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Harvard Forest Symposium Abstract 2009

  • Title: Soil Respiration at a Diel Time Scale
  • Primary Author: Kathleen Savage (Woods Hole Research Center)
  • Additional Authors: Eric Davidson (University of Maryland - Center for Environmental Science)
  • Abstract:

    Seasonal measurements of soil respiration (SR) are often used to derive a single Q10 temperature function, which is assumed also to apply to diel timescales. However, because SR is a combination of autotrophic (root) and heterotrophic (microbial) respiration, both of which may be influenced by differing environmental factors and at differing time scales, the assumption of similar temperature responses at seasonal and diel time scales may not be valid.

    Using an automated system, high frequency temporal SR and soil temperature measurements (8-10cm depth) were collected in a well-drained deciduous stand at the Harvard forest, MA and in a well drained coniferous dominated stand at the Howland forest, ME. SR was measured at 6 chambers every half hour from spring (early May) through fall (early Nov). Our objective was to utilize these two datasets to tease out process based responses of SR to environmental variables at a seasonal and diel time scale.

    The seasonal Q10 value calculated for the Harvard and Howland forests were similar; Q10 of 4.0 for Harvard and, 4.2 for Howland. High Q10 values (>3) maybe an indicator that factors other than temperature, but that covary with temperature, also contribute to temporal variation in soil respiration. These factors that covary with temperature may be related to the phenology of seasonal C inputs to the soil.

    To examine the diel response of SR to soil temperature at both sites, a diel Q10 was calculated for periods of several days within each season that were not influenced by precipitation (either wetting up or drying down) and that contained no missing SR measurements per day, Figure 1. The calculated diel Q10 values were unreasonably high (from 5 to 74) for all months at the evergreen Howland forest (Fig. 1a) and also high (6-11) at the deciduous Harvard Forest (Fig. 1b) during months when leaves were present. Very high diel Q10 values (>6) when a canopy was present indicate that processes other than the direct effects of varying soil temperature must be influencing the diel amplitude of SR during seasons when photosynthesis occurs. The most likely explanation is that substrate supply for root respiration probably covaries with both seasonal and diel soil temperature variation and contributes to the seasonal and diel amplitudes.

    To further investigate this, we (in collaboration with Jim Tang from MBL) have installed 6 automated SR chambers in a trenched and an un-trenched plot at Little Prospect Hill. Trenching is designed to exclude root growth within the trenched plot. The high frequency SR data from the trenched plot will allow us to examine the seasonal and diel pattern of the heterotrophic component of SR, while the un-trenched plot will provide us with the combined response of heterotrophic and autotrophic respiration.

  • Research Category: Forest-Atmosphere Exchange

  • Figures:
  • Harabs09_Fig1.gif