Each of the three forest types; oldgrowth, thinned pole and pole stands (OG. T, and P) used in this study were organized into triads to reduce confounding elements while comparing treatments. Individual sites within each triad were matched by elevation, aspect and proximity. Three triads were analyzed in each of the three regions. At each site, soil samples were collected to a depth of 10 cm every 5 m along a 250 m transect.
Sample Collection and Storage:
Mineral soil samples were collected with a trowel to a depth of 10 cm and transported to the laboratory in an ice chest. Soils were stored at 15°C until the initiation of the analyses which was within 16 h of their receipt.
Field measurements:
Field forest floor respiration rates, soil temperature, and litter depth were all measured at the study sites. The 24 h forest floor respiration was measured using a standard soda-lime technique (Edwards, 1982) . At the end of the incubation period, a 10 mL headspace gas sample was removed from the incubation chambers before the soda-lime jars were removed. Headspace samples from sealed controls were used to subtract out ambient CO2 concentrations at the site prior to initiation of the incubation period. Headspace CO2 that was not absorbed by the soda lime during the incubation period was added to the total CO2 adsorbed into the soda lime. Headspace CO2 concentrations were measured on a Hewlett Packard model 5890 gas chromatograph fitted with Hewlett Packard model 3396 integrator. The GC was fitted with a flame ionization detector and a methanizer in series. The integrator was calibrated using known gas standards and the external calibration method.
The other soil characteristics measured in this study were: soil organic matter, laboratory respiration, exchangeable ammonium, mineralizable nitrogen, water extractable organic carbon, denitrification potential, pH, bulk density, and moisture.
In preparation for laboratory analysis, all soils except those used in the water extractable organic carbon measurements were sieved through a 2-mm sieve. Soil organic matter was measured by loss-on-ignition at 550°C for 6 h following oven drying at 100°C. Laboratory respiration rates were measured by adding 5 g field moist soil to a 25 mL Erlenmyer flask. After being sealed with a serum bottle cap, the flasks were incubated at 24°C for one hr. Zero-time headspace-CO2 concentration measurements were made after an initial 1 h preincubation period. Flasks were subsequently incubated for 2 h at 24 °C and the headspace analyzed again for CO2. CO2 concentrations were assayed with a GC as described above. In the H2O respiration experiments, the same procedure was followed as for laboratory respiration rates except 2.0 mL of sterile H2O was added to the soils in the flasks; for SIR, 2 mL of sterile 1 mM glucose was added to the soils.
Exchangeable NH4+ concentration was determined by shaking 10 g field-moist soil with 50 mL 2 M KCl for 1 h (Keeney, 1982). After adding 0.3 mL 10 M NaOH to the slurry, NH4+ concentration was measured with a Orion model 95-12 ammonium electrode (Orion Research Inc. Boston, Ma, USA). Mineralizable N was measured using the waterlogged technique (Keeney, 1982). Ten g of field-moist soil was added to 53 mL of distilled water in a 20 x 125 mm screw cap test tube and incubating at 40°C. After 7 days, 53 ml of 4 M KCl was added to the slurry and NH4+ concentration was determined with the ammonium electrode. Mineralizable N was calculated from the difference between initial and final NH4+ concentrations.
Water extractable organic carbon (DOC) determinations were made using a Dormann Carbon Analyzer. DOC extractions were made on nonsieved samples. Five g of wet weight soil were slurried with 15 ml of water in 100 mL serum bottles and shaken for one hour at 24°C. Subsamples of the resulting slurry were centrifuged at 13,600 x g for 5 min. on a microfuge (Sorvall model MC12 V). Water-only blanks were run concomitantly to subtract out background organic carbon levels. Supernatants were extracted from the centrifuge tubes and frozen until analyzed.
Just before analyzing the samples, they were thawed and vortexed to suspend the precipitate. A series of experiments conducted with identical subsamples had the following treatments: (1) not frozen, (2) frozen with vortexing, and (3) frozen without vortexing. It was found that treatments 1 and 2 produced the same result therefore we concluded that the freezing and subsequent resuspension did not bias the resulting measurements.
Denitrification potential was measured using a method similar that used by Groffman and Tiedje (1989). Each reaction vessel (25 mL Erlenmeyer flask) contained 5 g of less than 2mm, field-moist soil. The flask was sealed with a rubber serum bottle stopper and purged with Ar to displace O2 in the headspace. After purging with argon, 2 mL of a 1mM solution of glucose and NO3- was added to the flask which were incubated at 25°C for one hr. This preincubation period was used because time series experiments on representative soils showed a lag in N2O production during this period. The same experiments have shown linear N2O production rates during the following 2-4 hr. After the preincubation period, 0.5 mL of headspace gas was removed from the reaction vessel and injected into a gas chromatograph fitted with an electron capture detector (Hewlett Packard model 5890 GC fitted with Hewlett Packard model 3396 integrator).
Another headspace N2O analysis was made after an additional two hr incubation at 25°C. The net N2O released over this 2 h period was used to estimate N2O production rates. Acetylene was not routinely added to the headspace to prevent the conversion of N2O to N2 because randomly selected samples (10% of the total) were also assayed with a 10% acetylene atmosphere. There were no significant differences between N2O production rates with and without acetylene.
Soil pH was measured in 1:10 (soil: distilled water) slurries of oven-dried (100°C) soil. These slurries were shaken for 1 h prior to reading pH values with Sigma model E4753 electrode.
REFERENCES
N. T. Edwards. 1982. "The use of soda-lime for mesuring respiration rates in terrestrial systems". Pedobiologia 23: 321-330
Groffman P. M. and Tiedje J. M. (1989) Denitrification in north temperate forest soils: relationships between denitrification and environmental factors at the landscape scale. Soil Biology and Biochemistry 21:621-626.
Keeney, D.R. (1982) Nitrogen-Availability indices. In: Methods of Soil Analysis, Part 2. Chemical and Microbiological Properties. 2nd Edition. Agronomy no. 9 (Ed. A.L. Page). American Society of Agronomy, Soil Science Society of America, Madison, pp. 711-733.
