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Summer Research Project 2017

• Title: Disentangling the Factors Behind Thermal Acclimation of Soil Respiration Observed in a Long-Term Soil Warming Experiment
• Group Project Leader: Michael Bernard
• Mentors: Michael Bernard; Jerry Melillo; William Werner
• Collaborators: Michael Bernard; Jerry Melillo; William Werner
• Project Description:

  Global climate change is expected to alter the biogeochemistry of terrestrial ecosystems as increased temperatures accelerate decomposition of soil organic matter (SOM) into atmospheric CO2 (i.e. heterotrophic soil respiration), generating a self-reinforcing feedback to climate change. Given that a significant portion of carbon in the terrestrial biosphere is stored in forest soils, it is important that we improve our understanding of the patterns and processes by which the terrestrial carbon cycle will feedback to the climate system as the world warms.
  
  Since 1991, we have warmed forest soil in experimental plots (6 m x 6 m) five degrees Celsius above control plot temperatures at the Harvard Forest Long-term Ecological Research (LTER) site using buried heating cables. During this time, we have measured a net loss of soil carbon in response to warming that has been characterized by a three-phase pattern in heterotrophic soil respiration (i.e. the flux of carbon from SOM to CO2): an ephemeral increase (relative to control plots) during the first ten years (Phase I), a seven-year period during which there was no difference between the heated and control plots (Phase II), and a resumed increase that is currently ongoing.
  
  During all three phases, we observed “thermal acclimation” in the heated plots. Prolonged exposure to higher temperatures causes heterotrophic soil respiration to respond less strongly to temperature, such that if returned to equal temperatures, the heated plots would respire less than the control plots. Since long-term soil warming has altered several soil characteristics (including but not limited to moisture, water holding capacity, microbial biomass, and both the quantity and quality of soil organic matter), determining which factor(s) underlie the acclimation response is difficult to do using the field data. By setting up a series of laboratory incubations of soil collected from the warmed and control plots, we can investigate this question more effectively by holding these factors constant or standardizing our results by their values.
  
  Last summer’s student incubated soils from the warmed and control plots at a range of temperatures (6 – 30 degrees Celsius) and was able to qualitatively reproduce the thermal acclimation pattern of soil respiration observed in the field. Reductions in organic matter quantity and microbial biomass in warmed soils accounted for much of the difference in soil respiration; however, a reduction in microbial biomass per gram organic matter in warmed mineral soil (relative to control) was an unanticipated, and as of yet unexplained, result. Our 2017 project will build upon these findings by exploring two questions.
  
  Our first research question is: Do differences in SOM quality or physical protection account for reductions in microbial biomass per unit organic matter in warmed mineral soil? We will address this by characterizing the chemical compounds and aggregate structure of mineral soil from our experimental plots as well as by performing lab incubations on soils following aggregate destruction to measure CO2 production.
  
  Our second research question is: How much does moisture influence the warming effect on soil respiration. Recent dry seasons suggest that low soil moisture may reduce the warming effect, but it is difficult to assess the effect of moisture independently of temperature in the field given inter-seasonal and inter-annual variation. We will quantify the relative importance of each effect using a temperature × moisture factorial lab incubation on warmed and control soils.
  
  The student will work closely alongside two soil warming research assistants on site at the Harvard Forest. We anticipate that this project will be comprised of approximately 50% laboratory work, 25% field work, and 25% data analysis. The student should be interested in learning new lab/field techniques as well as gaining experience programming and analyzing data using the “R” statistical program. Some portion of the work may take place at UMass Amherst laboratories operated by our collaborators, which would require travel using Harvard Forest vehicles.
  

• Readings:

  Allison, S.D., Wallenstein, M.D. and Bradford, M.A., 2010. Soil-carbon response to warming dependent on microbial physiology. Nature Geoscience, 3(5), pp.336-340.
  
  Bradford, M.A., Davies, C.A., Frey, S.D., Maddox, T.R., Melillo, J.M., Mohan, J.E., Reynolds, J.F., Treseder, K.K. and Wallenstein, M.D., 2008. Thermal adaptation of soil microbial respiration to elevated temperature. Ecology Letters, 11(12), pp.1316-1327.
  
  Bradford, M.A., Wieder, W.R., Bonan, G.B., Fierer, N., Raymond, P.A. and Crowther, T.W., 2016. Managing uncertainty in soil carbon feedbacks to climate change. Nature Climate Change, 6(8), pp.751-758.
  
  Melillo, J.M., Steudler, P.A., Aber, J.D., Newkirk, K., Lux, H., Bowles, F.P., Catricala, C., Magill, A., Ahrens, T. and Morrisseau, S., 2002. Soil warming and carbon-cycle feedbacks to the climate system. Science, 298(5601), pp.2173-2176.
  
  Frey, S.D., Lee, J., Melillo, J.M. and Six, J., 2013. The temperature response of soil microbial efficiency and its feedback to climate. Nature Climate Change, 3(4), pp.395-398.
  
  Zhou, X., Wan, S. and Luo, Y., 2007. Source components and interannual variability of soil CO2 efflux under experimental warming and clipping in a grassland ecosystem. Global Change Biology, 13(4), pp.761-775.

• Research Category: Soil Carbon and Nitrogen Dynamics, Large Experiments and Permanent Plot Studies, Forest-Atmosphere Exchange