Looking deeper into the soil: biophysical controls and seasonal lags of soil CO2 production and efflux

We seek to understand how biophysical factors such as soil temperature (T-s), soil moisture (theta), and gross primary production (GPP) influence CO2 fluxes across terrestrial ecosystems. Recent advancements in automated measurements and remote-sensing approaches have provided time series in which lags and relationships among variables can be explored. The purpose of this study is to present new applications of continuous measurements of soil CO2 efflux (F-0) and soil CO2 concentrations measurements. Here we explore how variation in T-s, theta, and GPP (derived from NASA's moderate-resolution imaging spectroradiometer [MODIS]) influence F-0 and soil CO2 production (P-s). We focused on seasonal variation and used continuous measurements at a daily timescale across four vegetation types at 13 study sites to quantify: (1) differences in seasonal lags between soil CO2 fluxes and T-s, theta, and GPP and (2) interactions and relationships between CO2 fluxes with T-s, theta, and GPP. Mean annual T-s did not explain annual F-0 and P-s among vegetation types, but GPP explained 73% and 30% of the variation, respectively. We found evidence that lags between soil CO2 fluxes and T-s or GPP provide insights into the role of plant phenology and information relevant about possible timing of controls of autotrophic and heterotrophic processes. The influences of biophysical factors that regulate daily F-0 and P-s are different among vegetation types, but GPP is a dominant variable for explaining soil CO2 fluxes. The emergence of long-term automated soil CO2 flux measurement networks provides a unique opportunity for extended investigations into F-0 and P-s processes in the near future.