In the 1990s, there have been two major objectives Bob Griffiths' soils work at the H.J. Andrews Forest: (1) to determine how climate change and (2) to determine how forest disturbance influences soil nitrogen and carbon cycling. This study is designed to do both. Because of the broad geographical representation of these sites, all climate zones as influenced by slope, aspect and elevation, are represented as well as differing vegetation types, and disturbance histories. As of 1998, we represent different climate zones primarily by elevation. As the climate models are perfected, soils data can be evaluated in terms of annual mean temperatures and precipitation.
Because of the wide range of disturbances represented on the Andrews, these data will be analyzed in terms of disturbance influences on soil N and C cycling. Because of the scope of this study, these data could be used to parameterize nutrient and carbon cycling models in which potential influence of climate change and forest disturbance could be predicted over the Central Oregon Cascade Mountains.
The objective of this survey was to obtain the most geographically extensive dataset on soil characteristics as possible on the HJA. To this end, sites were located at 0.5 km intervals along all passable roads on the HJA. Site sites were located by distance, therefore, the type of vegetation that happened to be on the site was randomized. The only exception were 4 sites were adjacent vegetation types were sampled to gain direct vegetation effects data at a given location. All measurements were made either directly or from soils collected at 5 m intervals along a transect located approximately 40 meters from the nearest road and parallel to it. This interval was chosen since other studies have shown that for all of the variables studied, the samples taken at the is spacing were not autocorrelated. The sample grid consisted of enough points that strong geostatistical analyses could be made on the data. The variables chosen for measurement are those that give good indications of nitrogen and carbon cycling in forest soils. In addition, we chose to measure ectomycorrizhal mats since these apparently play a key role in nutrient cycling in these forests.
At each sample site a transect was established parallel to and greater than 40 meters distant from the closest road. Five sample locations were made at 5 meter intervals along that transect. Soil respiration measurements were made at each of these sites. In addition, three 4.7 x 10 cm soil cores were taken in the center three locations and composited for subsequent analysis. The samples were transported to the laboratory in an ice chest and subsequently stored at 15 degrees C until the initiation of analyses, usually within 16 h of their receipt. The following measurements were made in the field: litter depth, mineral soil respiration, soil temperature and ectomycorrhizal mat characteristics. Field (forest floor) respiration rates were measured with a nondispersive, infrared CO2 analyzer (Li-Cor, LI-6200). Measurements were made over a period of 1 min after the chamber gas reached ambient CO2 concentration. The instrument was calibrated on site against a known standard at each location. A Q10 adjustment was made for ambient soil temperature. Soil temperature was measured by electronic thermometers calibrated at 0 degrees C with ice water. The temperature probes were inserted into the mineral soil to a depth of 10 cm. Light was measured with the Li-Cor photometer.
The distribution of ectomycorrhizal mats was determined visually in the field by inspecting the relative abundance of mats in 4.7 x 10 cm cores. Two distinct mat types were scored: (1) mats similar to those of the genus Hysterangium and (2) mats similar to those of the genus Gautieria. This approach has been used successfully in the past to document ectomycorrhizal mat distribution patterns in coniferous forests of the Pacific Northwest (Griffiths et al. 1996).
Soil depth was measured by driving a steel rod into the ground until it stopped. This was done in 10 locations along the above mentioned transect. The coefficient of variability was determined by dividing the standard deviation by the mean value multiplied by 100.
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 degrees C for at least 8 h. The percent soil moisture was calculated by dividing the difference between wet and dry samples and dividing that number by the dry wt., which was then multiplied by 100. Soil organic matter was measured by loss-on-ignition at 550 degrees C for 6 h after oven drying at 100 degrees C.
Soil pH was measured in 1:10 (soil:distilled water) slurries of oven-dried (100 degrees C) soil. These slurries were shaken for 1 h prior to reading pH values with a Sigma model E4753 electrode. Soil organic matter was measured by loss-on-ignition at 550 degrees C for 6 h after oven drying at 100 degrees C. Bulk density was measured by oven-drying cores of a known volume.
Denitrification potential was measured using a method by Groffman and Tiedje (1989) as modified by us (Griffiths et al., 1998). Each reaction vessel (25-mL Erlenmeyer flask) contained 5 g of less than 2 mm, field-moist soil. Flasks were sealed with rubber serum bottle stoppers and purged with Ar to displace O2 in the headspace gas. After purging with Ar, 2 mL of a 1 mM solution of glucose and NO3- was added to each flask. Flasks were subsequently incubated at 25 degrees C for 1 h. This preincubation period was used because previous time-series experiments showed a lag in N2O production during this period. The same experiments have shown linear N2O production rates during the following 2-4 h (unpublished data). After the preincubation period, 0.5 mL of headspace gas was removed from the reaction vessel and injected into a gas chromatograph (GC) fitted with an electron capture 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. A second headspace N2O analysis was made after an additional 2-h incubation at 25 degrees C. The net N2O released over this 2-h period was used to estimate N2O production rates.
Laboratory respiration measurements were made on field-moist, sieved soils (4 g dry weight). Soils were brought to 75% moisture content by the addition of enough sterile deionized water to equal 3 g water per 25-mL Erlenmeyer flask. Once sealed with serum bottle stoppers, the flasks were incubated at 24 degrees C for 14 days before the headspace CO2 measurement was made using gas chromatography. This was a measure of labile soil carbon. The same GC and integrator as were used for this assay as that used to measure N2O, but in this case a flame ionization detector and a methanizer in series were used. Substrate induced respiration (SIR) was also measured in these soils. The reaction vessels were prepared as before except 0.1 mL of 1 M glucose solution was added to the reaction vessel and the assay for CO2 evolution rates were the same as for laboratory respiration.
Extractable ammonium was determined by shaking 10 g of field-moist soil with 50 mL 2 M KCl for 1 h (Keeney and Nelson 1982), adding 0.3 mL 10 M NaOH to the slurry, and measuring ammonium concentration with an Orion model 95-12 ammonium electrode (Orion Research Inc., Boston, MA). Mineralizable N was measured by the waterlogged technique of Keeney and Bremner (1966). For each analysis, 10 g of field-moist soil were added to 53 mL of distilled water in a 20 x 125 mm screw-cap test tube, and incubated at 40 degrees C for 7 d. Then 53 mL of 4 M KCl were added to the slurry, and ammonium concentration was determined with the ammonium electrode. Mineralizable N was calculated as the difference between initial and final ammonium concentrations.
Beta-glucosidase activity was determined by the spectrophotometric assay of Tabatabai and Bremmer (1969), as modified by Zou et al. (1992). One mL of 10 mM p-nitrophenyl b-D glucopyranoside substrate was added to duplicate 1-mL subsamples containing a soil slurry (1 gdw in 1 mL deionized H2O). The tubes were shaken and then placed with duplicate controls without substrate in a 30 degrees C water bath for 2 h. After incubating, 1 mL of 10 mM p-nitrophenyl b-D glucopyranoside was added to the controls, and all reactions were immediately stopped by the addition of 2 mL of 0.1 M tris[hydroxymethyl]aminomethane at pH 12.0. The mixtures were centrifuged for 5 min at 500 x g. From the supernatant, 0.2 mL was diluted with 2.0 mL deionized water. The optical density was measured at 410 nm, and a standard curve was prepared from 0.02 to 1.0 micro-mol/mL p-nitrophenol (pNP).
Griffiths, R.P., Homann, P.S., and Riley, R. (1998) Denitrification enzyme activity of Douglas-fir and red alder forest soils of the Pacific Northwest. Soil Biology and Biochemistry 30, 1147-1157.
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 & Biochemistry 21, 621-626.
Keeney, D.R., and Nelson, D.W. 1982. Nitrogen-inorganic forms. In Methods of soil analysis. Edited by A.L. Page, R.H. Miller, and D.R. Keeney. American Society of Agronomy, Madison, Wis. pp. 643-698.
Zou, X., D. Binkley, and K. G. Doxtader. (1992) A new method for estimating gross phosphorus mineralization and immobilization rates in soils. Plant & Soil 147,243 -
References
Griffiths, R. P., Bradshaw, G. A., Marks, B., and G. W. Lienkaemper. 1996. Spatial distribution of ectomycorrhizal mats in coniferous forests of the Pacific Northwest, USA. Plant and Soil 180:147-158.
