Adjustment of fine root respiration rates to soil warming in hardwood forests

Background/Question/Methods Soil warming experiments typically show significant increases in soil respiration during their first one to three years, with the enhancement then lessening. This transient response is attributed to rapid decomposition of labile soil C compounds in the first years of warming, but the role of root respiration in changes in soil CO₂ efflux from warmed soils is not well understood. To assess the degree to which root respiration adjusts to warmer soil temperature regimes, we measured specific respiration rates (nmol CO₂ g^-1 s^-1) in three soil warming experiments at Harvard Forest in September, 2007. Soils in the experiments had been warmed for two, five, and sixteen years. Respiration rates for fine roots (< 1 mm) from control (15 ^oC on measurement date) and warmed (21 ^oC) plots were measured both at ambient soil temperature and at a common reference temperature of 18 ^oC. Results/Conclusions Respiration rates at field soil temperature for warmed plots were 45% greater than for unwarmed plots, but this enhancement is lower than the 52 to 77% increase that would be predicted by typical Q₁₀'s for forest root respiration of 2 to 2.6.  As a result, respiration rates at the constant reference temperature were significantly lower for the warmed plots, by an average of 23%. Root N concentration did not differ among treatments, thus respiration rate per unit N at the reference temperature was significantly lower for roots from warmed plots. Lower respiration rates for roots from warmed soils may indicate physiological acclimation to warmer temperatures or may simply be a consequence of drier soils on the warmed plots. Soil moisture contents were 11 to 18% lower for the warmed plots, with the treatment effect being significant for two of the three experiments. Continued root respiration measurements during the 2008 growing season will help us separate effects of dry soils from true temperature acclimation and assess the combined effects of warmer and drier soils on the contribution of roots to growing season soil respiration flux.