Disentangling the Effects of Temperature, Moisture, and Substrate Availability on Soil CO 2 Efflux
Soil respiration (Rs), the largest carbon emission flux in ecosystems is usually modelled as an empirically parameterized function of temperature, and sometimes water availability. The likely contribution by other factors, such as carbohydrate substrate supply from photosynthesis has been recognized, but modeling capacity to use this information is limited. Wavelet transformations of the residuals of a seasonal Q10 temperature response model indicated structure at different temporal scales. We hypothesize this indicates the lack of explicit representation of relevant processes in the models. Using cross-spectral analysis, we found that time series of photosynthetically active radiation (PAR) explained most of the diurnal variation, temperature explained variability at multiple time scales (diurnal - synoptic) whereas volumetric soil water content correlated with variability in Rs at scales 15-30 days. The results suggest that the time domains of influence for different driving variables of Rs are discrete, and largely non-overlapping, and represent functional relationships between soil biological activity and its constraints. Analysis of phase angles showed that Rs lagged PAR by 1.5-3.0 hours. As this time-lag was the same in both young and mature trees, with more than 5-fold difference in transport distances, we hypothesize that this finding adds to the body of literature that support the pressure-concentration-wave model of carbohydrate availability in plants.