Responses of Fine Root Respiration and Root N to Soil Warming in Hardwood Forests.

Soil warming typically causes large initial increases in soil respiration, with the enhancement lessening over time.  Much of this transient response has been attributed to rapid decomposition of labile soil C 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 soil warming, specific root respiration rates (nmol CO₂ g^-1 s^-1) were measured in long-term warming experiments at Harvard Forest initiated in 1991 and 2003.  On twelve dates between September, 2007 and November, 2009, respiration rates for fine roots (< 1 mm) from control and heated (+ 5 ^oC) plots were measured at both a reference temperature of 18 ^oC and at the ambient soil temperature of the control or heated plots.  Average fine root respiration rates at the constant reference temperature of 18 ^oC were similar for control and heated plots for the 1991 warming experiment and 20% higher for the 2003 warming experiment.  As a result, respiration rates at ambient soil temperature were higher for the heated plots throughout the year, and for the 2003 warming experiment were often more elevated for the 5 ^oC increase than would be predicted using typical Q₁₀s for root respiration of 2 to 3.  Higher fine root metabolic capacity (rate at the 18 ^oC reference temperature) for the 2003 warming treatment was accompanied by higher root N concentrations.  However, respiration rates per unit N tended to be lower for the heated plots, suggesting that a common relationship between fine root N and respiration can not be applied to both treatments.  Overall, the data showed no evidence for acclimation of specific root respiratory capacity in response to warming, but field observations made during fine root sampling suggest reductions in root biomass with warming may play an important role in regulating annual ecosystem root respiration response.