Eight Mile Lake gradient sites: Growing season soil profile CO2 production at 10, 20, 30, and 40 cm from 2005 to 2007

Methods We used soil gas wells to collect gas from the soil profile in order to estimate CO2 production from different soil layers. Gas wells were constructed from 1/8” diameter stainless steel tubing. The tubing had the length of the designated depth (10, 20, 30, and 40 cm) plus an additional 30cm for the aboveground and an additional 30 cm, which was bent into an L-shape and pushed horizontally into the face of the soil profile at designated depths within the soil profile. At the soil surface, an airtight stopcock was glued to the end of the stainless steel tubing and was kept closed, except during sampling to minimize ambient air exchange with soil gas. Gas wells had three holes near the tip that went in to the soil profile, which was wrapped around with mesh cloth to decrease the chance of clogging by soil organic matter or silt. Five gas well locations were established at each site in 2004, with each location having wells at four depths in the soil profile (10, 20, 30, and 40 cm). In 2005, we installed gas wells at 6 additional locations at each site, with each site having shorter depth intervals (5, 10, 15, 20, 25, and 30 cm) to obtain finer scale estimates of soil profile CO2 flux. These gas wells were left in place during the three years of study to minimize the disturbance created by installation of the gas wells. We used airtight plastic syringes to sample 20 mL of gas from each gas well. The gas wells were not purged before sampling, but we assumed that the CO2 concentration inside the tube was same as the CO2 concentrations in the soil layer because it was kept closed. The gas samples were analyzed using an injection valve and loop (Valco Instrumental Company Inc., Houston, TX, Valco 6-port injection valve, 1 mL injection loop) attached to an infrared gas analyzer (Li-820, LI-COR Biosciences, Lincoln, Nebraska). The injection loop attached to the infrared gas analyzer was a closed system that used a pre-existing gas scrubbed through a 1 L jar of soda lime. The loop trapped 1 mL of the injected sample, which was later mixed with larger volume of CO2-free air and circulated through an infrared gas analyzer. We used 1,000, 10,000, and 25,000 ppm certified CO2-in-air standards to generate standard curves for the samples measured with the infrared gas analyzer through the injection loop. Samples were taken weekly throughout the growing season (May to September) from 2005 to 2007 on calm mornings, to minimize pressure-driven soil gas exchange between ambient air and soil that occurs as a result of wind [Hirsch et al., 2004]. We were able to sample CO2 from deeper soil layers in the beginning of the growing season, which was taken from air pockets within frozen soil. These air pockets trapped CO2 during soil surface freeze in the fall and CO2 produced over the winter, which often contained high CO2 concentrations up to 10%. However, since these air pockets were not subject to the standard assumptions of the normal diffusion law, we did not include these measurements in the growing season soil CO2 flux estimates. On average, across years and sites, we were able to sample gas from ice-free soil starting at day of year 150 for the 10 cm depth, starting at day of year 165 for the 20 cm depth, and starting at day of year 185 for the 30 cm depth, as the seasonal thaw front descended within the active layer. At the EML gradient sites, deeper soil layers were saturated with water most of the time due to a water table that was perched on the ground ice surface. During rain events, even surface layers (10 cm) could become saturated with water. When soils were waterlogged, we collected 10 mL of soil water and mixed it with 10 mL of ambient air, shook it for 1 minute to equilibrate, then measured CO2 from the headspace of the mixed gas sample.