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Harvard Forest Symposium Abstract 2015

• Title: Sustained, self-reinforcing feedback between terrestrial carbon cycle and climate system based on 24 years of soil warming
• Primary Author: Jerry Melillo (Marine Biological Laboratory)
• Additional Authors: Michael Bernard (Marine Biological Laboratory); Frank Bowles (Research Designs); Kristen DeAngelis (University of Massachusetts Amherst); Serita Frey (University of New Hampshire - Main Campus); William Werner (Marine Biological Laboratory)
• Abstract:

  Over 24 years, we have measured changes in C and N cycles and soil microbial activity at the Prospect Hill soil warming experiment to explore how the terrestrial carbon-cycle will feedback to the climate system as the world warms. We have observed a pattern of soil carbon fluxes that can be broken into three distinct phases (Fig. 1): an ephemeral increase (relative to control plots) during the first ten years (Phase I), a seven-year period during which there was no significant response (Phase II), and a resumed increase that is currently ongoing (Phase III). In 2013, we measured a 31% reduction (800 ± 300 g C m-2) in forest-floor carbon stock and estimated a 16% loss (1350 ± 140 g C m-2) of soil carbon found in the top 60 cm of soil at the start of the experiment. We also observed a transient increase in rates of nitrogen mineralization during Phase I, but rates have remained at control levels when remeasured at later dates (Fig. 2).
  
  
  
  The decline in soil respiration during phase II is hypothesized to occur due to the depletion of fast-cycling labile SOM pools and, to some extent, a physiological acclimation (Fig. 3) of microbes to warmer temperatures. The reemergence of elevated soil respiration during phase III cannot be explained by either hypothesis; however, and our current thinking is that the microbial community may have reorganized in composition or function to better access more recalcitrant SOM pools. Testing the ability of soil microbes to utilize various carbon substrates as an indicator of functional capacity, we observed a decrease in substrate utilization in heated soils during phase II and an increase in mineral soils during phase III (Fig. 4).
  
  
  
  Our results suggest that soil warming may create a self-reinforcing feedback between the terrestrial carbon-cycle and climate system, but two mechanisms may reduce the magnitude of the feedback. Increased net nitrogen mineralization resulting from increased SOM decay could promote increased tree growth and woody storage of carbon in N-limited systems (see companion poster by Werner et al.); however, the return of N mineralization to control levels during phases II and III suggests that this fertilization response may be temporary. Starting in the second year of our study, we observed a reduced response of soil respiration to soil temperature. This “thermal acclimation” can be observed throughout all phases (Fig. 3) and its effect is apparent during three years when the heating system was inactive (Fig. 1), when soil carbon fluxes from heated plots were less than control plots. Without acclimation, we calculate that our estimate of carbon losses from the heated plots would be four times greater.
  
  

• Research Category: Soil Carbon and Nitrogen Dynamics


• Figures:
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