We present an hourly time series of the CO₂ concentration profile in the top 20 cm of a boreal forest litter layer at a site in northern Manitoba, Canada. The profile data, measured with an automated sampling system during the summer of 1999, show a pronounced daily cycle, with a small surface CO₂ gradient and low concentrations during the day and a large surface gradient and high concentrations at night. The CO₂ profile measurements allow us to test two current assumptions built into measurements of ecosystem carbon fluxes. The first assumption is that the flux from the surface to the atmosphere can be calculated using the measured CO₂gradient and a calculated value of the diffusive transport coefficient. The behaviour of the surface CO₂ gradient suggests that one cannot assume diffusive transport across the moss surface at this site when the friction velocity measured at 30 m exceeds 0.4 m s⁻¹. This condition, associated with turbulent mixing generated by wind shear and/or solar heating of the surface, was often encountered during the day at this site, though rarely at night. During the day, friction velocity and wind speed measured at 30 m height are linearly related, with friction velocity exceeding 0.4 m s⁻¹ when wind speed exceeds about 2 m s⁻¹. At night, wind at the top of the canopy may be laminar, so that the wind speed must exceed 4 m s⁻¹ to cause enough turbulence to raise friction velocity above the 0.4 m s⁻¹ threshold. The second assumption is that changes in soil pore-space CO₂ storage can be neglected when correcting eddy covariance measurements for ecosystem respiration that is stored in the ecosystem rather than being mixed into the overlying atmosphere. Our results show that the soil pore-space CO₂ profile is not in steady state at the site, but that the magnitude of the corresponding storage flux is small relative to the below-canopy CO₂ storage flux. The soil pore-space CO₂ storage flux ranges between ±0.4 μmol m⁻² s⁻¹, while the below-canopy storage flux ranges between ±20 μmol m⁻² s⁻¹. However, the soil pore-space storage flux could be significant relative to the CO₂ respiration flux across the soil surface, which we estimate to be in the range of 1–4 μmol m⁻² s⁻¹.

This paper presents data collected by an automated system designed to measure the seasonal cycle of both the quantity and isotopic composition of soil respiration. The results support the hypothesis that deep soil respiration at the BOREAS Northern Old Black Spruce site is sensitive to soil thaw and that much of this CO₂ represents decomposition of old organic matter, rather than simply root respiration. This is the first study to use a completely automated soil gas sampling system in the field to measure the soil CO₂ profile and collect samples of soil air for subsequent isotopic analysis. During the summer of 1999, deep soil respiration varied linearly with 50 cm temperature, with a slope of 0.2 kg C ha⁻¹ d⁻¹ °C⁻¹. Late in the season, this respiration was dominated by decomposition of old soil organic matter (fixed from the atmosphere centuries ago). This study has taken an important step toward clarifying the temperature sensitivity of deep soil CO₂ production in black spruce ecosystems through direct measurements within the soil and to quantify the seasonal variation of the relative contributions of recently fixed versus old carbon in deep soil respiration.

The response of large stores of carbon in boreal forest soils to global warming is a major uncertainty in predicting the future carbon budget. We measured the temperature dependence of decomposition for upland boreal peat under black spruce forest with sphagnum and feather moss understory using incubation experiments. CO₂ efflux rates clearly responded to temperature, which ranged from −10° to +8°C by ∼2°C increments. At temperatures below 0°C, significant decomposition was observed in feather moss peat but not in wetter sphagnum peat. Above 0°C, decomposition was exponentially related to temperature, corresponding to a Q(10) (the ratio of the rate of CO₂ evolution at one temperature divided by that at a temperature 10°C cooler) of 4.4 for feather moss and 3.1 for sphagnum peat. The greatest change in CO₂ evolution rate with temperature occurred between −2° and 0°C, which coincided with the phase transition of soil water. We saw no large change in the rate of CO₂ evolution between incubation experiments separated by a 6 month storage period for feather moss peat. Stable C isotope measurements of evolved CO₂ and the rate of change of CO₂ evolution with time suggest different substrates are used to sustain heterotrophic respiration above and below freezing. Radiocarbon signatures of CO₂ respired from both types of peat reflected significant contributions from C fixed in the last 35 years (“bomb” ¹⁴C) as well as C fixed prior to 1950. We observed no change in the Δ¹⁴C of respired CO₂ with temperature. Isotopic signatures of peat components showed that a combination of substrates must contribute to the CO₂ evolved in our incubations. Decomposition of fine roots (which made up less than 7% of the total peat C) accounted for ∼50% of respired CO₂ in feather moss peat and for ∼30% of respired CO₂ in sphagnum peat. Fine-grained (<1 mm), more humified material that makes up 60–70% of the bulk peat organic carbon contributed significantly to heterotrophic respiration (∼30% in feather moss and ∼50% in sphagnum moss peat), despite slow decomposition rates. Increased temperatures caused enhanced decomposition from all pools without changing their relative contributions. Because the contribution of peat decomposition is a small portion of total soil respiration at the study site, increased respiration rates would be difficult to measure as increased fluxes in the field. Nonetheless, sustained warming could lead to significant loss of C from these peat layers.

We present an estimate of net CO₂ exchange between the terrestrial biosphere and the atmosphere across North America for every week in the period 2000 through 2005. This estimate is derived from a set of 28,000 CO₂ mole fraction observations in the global atmosphere that are fed into a state-of-the-art data assimilation system for CO₂ called CarbonTracker. By design, the surface fluxes produced in CarbonTracker are consistent with the recent history of CO₂ in the atmosphere and provide constraints on the net carbon flux independent from national inventories derived from accounting efforts. We find the North American terrestrial biosphere to have absorbed −0.65 PgC/yr (1 petagram = 10¹⁵ g; negative signs are used for carbon sinks) averaged over the period studied, partly offsetting the estimated 1.85 PgC/yr release by fossil fuel burning and cement manufacturing. Uncertainty on this estimate is derived from a set of sensitivity experiments and places the sink within a range of −0.4 to −1.0 PgC/yr. The estimated sink is located mainly in the deciduous forests along the East Coast (32%) and the boreal coniferous forests (22%). Terrestrial uptake fell to −0.32 PgC/yr during the large-scale drought of 2002, suggesting sensitivity of the contemporary carbon sinks to climate extremes. CarbonTracker results are in excellent agreement with a wide collection of carbon inventories that form the basis of the first North American State of the Carbon Cycle Report (SOCCR), to be released in 2007.