This synoptic sampling grid was established to conduct basin-scale surveys of forest soil properties. In the first survey conducted in July 1993, forest floor respiration rates were measured at 130 sites covering a large portion of the HJA. The measurements made in this (SP005) were in situ field respiration. In addition to field respiration rates measured by the soda-lime method, soil and air temperatures were recorded along with maximum and minimum soil temperatures for the 24 hr CO2 collection period. Only one measurement was made per site. These data and associated metadata can be found in database SP005.
In 1994, a much more comprehensive study was conducted. The number of sites studied was expended to 183 sites located at 0.5 km intervals along all passable roads on the HJA. With the exception of the max-min soil temperature data, all variables measured in 1993 were repeated in 1994. In addition, the following variables were measured: pH, soil moisture, bulk density, soil organic matter, laboratory respiration (both long and short-term; with and without amendments), denitrification potential, extractable ammonium, and mineralizable nitrogen. In addition, we scored for the presence of moss on the surface and mycorrhizal mats found in 4.7 x 10 cm cores. These data and associated metadata are located in database SP006.
For this study, we measured CO2 evolution in the same soil samples over a period of 41 months to determine the rate at which respiration reduced over this time period. We also wanted to determine if respiration rates remained the same relative to each other over time.
Soil samples were collected to a depth of 10 cm during the 1994 REU synoptic grid study (see SP006 and SP028). While conducting autocorrelation analyses, we found that for the variables studied, samples collected at 5 m intervals were not autocorrelated and could thus be considered independent for statistical analysis. For this reason, replicate samples were taken in plots separated by 5 meters along a transect parallel to the road.
In preparation for laboratory analyses, all soils were sieved through a 2-mm sieve. Soil moisture was determined by drying duplicate 10 g field-moist sieved soils at 100°C for at least 8 h. Dividing the difference between wet and dry samples and dividing that number by the dry wt., which was then multiplied by 100, calculated the percent soil moisture.
Laboratory respiration measurements were made on field-moist, sieved soils (4 g dry weight). Soils were brought to 43% moisture content by the addition of enough sterile deionized water to equal 1.71 g water per 50-mL serum bottle. Once sealed with serum bottle stoppers, the serum bottles were incubated at 15°C. Weekly, the CO2 in the headspace was measurement using a gas chromatograph fitted with thermal conductivity detector (Hewlett Packard model 5890 GC, connected to a Hewlett Packard model 3396 integrator). The integrator was calibrated by the external calibration method with known gas standards. After the headspace was analyzed, the headspace was purged with lab air and the bottles weighed. Water loss was replenished as needed with sterile deionized water. Weekly measurements were made during the first 15 weeks. The headspace analysis was conducted every other week after the 15th week.
