecosystem functions. This research will advance global change and carbon cycle research with the NACP, BASIN, and DOE-TCP Programs.