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Harvard Forest Symposium Abstract 2014

  • Title: Partitioning Autotrophic and Heterotrophic Contributions to Soil Respiration
  • Primary Author: Kathleen Savage (Woods Hole Research Center)
  • Additional Authors: Eric Davidson (University of Maryland - Center for Environmental Science)
  • Abstract:

    Soil respiration (Rt) is an aggregation of two belowground processes, autotrophic (live root and mycorrhizal fungal symbionts: Ra) and heterotrophic (live microbial decomposition of soil organic matter: Rh) respiration. Rt is often measured and modeled as a single process, however Rh and Ra typically originate from a variety of species and may respond differently to temperature, soil water content and substrate availability at diel and seasonal time scales.

    Within the footprint of the EMS tower at Harvard forest, automated respiration chambers were utilized in conjunction with the trenching method to partition Rt into its components, Rh and Ra. In the late fall of 2012, a trench was dug (to 1m depth) around a 5x5m area, severing all roots leading into the treatment plot. Plastic tarp was placed along the walls of the trench and then backfilled. Four automated chambers were place in each of the trenched and un-trenched plots. Fluxes, along with concurrent soil temperature and moisture, were measured at hourly rates from early April through mid Nov. of 2013. The fluxes from the non-trenched plot represent the combined heterotrophic and autotrophic components, Rt. Fluxes from the trenched plot represent the heterotrophic component, Rh, and the difference (Rt-Rh) represents the autotrophic component, Ra.

    Figure 1 shows the mean daily flux for 2013 from the trenched and un-trenched plots. There is a strong seasonal pattern to the contribution of Ra to Rt (Figure 1a, b). Peak daily contributions of≈ 30% were observed in mid-summer when tree activity was greatest and the lowest % daily contribution of Ra to Rt occurred in early spring and late fall at around 10% (Figure 1b). Summing the daily fluxes over the entire sampling season, Ra contributed 23% to the total seasonal estimate of Rt. This is similar to our findings at the Little Prospect Hill tract of the Harvard forest, in which we conducted a similar trenching experiment and found that Ra contributed 22% to Rt over the entire season with daily peaks in Ra contribution to Rt during the mid-summer of 46%.

    Flux data were binned into phenological stages, spring (Apr-May), summer (June, July, Aug to Sept 15) and fall (Sept 16, Oct, Nov). Looking at the diel pattern during differing phenological stages (Figure 2, diel model equation presented in Figure 2) the mean daily respiration (Y) of Rt was greater than Rh during all seasonal periods with the greatest difference during the summer when the autotrophic component (Ra) of Rt was greatest (Figure 1b). Diel amplitudes were greater for Rt than Rh in the summer period, but showed similar amplitudes in the spring and fall. This difference in diel amplitude can be attributed to the greater influence of Ra in the summer months, compared to spring and fall.

    The contribution of Ra to Rt changed seasonally and on a diel time step depending on the phenological stage of the forest, with the greatest influence of Ra on Rt occurring during the summer months, when trees are most active.

  • Research Category: Forest-Atmosphere Exchange

  • Figures:
  • Fig1_2014.pdf