CARBON-DIOXIDE EFFLUX FROM THE FLOOR OF A BOREAL ASPEN FOREST - I - RELATIONSHIP TO ENVIRONMENTAL VARIABLES AND ESTIMATES OF C RESPIRED

Soil CO2 efflux (Soil Fc) in a boreal aspen (Populus tremuloides) fore st was related to environmental variables to estimate the mass of carb on (C) annually respired. Measurements of soil Fc were made between Ap ril and September with a dynamic closed chamber via an infra-red gas a nalyzer. Over the course of the study day-time soil Fc ranged from 0.0 27 to 0.411 mg CO2 m(-2) s(-1). Summertime estimates of soil Fc were h igher than expected, and this was attributed to underestimation by sta tic chamber methods. Strong relationships were found between seasonal patterns of (1) soil Fc and soil temperature (Ts) at various depths (R -2 = 0.77 to 0.87, Q(10) = 3.9 to 5.1), (2) soil Fc and humus [CO2] an d volumetric moisture (theta(v); R-2 = 0.76 to 0.88) and (3) humus [CO 2] and humus Ts and theta(v) (R-2 = 0.74). On their own, theta(v), lit ter and soil organic matter content explained little (<5%) of the vari ation in soil Fc. Ts was the single most effective variable used to pr edict soil Fc. Annual masses of soil Fc C were estimated from the rela tionship between soil Fc and 0.1 m Ts in 1994. Long-term (49-yr) estim ates were generated from monthly mean air temperatures (Ta) using the relationship between Ts (0.10 m) and Ta (2 m) at the site throughout 1 994-1995. In both years, Ta explained about 70% of the variability in Ts. Estimates of annual soil Fc C were 905 (1994), 870 (1995) and 809 (long-term) g m(-2). Growing season soil Fc approximated 75% of the an nual C mass respired. These annual C fluxes far exceed current estimat es for boreal and most temperate forest ecosystems. 1994 soil Fc C als o exceeded the site estimate of aboveground net primary productivity ( 361 g C m(-2) yr(-1)). This extreme shortfall, along with the inabilit y of soil organic matter or litter mass to explain soil Fc, suggests t hat about 60% of annual soil Fc C is attributable to the presence of r oots. If soil Fc in vegetated environments is primarily due to the pre sence of roots then the significance of soil Fc to global C cycling ca n only be understood with concurrent estimates of net ecosystem C exch ange.