Soil respiration and soil carbon balance in a northern hardwood forest ecosystem

Soil C fluxes were measured in a northern hardwood forest ecosystem at the Hubbard Brook Experimental Forest to provide insights into the C balance of soils at this long-term study site. Soil CO2 emission (F(CO2)) was estimated using a univariate exponential model as a function of soil temperature based on 23 measurement dates over 5 years. Annual F(CO2) for the undisturbed northern hardwood forest was estimated at 660 +/- 54 g C.m(-2).year(-1). Low soil moisture significantly reduced F(CO2) on three of the measurement dates. The proportion of F(CO2) derived from the forest floor horizons was estimated empirically to be about 58%. We estimated that respiration of root tissues contributed about 40% of F(CO2), with a higher proportion for mineral soil (46%) than for forest floor (35%). Soil C-balance calculations, based upon evidence that major soil C pools are near steady state at this site, indicated a large C flux associated with root exudation plus allocation to mycorrhizal fungi (80 g C.m(-2).year(-1), or 17% of total root C allocation); however, uncertainty in this estimate is high owing especially to high error bounds for root respiration flux. The estimated proportion of F(CO2) associated with autotrophic activity (52%) was comparable with that reported elsewhere (56%).