Seasonal relationships between soil respiration and water-extractable carbon as influenced by soil temperature and moisture in forest soils of the Andrews Experimental Forest, 1992-1993

ORIGINATOR: Robert P. Griffiths
ABSTRACTOR: Robert P. Griffiths
1 Dec 1992
22 Mar 2013
Inorganic nutrients, Organic matter, Long-Term Ecological Research (LTER), meteorology, soil chemistry, microclimate, soil respiration, respiration, carbon cycling, inorganic nutrients, carbon
Field Methods - SP004:

1. Field studies; monthly: There are 10 plots within each of the twenty sites. These consist of locations along a 50 meter transect 5 meters apart where the gas flux chambers are placed and soil samples taken. Soil respiration was measured using indicating soda lime with a mesh of 6/12 (Fisher #S201-3). The increase in weight is directly proportional to the amount of CO2 adsorbed during the incubation period. Thirty grams of soda lime was added to 8 oz glass jars fitted with gastight metal lids. They were heated uncovered for 8 hr at 100°C to remove moisture. At time 0 the jars were placed on metal mesh stands within a incubation chamber (3 gal plastic ice cream tubs with a 23.5 cm opening), the lids were removed and the chambers sealed into the soils. Twenty four hours later, the lids were placed back on the jars and returned to the laboratory where they were again dried (with lid off) at 100°C for 8 hr.

At each site, there was one control jar used to determine background CO2 levels and the uptake of CO2 during drying (jar was placed in a sealed incubation vessel). At every other location along the transect, about 800 g of soil was collected from the top 10 cm of mineral soil and put into plastic bags. These soils were sieved through a 2 mm sieve and used for the laboratory analyses. In the field, maximum and minimum 24 hr air temperatures were recorded using max-min thermometers or by traces using small portable thermographs in plastic bags. In addition, air and soil temperatures were taken at the beginning and end of each field respiration period.

2. Field studies; July: At each of 10 plots within each site, there were two chambers placed; one to measure total soil respiration as was the case in the monthly samplings and one next to it in which a plastic sheet was installed just above the mineral soil. The plastic sheets were installed 24 hr before the start of the actual experiment. By comparing the CO2 released by the total soil including both litter and mineral soil and litter alone, the contribution of mineral soil (and roots) could be calculated.

At each of the ten plots along the transect, 4.7 x 10 cm mineral soil cores were taken in the top 10 cm of mineral soil to measure bulk density. Another set of cores was used to estimate litter concentrations.

Laboratory Methods - SP004:

a. For laboratory respirations, 5 gr (wet weight) of 2 mm sieved soil was added to a 25 mL Erlenmeyer flask and sealed with a serum bottle stopper. After the flask was sealed, the flask was preincubation for at least 1 hr at 15°C prior to taking the first headspace measurement. A sample of the headspace was then taken and analyzed using a gas chromatograph fitted with a TCD detector; two hours later, a second reading was taken. The resulting data were normalized to incubation time and g dry weight.

b. The methane oxidation experiments were conducted in much the same way as the respiration experiments except about 30 ppm methane was added to the incubation chambers and the flasks were incubated for one hour at 15°C. The reduction in methane was measured with a gas chromatograph fitted with a flame ionization detector. The same approach was taken with the nitrous oxide consumption but an electron capture detector was used. During the second July sampling denitrification potentials were measured by adding 5 g wet weight of sieved soil to a 25 mL Erlenmeyer flask which was stoppered and the headspace was purged with Ar for a minimum of 3 mins. at over 100 mL per min. Two mL of a sterile solution containing 10-3 M glucose and 10-3 M nitrate is injected through the stopper and an equal volume of headspace was removed. The flasks were incubated at 15°C for a least one hour before the first headspace sample is taken for N2O analysis. The sample is then incubated at the same temperature for two hours and a second headspace sample is taken using the ECD detector. During the second July, methane production measurements were also taken by purging the headspace of the flask with Ar before methane production was measured after at least a 1 hr preincubation period.

c. Dissolved Organic Carbon (DOC) was determined by adding 10 gr of sieved soil to 30 mL DH20 within a 125 mL Erlenmeyer flask. The flasks were shaken on a rotary shaker for 1 hr at 22°C. The soil slurries were allowed to settle at 22°C for 1 hr and then 1.5 mL was removed and added to a 1.5 mL capped microfuge tube. The slurry was spun for 5 min. in a Beckman microfuge and 0.5 mL was removed and placed into a 0.5 mL microfuge tube and frozen for future analyses. DOC measurements were made on a Dormann Carbon Analyzer using 100 ppm standards to calibrate the instrument.

d. Ten gr. sieved soil samples were used to measure extractable ammonium and mineralizable nitrogen. The extractable ammonium was measured by adding 50 mL of 2 M KCl to the soil in a 250 mL Erlenmeyer flask and then shaken on a rotary shaker for 1 hour at which time ammonium concentrations were determined using an Orion ammonium electrode. The minerlizable nitrogen was measured by adding 10 g of soil to a large 53 mL test tube that was then filled with DH20, sealed and incubated for 7 days at 40°C. Ammonium concentrations were determined in incubate soil slurries to which an equal volume of 4 M KCl was added. All of the above measurements except the DOCs were done in duplicate.

e. Ten gr samples were dried for 8 hr at 80°C to determine dry wts. One of these was heated in a furnace to 280°C for at least 4 hr and reweighed to determine level of labile carbon present. They were then heated again to 550°C for a least 4 hr to determine SOM. The other dried sample was used to determine pHs by adding 10X wt./vol. water.

Experimental Design - SP004:

These sites were selected to give a wide representation of different microclimates and vegetation types found at the HJA. The microclimate sites were selected along a rough N/S gradient from the top of a ridge to the North to another ridge to the South. Another set was selected along an roughly E/W gradient which gave representative sites of different elevations and aspects including sites that were hot and dry and those that are typically wet and cold. Also included were sites near streams that experience cooling. The vegetation sites included clearcut sites, and regenerated sites of different age classes. In addition, we included a Sitka alder and a broad-leave maple site.

Two types of studies were conducted; (1) monthly measurements of field respiration, temperature, moisture, laboratory respiration, soil organic matter by combustion to 550 C, labile carbon by combustion to 280 C, and dissolved organic carbon and (2) an extended measurements of other variables in addition to those normally measured during monthly studies. These were conducted at every other site along transects during the month of July 1992 and 1993. In July 1992, we measured mineralizable nitrogen, extractable ammonium, litter concentrations, litter respiration (separate from the normal total soil respirations taken monthly), and both nitrous oxide and methane consumption rates using gas concentrations approximately 10X ambient. In July 1993, we made the following changes from the 1992 protocol; bulk density was also measured as was denitrification potentials and methane production rates. The nitrous oxide and methane consumption experiments were dropped.

Andrews Experimental Forest
Ground condition
T:\GIS\oregon\willamette\hja27\gfluxpts: locations of sites obtained with gps Pathfinder unit and compass/tape