Soil respiration measurements along climate and black spruce productivity gradients in interior Alaska

Soil respiration measurements were made using an infrared gas analyzer (IRGA; EGM-4 gas analyzer, PP Systems, Haverhill, MA) with a dynamic soil respiration chamber (SRC-2, PP Systems). Respiration collars (18) were randomly located on three 20 x 20 m sampling grids (separate from WSOC collection) at each site. The respiration collars (10.2 cm diameter schedule 40 polyvinyl chloride) were inserted to a depth of 2.5 cm at least 1 week prior to initial measurements and were left in place for the duration of the experiment. Care was taken to ensure that the collars remained at the same depth throughout the measurement period. All vascular plants were removed from the collars prior to measuring. Measurements were made approximately bi-weekly at each site, from May through September in 2004 (n = 5 or 6). A portable thermometer was used to measure soil temperature at 10 cm below the surface concurrently with soil respiration measured at each collar. Changes in collar-specific air volumes (caused by changes in microtopography and the presence of moss) were measured directly through use of the ideal gas law by injecting 25 cm-3 of 3500 mg m-3 CO2 into the soil respiration chamber and measuring the subsequent dilution of CO2 concentration with the IRGA. All flux values have been adjusted for these collar specific volumes.