Comparing ecosystem and soil respiration: Review and key challenges of tower-based and soil measurements

Authors: BARBA, J. - University Of Delaware; CUEVA, A. - University Of Delaware; BAHN, M. - University Of Innsbruck; BARRON-GAFFORD, G. - University Of Arizona; BOND-LAMBERTY, B. - Pacific Northwest National Laboratory; HANSON, P. - Oak Ridge National Laboratory; JAIMES, A. - University Of Delaware; KULMALA, L. - University Of Eastern Finland; PUMPOANEN, J. - University Of Eastern Finland; Scott, Russell - Russ; WOHLFAHRT, G. - University Of Innsbruck; VARGAS, R. - University Of Delaware

Submitted to: Agricultural and Forest Meteorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/22/2017
Publication Date: 2/15/2018
Citation: Barba, J., Cueva, A., Bahn, M., Barron-Gafford, G., Bond-Lamberty, B., Hanson, P., Jaimes, A., Kulmala, L., Pumpoanen, J., Scott, R.L., Wohlfahrt, G., Vargas, R. 2018. Comparing ecosystem and soil respiration: Review and key challenges of tower-based and soil measurements. Agricultural and Forest Meteorology. 249:434-443. https://doi.org/10.1016/j.agrformet.2017.10.028.
DOI: https://doi.org/10.1016/j.agrformet.2017.10.028

Interpretive Summary: A major challenge in quantifying the net exchange of carbon dioxide between the land and atmosphere is determining the amount of carbon dioxide that is released from ecosystems via plant and microbial respiration. Ecosystem respiration is the release of carbon dioxide from the ecosystem to the atmosphere and includes both soil respiration and above-ground plant respiration. Here we compare independent measurements of soil respiration with that of the entire ecosystem at four different sites. While both soil and ecosystem respiration show similar seasonal fluctuations, we found that respiration from the soil was erroneously not always lower than from the ecosystems. We identified several issues in measuring ecosystem respiration and soil respiration that could result in an underestimation of ecosystem respiration and/or an overestimation of soil respiration. Addressing these issues should help to improve our methods for measuring these important greenhouse gas exchanges.

Technical Abstract: The net ecosystem exchange (NEE) is the difference between ecosystem CO2 assimilation and CO2 losses to the atmosphere. Ecosystem respiration (Reco), the efflux of CO2 from the ecosystem to the atmosphere, includes the soil-to-atmosphere carbon flux (i.e., soil respiration; Rsoil) and aboveground plant respiration. Therefore, Rsoil is a fraction of Reco and by definition has to be smaller than Reco at annual, seasonal and daily scales. However, several studies estimating Reco with the eddy covariance technique and measuring Rsoil within the footprint of the tower have reported higher Rsoil than Reco at different time scales. Here, we compare four different and contrasting ecosystems (from forest to grasslands, and from boreal to semiarid) to study whether, and under what conditions, measurements of Reco are lower than Rsoil. In general, both fluxes showed similar temporal patterns, but Reco was not consistently higher than Rsoil from daily to annual scales across sites. We identified several issues that apply for measuring NEE and measuring/upscaling Rsoil that could result in an underestimation of Reco and/or an overestimation of Rsoil. These issues are discussed based on (a) nighttime measurements of NEE, (b) Rsoil measurements, and (c) the interpretation of the functional relationships of these fluxes with temperature (i.e., Q10). We highlight that there is still a need for better integration of Rsoil with eddy covariance measurements to address challenges related to spatial and temporal variability of Reco and Rsoil.