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 (FCO2) was estimated using a univariate exponential model as a function of soil temperature based on 23 measurement dates over 5 years. Annual FCO2 for the undisturbed northern hardwood forest was estimated at 660 ± 54 g C·m2·year1. Low soil moisture significantly reduced FCO2 on three of the measurement dates. The proportion of FCO2 derived from the forest floor horizons was estimated empirically to be about 58%. We estimated that respiration of root tissues contributed about 40% of FCO2, 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·m2·year1, 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 FCO2 associated with autotrophic activity (52%) was comparable with that reported elsewhere (56%).
The Northern hardwood forest ecosystem: ten years of recovery from clearcutting