High-frequency analysis of the complex linkage between soil CO2 fluxes, photosynthesis and environmental variables

High-frequency soil CO2 flux data are valuable for providing new insights into the processes of soil CO2 production. A record of hourly soil CO2 fluxes from a semi-arid ponderosa pine stand was spatially and temporally deconstructed in attempts to determine if variation could be explained by logical drivers using (i) CO2 production depths, (ii) relationships and lags between fluxes and soil temperatures, or (iii) the role of canopy assimilation in soil CO2 flux variation. Relationships between temperature and soil fluxes were difficult to establish at the hourly scale because diel cycles of soil fluxes varied seasonally, with the peak of flux rates occurring later in the day as soil water content decreased. Using a simple heat transport/gas diffusion model to estimate the time and depth of CO2 flux production, we determined that the variation in diel soil CO2 flux patterns could not be explained by changes in diffusion rates or production from deeper soil profiles. We tested for the effect of gross ecosystem productivity (GEP) by minimizing soil flux covariance with temperature and moisture using only data from discrete bins of environmental conditions (+/- 1 degrees C soil temperature at multiple depths, precipitation-free periods and stable soil moisture). Gross ecosystem productivity was identified as a possible driver of variability at the hourly scale during the growing season, with multiple lags between similar to 5, 15 and 23 days. Additionally, the chamber-specific lags between GEP and soil CO2 fluxes appeared to relate to combined path length for carbon flow (top of tree to chamber center). In this sparse and heterogeneous forested system, the potential link between CO2 assimilation and soil CO2 flux may be quite variable both temporally and spatially. For model applications, it is important to note that soil CO2 fluxes are influenced by many biophysical factors, which may confound or obscure relationships with logical environmental drivers and act at multiple temporal and spatial scales; therefore, caution is needed when attributing soil CO2 fluxes to covariates like temperature, moisture and GEP.