Ventilation of subterranean CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; and Eddy covariance incongruities over carbonate ecosystems

A Were,F Domingo,L Villagarcía,C Moreno De Jong,P Serrano-Ortiz,A S Kowalski

doi:10.5194/bg-7-859-2010

Abstract

Abstract. Measurements of CO2 fluxes with Eddy Covariance (EC) systems are ongoing over different ecosystems around the world, through different measuring networks, in order to assess the carbon balance of these ecosystems. In carbonate ecosystems, characterized by the presence of subterranean pores and cavities, ventilation of the CO2 accumulated in these cavities and pores can act as an extra source of CO2 exchange between the ecosystem and the atmosphere. In this work we analyse the effect of the subterranean heterogeneity of a carbonate ecosystem on measurements of CO2 fluxes by comparing measurements from two EC systems with distinct footprints. Results showed that both EC systems agreed for measurements of evapotranspiration and of CO2 in periods when respiratory and photosynthetic processes were dominant (biological periods), with a regression slope of 0.99 and 0.97, respectively. However, in periods when the main source of CO2 comes from the ventilation of subterranean pores and cavities (abiotic periods) agreement is not good, with a regression slope of 0.6. Ground-penetrating radar measurements of the sub-surface confirmed the existence of high sub-surface heterogeneity that, combined with different footprints, lead to differences in the measurements of the two EC systems. These results show that measurements of CO2 fluxes with Eddy covariance systems over carbonate ecosystems must be taken carefully, as they may not be representative of the ecosystem under consideration.

Highlights

The importance of characterising the global carbon cycle is clear, since CO2 is the principal greenhouse gas after water vapour (Schimel, 1995)
These results show that measurements of CO2 fluxes with Eddy covariance systems over carbonate ecosystems must be taken carefully, as they may not be representative of the ecosystem under consideration
For the period where both Eddy Covariance (EC) systems were on separate towers, the values of LE measured ranged between ca. 160 W m−2 and −60 W m−2, while Fc ranged between ca. 6 μmol m−2 s−1 and −5 μmol m−2 s−1 (Fig. 2)

Summary

Introduction

The importance of characterising the global carbon cycle is clear, since CO2 is the principal greenhouse gas after water vapour (Schimel, 1995). In this context, accurate measurements of net carbon exchange between terrestrial ecosystems and the atmosphere are very important as terrestrial ecosystems are the main driver of global interannual variability in atmospheric CO2 (Friend et al, 2007). Pores, fissures and cavities near the surface can accumulate high concentrations of soil-derived CO2 (Bourges et al, 2001; Wood, 1985) that can be isolated from soil-atmosphere exchange flows.

Methods

Results

Conclusion