The effects of elevated soil temperatures on root and microbial respiration in a mixed hardwood forest

Jonathan Sanderman

To examine the belowground carbon responses to soil warming, we have been conducting a trenched and heated factorial experiment at the Harvard Forest, in Petersham, MA. In the fall of 1994, triplicate 4 m² plots of four treatments were established: (1) trenched and heated; (2) trenched and unheated; (3) untrenched and heated; (4) untrenched and unheated. CO₂ effluxes, soil temperature, and soil moisture were measured in each plot during 1996 and 1997.

A 5° C increase in soil temperature raised total soil respiration (untrenched) by 20%, microbial decomposition (trenched) by 27%, but did not appear to affect root respiration which remained constant at 20% of total soil respiration. Soil respiration increased exponentially with soil temperature (R² = 0.70) and linearly with soil moisture (R² = 0.20). Heating depressed the Q₁₀ for total soil respiration from 3.1 to 2.2, while the Q₁₀ for the microbial component remained constant at 2.1.

Cumulative CO₂ effluxes for all treatments and the increases due to heating were less in 1997 than in 1996. Monthly precipitation differences between 1997 and 1996 could account for 20% of the variance in the monthly CO₂ efflux. The lowered response to heating in 1997 could also be a result of a decreasing labile carbon pool due to increased microbial decomposition associated with reduced belowground litter and root inputs.