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Harvard Forest Symposium Abstract 2009

• Title: Response of fine root respiration rates and root N to soil warming in hardwood forests
• Primary Author: Andrew Burton (Michigan Technological University)
• Additional Authors: Serita Frey (University of New Hampshire - Main Campus); Jerry Melillo (Marine Biological Laboratory)
• Abstract:

  Soil warming typically causes large initial increases in soil respiration, with the enhancement lessening over time. Much of this transient response is attributed to rapid decomposition of labile soil C compounds in the first years of heating, but the potential role of changes in root respiration is not well understood. To assess the degree to which root respiration adjusts to warmer soil temperature regimes, specific root respiration rates (nmol CO2 g-1 s-1) were measured in three soil warming experiments at Harvard Forest in September, 2007, and six times from May through October, 2008. Soils in the experiments had been heated since 1991, 2003 and 2006. Respiration rates for fine roots (< 1 mm) from control and heated plots were measured both at a common reference temperature of 18 oC and at the ambient soil temperature of the control or heated (+ 5 oC) plots in each experiment.
  
  
  
  In the 1991 (6 m x 6 m) and 2006 (3 m x 3 m) small plot experiments, fine root respiration rates at the constant reference temperature of 18 oC were significantly lower for the heated plots through June 2008, and thereafter were usually similar for control and heated plots (Figure 1). As a result, respiration rates at ambient soil temperature for these experiments were higher for the heated plots throughout the year (Figure 2), but were often not as elevated, for 5 degree C warmer conditions, as would be predicted using typical Q10s for root respiration of 2 to 3. In the 2003 Mega plot (30 m x 30 m) experiment, fine root respiration rates at the 18 oC reference temperature were higher for the heated plot than the control plot from July through October (Figure 1), which was a wetter period than the previous months during which respiration rates were lower for the heated plots. At ambient soil temperatures, rates were much higher for the heated Mega plots than for the control (Figure 2). Higher rates for the heated Mega plots were accompanied by higher root N concentrations. However, respiration rates per unit N were consistently lower in the heated plots of all three experiments than for the control plots (Figure 3), suggesting that a common relationship between fine root N and respiration could not be applied to both treatments.
  
  
  
  Overall, the data showed no consistent trend for lower fine root metabolic capacity (i.e. respiration rate at the reference temperature) for the heated treatments of the three experiments, with higher rates in the heated plots almost as common as lower rates. Other research at the sites, exploring the effects of warming on fine root biomass, will be utilized in conjunction with the specific respiration rates to assess how the two factors combine to regulate changes in fine root respiration at the ecosystem level in warmed soil. Field observations made during fine root sampling suggest reduced root biomass may play an important role in regulating annual ecosystem root respiration, especially in the Mega plots. Future work will also include comparisons of seasonal changes in root respiration on the Mega plots with pulses of soil moisture and N availability, as it appears warming in these large plots has made root system activity much more dynamic than warming in the smaller plots, which do not encompass trees’ entire root systems.
  
  

• Research Category: Large Experiments and Permanent Plot Studies
  Soil Carbon and Nitrogen Dynamics


• Figures:
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HarvardForest-Burton2009Abstract-Figure3.jpg