SITE DESCRIPTION
Samples were collected in Watershed 6 of the Hubbard Brook
Experimental Forest. Watershed 6 is forested by typical northern
hardwood species (sugar maple, beech and yellow birch) on the lower
90% of its area and by a montane boreal transition forest of red
spruce, balsam fir and white birch on the highest 10%. Hydrology and
stream water chemistry have been monitored since 1963. A grid system
of 208 plots, each 25 x 25 m square, was created in Watershed 6 in
1965. More detailed information on the elevation zones and plot layout
can be found under the 'watershed/research site' tab on
https://hubbardbrook.org.
SAMPLING DESIGN
From 1976 to 2002, each row of the Watershed 6 grid system of 25 x 25
m plots was considered a stratified sampling set and one or more plots
was randomly chosen from these lines as plots to be sampled. Some rows
are short, with only 1 to 4 plots, and others are longer, up to 10
plots across. The longer rows may have 2 or 3 plots sampled. The short
rows may not have any plots sampled. After 2002, a random number
generator was used to identify 100 plots over the entire watershed to
sample. The location of the sampling site within each plot was
determined by selecting two random integers between 0 and 24
("x" and "y") and navigating to the selected
coordinates from the northwest corner of the 25 x 25 m plot. The
target number of samples changed year by year, and the number of plots
sampled varies from 58 to 101.
FIELD COLLECTION PROCEDURES
Once a sampling point was located by taping or pacing, a 15 x 15 cm
template was placed on the forest floor and 4 nails were inserted
through pre-drilled holes, one on each corner. The nails hold the
template in place on the forest floor and retain the forest floor
material within the sample area beneath the template. A keyhole saw,
pruning shears and a trowel were used to make a clean cut around the
template, thus isolating a block of forest floor. The surrounding
forest floor material was scraped back from the block leaving it as a
pedestal. This block was then undercut in the mineral soil with a saw
or trowel to sever any roots. Using a trowel inserted beneath the
block and hands on top, the block was carefully picked up and turned
over such as to rest on the template. Any remaining mineral soil was
carefully scraped away "up" to the bottom of the Oa layer of
the forest floor. After all mineral soil was removed, the thickness of
the block was measured at the highest and lowest point on each of the
4 sides (1976-1982) or at 5 cm intervals along each of the four sides
(1987 onward) for a total of 8 measurements. Beginning in 2002, the
total thickness and the thickness of the combined Oi and Oe horizons
(referred to in this data set as "Oie") were measured at
each point. The average thickness of the Oa horizon could then be
determined by subtraction of the average Oie thickness from the
average total thickness for each block. In 1976, 1977, and 1982, the
entire block was returned to the laboratory for analysis. In 1978, the
sample was split into Oi, Oe, and Oa horizons in the field. From 1987
on, the block was separated into Oie and Oa horizons in the field.
Each horizon was placed in a separate plastic bag for return to the
laboratory.
Beginning in 1997, the upper 10 cm of the mineral soil was sampled by coring.
After sampling the O horizons, a stainless steel corer (diameter 1.362 inches 1997-2002,
diameter 1.374 inches 2013 onward) was inserted in the soil below the forest floor.
If the core length exceeded 10 cm, the core was cut to 10 cm and bagged. If the core
length was less than 10 cm, usually due to refusal by stones, the entire core was bagged.
Two full 10-cm cores, or up to four partial cores, were collected at each site, unless mineral soil was absent.
LABORATORY PROCEDURES
The whole sample (1976, 1977, and 1982), all horizons (1978 and 1987),
or Oie samples only (1992 onward) were oven dried at 80oC to a
constant weight, and ground in a Wiley mill to pass a 2-mm screen.
Large solid sticks and roots larger than the size of a pencil and
obviously alive or fresh material were removed and weighed. These were
not ground and were not included in the mass calculations.
The Oa horizon was air dried (1992 onward), weighed and then sieved
through a 5-mm stainless steel screen. Sticks, roots and debris
(coarse fraction) not passing through the screen with mild pressure
were weighed. Stones, which were rarely present in the Oa, were also
included in the coarse fraction. Material passing through the screen
was weighed and rebagged. A subsample of this air dry screened soil
was weighed, oven dried at 80oC, and weighed again. An air dry to oven
dry conversion factor was determined in order to express the total Oa
soil mass on an oven dry weight basis.
The mineral soils (1997 onward) were air dried, weighed, and passed
through a 2-mm stainless steel screen. Material passing through the
screen was weighed and rebagged. Material not passing through the
screen, including sticks, roots, and stones, was weighed. A subsample
of this air dry screened soil was weighed, oven dried at 80oC, and
weighed again. An air dry to oven dry conversion factor was determined
in order to express the total mineral soil mass on an oven dry weight
basis.
Samples of the ground/screened Oie, Oa, and mineral soils were put in
4 oz. glass jars for long-term preservation in the Hubbard Brook
archives.
For determination of organic content and acid-extractable metals
determination, a known mass of the oven dry ground material was placed
in a pre-weighed crucible and ashed overnight at 450-500oC. The ashed
material was cooled and weighed. The loss-on-ignition was computed and
used as a measure of organic matter content. The ash was eluted with
10 ml of 6N HNO3 and heated for a few minutes (to a simmer) on a hot
plate. The hot extract solution was poured into a filter paper
(Whatman # 41 ashless) in a funnel and into a 50 ml volumetric flask.
The crucible and the material on the filter paper were rinsed several
times with distilled deionized water. The volume in the flask was
brought up to 50 ml with distilled deionized water. The 50 ml of
extract solution was stored at room temperature until chemical
analysis. The mass of sample, analytes measured, and instrument used
each year are listed below. Analysis of plant tissue standard
reference materials (e.g. apple leaves, pine needles) has consistently
yielded near-100% recoveries, indicating that this digestion procedure
is effective for digesting organic matter. However, digestion of Oa
horizon and mineral soil samples is incomplete, as evidenced by
residual mineral matter on the filter paper. It is therefore safest to
refer to the results as "acid-extractable" rather than
"total" element content.
Total carbon and nitrogen were determined using oven-dried ground
material on a CHN analyzer.
CALCULATIONS
For organic horizons, total mass is the sieved mass of soil for the horizon multiplied
by the ratio of oven dry soil to air dry soil masses and divided by the area of the
template used for sampling (15 x 15 cm). For the mineral horizon, total mass is the
average sieved mass of soil in the cores multiplied by the ratio of oven dry soil to air
dry soil masses and divided by the diameter of the soil corer. Organic mass is the total
mass of soil in the horizon multiplied by the loss on ignition for the horizon. Concentrations
of elements in the forest floor were calculated on an oven dry weight basis. Total amounts of the
elements were estimated on a per square meter basis by extrapolating up from the 15 x 15 cm sample mass.
INSTRUMENTATION
Sample analysis varied between years in the mass of soil used,
analytes measured and instrumentatation used for each element.
Year_______Soil mass (g)__Analytes__Instrument
1976-1978____2.0_________Zn, Pb, Cu: AA
1976-1978____0.5_________Ca, Mg: ICP (analyzed in 1995)
1982_________2.0_________Zn, Pb, Cu: AA *
1982_________0.5_________Ca, Mg, K, P, Mg, Cu, Pb, Zn: ICP (analyzed
in 1998)
1987_________2.0_________Zn, Pb, Cu: AA
1987_________0.5_________Ca, Mg: ICP
1992_________0.5_________Ca, Mg, K, P, Mn, Zn, Pb, Cu: ICP
1997_________0.5_________Ca, Mg, K, P, Mn, Zn, Pb, Cu: ICP
2002_________0.5_________Ca, Mg, K, P, Mn, Zn, Pb, Cu: ICP
2013_________0.6_________Mg, K, P, Mn, Zn, Pb, Cu: ICP-MS
2013_________0.6_________Ca: AA
2018_________0.6_________Ca, Mg, K, P, Mn, Al, Fe: ICP; Cu, Pb, Zn:
ICP-MS
* All data included in the data table are from ICP analysis, except
plot 27 which had insufficient volume available for reanalysis.
NOTES
The primary objectives of the forest floor samplings in 1976-1978 were
1) the determination of the amount of lead (Pb) in the forest floor,
2) monitoring the total mass of the forest floor over time and 3)
development of a field collection methodology that was efficient and
easily implemented. It was determined that separation of the forest
floor at least into Oie and Oa horizons, would have been better than
no separation whatsoever, so this procedure was implemented from 1987
onwards. It was also determined that air drying the Oa horizon would
have been preferable to oven drying. Air drying makes possible the
determination of exchangeable nutrients, whereas oven drying may alter
the cation exchange complex. Air drying of the Oa horizon samples was
implemented from 1992 onward.
The forest floor of Watershed 6 was sampled twice in 1969 and once in
1970. Since sampling technique differed significantly from the
sampling technique used from 1976 onward, the earlier data are not
included in this dataset. They may be requested in hard copy format
from Chris Johnson at Syracuse University.
