Three scales of temporal resolution from automated soil respiration measurements

Soil respiration (R s) is a combination of autotrophic and heterotrophic respiration, but it is often modeled as a single efflux process, influenced by environmental variables similarly across all time scales. Continued progress in understanding sources of variation in soil CO₂ efflux will require development of R s models that incorporate environmental influences at multiple time scales. Coherence analysis, which requires high temporal frequency data on R s and related environmental variables, permits examination of covariation between R s and the factors that influence it at varying temporal frequencies, thus isolating the factors important at each time scale. Automated R s measurements, along with air, soil temperature and moisture were collected at half hour intervals at a temperate forest at Harvard Forest, MA in 2003 and a boreal transition forest at the Howland Forest, ME in 2005. As in other temperate and boreal forests, seasonal variation in R s was strongly correlated with soil temperature. The organic and mineral layer water contents were significantly related to R s at synoptic time scales of 2-3 days to weeks, representing the wetting and drying of the soils as weather patterns move across the region. Post-wetting pulses of R s were correlated with the amount of precipitation and the magnitude of the change from pre-wet-up moisture content to peak moisture content of the organic horizon during the precipitation events. Although soil temperature at 8-10cm depth and R s showed strong coherence at a 24-h interval, calculated diel Q ₁₀ values for R s were unreasonably high (6-74) during all months for the evergreen forest and during the growing season for the deciduous forest, suggesting that other factors that covary with soil temperature, such as canopy assimilatory processes, may also influence the diel amplitude of R s. Lower diel Q ₁₀ values were obtained based on soil temperature measured at shallower depths or with air temperature, but the fit was poorer and a lag was needed to improve the fit (peak R s followed peak air temperature by several hours), suggesting a role for delayed substrate supply from aboveground processes to affect diel patterns of R s. High frequency automated R s datasets afford the opportunity to disentangle the temporal scales at which environmental factors, such as seasonal temperature and phenology, synoptic weather events and soil moisture, and diel variation in temperature and photosynthesis, affect soil respiration processes.