Distinct patterns in the diurnal and seasonal variability in four components of soil respiration in a temperate forest under free-air CO 2 enrichment

Abstract

Soil respiration (R S) is a major flux in the global carbon (C) cycle. Responses of R S 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 oxidizing soil C from different pools, age and chemistry. The mechanisms underlying the temporal variability of R S components are poorly understood. In this study, we used the long-term whole-ecosystem 13C tracer at the Duke Forest Free Air CO 2 Enrichment site to separate forest R S into its autotrophic (R R) and heterotrophic components (R H). The contribution of R H to R S was further partitioned into litter decomposition (R L), and decomposition of soil organic matter (R SOM) of two age classes - up to 8 yr old and SOM older than 8 yr. Soil respiration was generally dominated by R SOM during the growing season (44% of daytime R S), especially at night. The contribution of heterotrophic respiration (R SOM and R L) to R S was not constant, indicating that the seasonal variability in R R alone cannot explain seasonal variation in R S. Although there was no diurnal variability in R S, there were significant compensatory differences in the contribution of individual R S components to daytime and nighttime rates. The average contribution of R SOM to R S was greater at night (54%) than during the day (44%). The average contribution of R R to total R S was ∼30% during the day and ∼34% during the night. In contrast, R L constituted 26% of R S during the day and only 12% at night. About 95% of the decomposition of soil C older than 8 yr (R pre-tr) originated from R SOM and showed more pronounced and consistent diurnal variability than any other R S component; nighttime rates were on average 29% higher than daytime rates. In contrast, the decomposition of more recent, post-treatment C (R pre-tr) did not vary diurnally. None of the diurnal variations in components of R H could be explained by only temperature and moisture variations. Our results indicate that the variation observed in the components of R S is the result of complex interaction between dominant biotic controls (e.g. plant activity, mineralization kinetics, competition for substrates) over abiotic controls (temperature, moisture). The interactions and controls among roots and other soil organisms that utilize C of different chemistry, accessibility and ages, results in the overall soil CO 2 efflux. Therefore understanding the controls on the components of R S is necessary to elucidate the influence of ecosystem respiration on atmospheric C-pools at different time scales. © Author(s) 2011.