Abstract. Soil respiration (RS) is a major flux in the global carbon (C) cycle and its responses to changing environmental conditions may exert a strong control on the residence time of C in terrestrial ecosystems and in turn influence the atmospheric concentration of greenhouse gases. Soil respiration consists of several components returning C of different nature and age to the atmosphere, with root/rhizosphere respiration often assumed to be the dominant and variable one. Rates of RS vary greatly in time and space and the mechanisms underlying this temporal variability, or the RS components responsible for it, are poorly understood. It is often assumed the Rs and its components are under abiotic control at almost all time scales. In this study, we used the ecosystem 13C tracer at the Duke Forest Free Air CO2 Enrichment site to separate forest RS into four components: root/rhizosphere respiration (RR), litter decomposition (RL), and decomposition of soil organic matter (SOM) of two age classes – up to 8 years old and SOM older than 8 years. We then examined and found that diurnal and seasonal variability in the components of Rs occurred at different magnitudes and directions than total RS. Soil respiration was generally dominated by RSOM during the growing season (44% of daytime RS), especially at night. The contribution of heterotrophic respiration (RSOM and RL) to RS was not constant during the growing season, indicating that the seasonal variability seen in RR alone cannot explain the seasonal variability in RS. Although there was no diurnal variability in RS, there were significant compensatory differences in the contribution of individual RS components to daytime and nighttime rates. The average contribution of RSOM to RS was greater at night (54%) than during the day (44%) whereas the average contribution of RR to total RS was ~30% during the day and ~34% during the night. In contrast, RL constituted 26% of RS during the day and only 12% at night. Interestingly, the decomposition of C older than 8 years (Rpre-tr), which could contain the most recalcitrant C-pools in this forest, showed more pronounced and consistent diurnal variability than any other RS component, with nighttime rates on average 29% higher than daytime rates. In contrast, the decomposition of more recent, post-treatment C (Rpre-tr) did not vary diurnally. None of this diurnal variation in components of Rs could be explained by temperature and moisture variations and were likely due to biological controlling mechanisms. On growing season time scales some components of Rs varied with temperature moisture variations that also affect plant photosynthetic activity. Our results indicate that the variation observed in this forest on the components of RS is the result of complex interaction between dominant biotic controls (plant activity, mineralization constants, competition for substrates) over abiotic controls (temperature, moisture) in diurnal and seasonal time scales. Because RS integrates biological activity of several types of organisms, utilizing C of different chemistry, accessibility and ages, considering the controls and interaction among soil pools that result in the overall soil CO2 efflux is important in elucidating the controls on RS on ecosystem and atmospheric C-pools at different time scales.
Distinct patterns in the diurnal and seasonal variability in four components of soil respiration in a temperate forest under free-air CO<sub>2</sub> enrichment